PHY 504 Exam 3

Kidney Handling of Filtered Substances

1) A substance can be filtered at the glomerulus, and that's it --This occurs when there are no channels or transporters expressed in the tubule for the filtered substance --So whatever gets filtered will show up in the urine 2) A substance can be filtered and partially reabsorbed --A substance is filtered at the glomerulus and then some of the substance is reabsorbed by the peritubular capillaries --But some still remains in the tubular fluid to be excreted in the urine 3) A substance can be filtered and then completely reabsorbed (this is what should happen with glucose) --Glucose is small enough to be readily filtered in the nephron, but all filtered glucose is normally reabsorbed so that none of it will be excreted in the urine 4) A substance can be filtered and then secreted --Means that the amount of this substance in the urine will be greater than the amount that was filtered The handling of different substances by the kidney can be used to evaluate renal function

Filtration at the Renal Corpuscle

A highly regulated process that ensures that only what we want to get filtered is actually filtered --The filtration setup consists of a 3-stag filter with progressively smaller pores: the capillary endothelium, basement membrane, and epithelial lining of Bowman's capsule Capillary Endothelium --The fenestrations between cells are typically around 60-70 nm wide, and these openings exclude cells, but not macromolecules, from the ultrafiltrate --The luminal surface of the endothelium is covered with negatively-charged sialoglycoproteins, which contribute to the selectivity of the filtration barrier based on charge Basement Membrane --There are three layers to the glomerular basement membrane: endothelial cells attached to the lamina rara interna, the central lamina densa, the lamina rara externa on the epithelial side --The lamina rara interna and lamina rara externa contain polar, noncollagenous proteins (like heparan-sulfate proteoglycans) that contribute to the negative charge of the filtration barrier --The lamina densa contains nonpolar collagenous proteins that primarily contribute to size selectivity --The filtration barrier is permeable to molecules with an effective molecular radius that is less than 2 nm and impermeable to those with radii greater than 4 nm Bowman's Capsule Epithelial Cells --These cells have foot-like processes called podocytes that are spaced out to let certain substances pass through and prevent the filtration of others --Podocytes wrap around the glomerular capillaries and basement membrane --The capillary endothelial cells are almost entirely surrounded by the glomerular basement membrane and the layer of podocyte foot processes --The one exception to this setup is a small region near the center of the glomerulus where endothelial cells come into direct contact with mesangial cells (their contractile properties enable mesangial cells to modulate intraglomerular capillary flow and ultrafiltration surface area--this helps to regulate the single nephron glomerular filtration rate) --Between podocytes is a slit diaphragm that contains pores between 4-14 nm ---The diaphragm is essential for regulating the filtration process ---There are several proteins that expressed to control slit permeability: if we lose function of slit proteins, the impact can be profound on what is and is not filtered ---Nephrin and podocin are two such proteins: the genetic absence of nephrin leads to several proteinuria and Finnish-type nephrosis; mutation of podocin causes steroid-resistant nephrotic syndrome If a substance can cross all three layers, it will appear in Bowman's capsule

RAAS

A long-term mechanism for increasing BP by way of increasing blood volume There are specialized muscle cells at the afferent arterioles of the kidney called juxtaglomerular cells --These synthesize, store, and release renin --When baroreceptors sense a drop in BP, these cells are signaled through sympathetic outflow from the medulla to release renin --Macula densa cells in the distal tubule also stimulate juxtaglomerular cells to release renin, but this is typically accomplished through the release of prostaglandins after sensing a decrease in NaCl

Regulation of Effective Circulating Volume--Angiotensin II

A secretagogue for aldosterone as part of RAAS --Secretagogue: any substance that causes another substance to be released Also has effects on sodium and water reabsorption that are independent of aldosterone Can directly stimulate sodium reabsorption along the renal tubule --Occurs by upregulating certain channels: Na/H exchanger on the apical membrane, sodium-bicarb cotransporter, and Na/K pump on basolateral membrane Can also regulate fluid movement by its constrictive effects on the efferent arterioles --When efferent arterioles are constricted, hydrostatic pressure of the peritubular capillaries drops --Constriction also causes renal blood flow to slow down ---A lower blood flow means that filtration fraction increases ---As filtration fraction goes up, plasma proteins become more concentrated, thereby increasing the colloid osmotic pressure --All of these changers promote an increase in sodium and fluid absorption in response to the hemodynamic effects of angiotensin II If there is an increase in ECF and BP, then normal volume and pressure levels can be restored by inhibiting the production or action of angiotensin II --Several compounds that target different segments of the RAAS can reduce the activity of angiotensin on the body (include ACE inhibitors, angiotensin II antagonists, renin inhibitors) --When angiotensin II activity is blocked, we see an increase in sodium and fluid excretion, which yields a reduction in BP

Sample Calculation: You want to measure the total body water in a patient. You inject intravenously 10 mL of a solution of antipyrine with a concentration of 150 mg/mL. After equilibrium, the concentration of antipyrine in the blood is 0.03 mg/mL. What is the total body water of this patient?

Amount of antipyrine injected (Q) = 10 mL * 150 mg/mL = 1500 mg Total body water = Q/C = 1500 mg/(.03 mg/mL) = 50,000 mL = 50L

Diuretics

Acetazolamide --Blocks carbonic anhydrase and mainly acts in the proximal tubule --Important for renal adaptation to maintain acid-base balance at high altitudes by promoting the excretion of bicarbonate Furosemide --Regulates blood pressure by changing the ability of the thick ascending loop of Henle to exchange sodium for water Chlorothiazide and Amiloride --Work in the distal convoluted tubule to regulate blood pressure --Chlorothiazide: reduces salt reabsorption from the tubule, which means that more water is retained in the tubular fluid --Amiloride: known as a potassium-sparing diuretic because it reduces the excretion of potassium by blocking apical epithelial sodium channels directly; leads to hyperpolarization of the membrane, thereby reducing the electrochemical gradient that would normally promote distal potassium secretion

Metabolic Responses

Acidosis ([HCO3-] < 22 mEq/L) --Causes 1. Carbonic anhydrase inhibitors (i.e. acetazolamide) 2. Diarrhea (lot of bicarb secrete into duodenum from pancreas during digestion) 3. Severe diabetic ketoacidosis The body can compensate for metabolic acidosis with respiratory alkalosis due to hyperventilation, but only to a certain extent Alkalosis ([HCO3-] > 26 mEq/L) --Causes 1. Vomiting (loss of stomach acid) 2. Excessive ingestion of antacids 3. Overconsumption of bicarb (diuretic or laxative abuse)

Disorders of Aldosterone Supply

Addison's Disease (Adrenal Insufficiency) --A condition in which patients do not produce sufficient cortisol (and sometimes not enough aldosterone) --Hyperkalemia results from aldosterone deficiency because fewer Na/K pumps and apical potassium channels means that there is less potassium secretion into the tubular lumen--so more stays in the plasma --Normally, around 2-3% of filtered sodium is under control of aldosterone: but the sustained loss of even such a small fraction would exceed the daily sodium intake and lead to severe sodium depletion, loss of ECF volume, and circulatory insufficiency/hypotension Conn's Syndrome (Primary Aldosteronism) --Can result from a tumor or other abnormal growth on the adrenals that cause an overproduction of aldosterone --The resulting sodium retention and potassium excretion leads to severe electrolyte and fluid imbalance in the body --You can also get an overproduction of aldosterone through diuretic abuse (often abused for weight loss) ---When diuretics are abused, there is excessive urine production, so the ECF volume is decreased ---Volume depletion means a lower GFR, which triggers the macula densa to stimulate the RAAS --The more aldosterone is secreted, the more reabsorption of sodium and water there will be at the expense of potassium secretion ---This means that, as sodium accumulates in the blood, plasma levels drop for potassium ---Increased potassium secretion in the distal tubule also means increased tubular acid secretion: the more acid gets secreted, the more bicarb gets reabsorbed, which will tip the scales towards metabolic alkalosis

Aldosterone Role in RAAS

Angiotensin II stimulates the cortical adrenal gland to release aldosterone Aldosterone then acts on principal cells in the distal nephron to promote potassium secretion and sodium and water reabsorption Aldosterone directly stimulates the basolateral Na/K pump with enhances sodium reabsorption and potassium secretion Aldosterone also promotes the placement of additional potassium channels in the apical principal cell membrane, thereby boosting permeability for secretion of potassium --High aldosterone levels can therefore lead to hypokalemia --Urine acidification is also promoted indirectly by aldosterone due to its action to stimulate epithelial sodium channels on the principal cell ---Increased sodium reabsorption enhances the lumen-negative transepithelial voltage that facilitates proton secretion by the neighboring intercalated cells in the cortical collecting tubule In intercalated cells, the proton pump is stimulated by aldosterone to increase the secretion of protons into the lumen to acidify it

RAAS Mechanism

Angiotensinogen is release from the liver and renin from the kidney Renin functions as an enzyme that cleaves angiotensinogen to form angiotensin I Endothelial cells in several parts of the body, but particularly the lungs, release the ACE enzyme (angiotensin-converting enzyme) --ACE converts angiotensin into angiotensin II Angiotensin II exerts effects on 4 cell types 1. Smooth muscle cell constriction --Preferentially affects the efferent arterioles in the kidneys, which causes glomerular hydrostatic pressure to increase in response to elevated efferent arteriolar resistance; the result is an increase in GFR 2. Direct action on the kidneys to release water --Accomplished by enhancing sodium retention, both directly by stimulating apical sodium-proton exchange in the proximal nephron and indirectly by lowering the renal plasma flow 3. Angiotensin II receptors on the hypothalamus stimulate the posterior pituitary to release ADH 4. Actions of angiotensin II on adrenal glands leads to aldosterone release Angiotensinogen and ACE are continuously available --The RAAS cascade can only proceed when renin is released to cleave angiotensinogen into angiotensin I, thereby providing the substrate for ACE --The final result is an increase in sodium and water reabsorption in the kidney, which increases the fluid in the body to correct the change in effective circulating volume and pressure

Potassium Balance

Around 98% of all potassium in the body resides within cells --Because of its importance in the membrane potential of electrically excitable cells, maintaining the balance of potassium between the intracellular and extracellular environments in critical There are two steps to regulating potassium balance --Rapid regulation: largely dependent on the exchange of potassium between the intracellular and extracellular environments (internal balance mechanism) --Long-term regulation: done by modulating urinary excretion of potassium (external balance mechanism) There are several scenarios that could upset potassium balance --Ingestion of a potassium-rich meal --Cell lysis: when cells in the body rupture, the intracellular potassium gets release and can contribute to an increase in the potassium content of the ECF --Strenuous exercise: triggers an increase in potassium due to the high rate of skeletal muscle APs, which require large effluxes of intracellular potassium stores --Acid-base imbalance: there are proton-potassium pumps in the membranes of some cell types ---When the pH of the plasma changes, the activity of the H-K pump changes which will cause a change in potassium balance (acidosis tends to move protons into cells in exchange for potassium being shuttled out) Normal plasma concentration of potassium is 3.5-5.0 mEq/L --Severe hyperkalemia (>7/0 mEq/L): partial depolarization of cell membranes, cardiac toxicity (vfib or asystole) --Severe hypokalemia (<3.0 mEq/L): hyperpolarization of cell membranes, fatigue, muscle weakness, hypoventilation, delayed ventricular repolarization

Regulation of GFR

Autoregulation --Resistance, renal plasma flow rate Extrinsic regulation --ANS control of arteriolar smooth muscle contraction --Smooth muscle cells of the afferent and efferent arterioles play a main role in controlling renal plasma flow by varying resistance --This can be accomplished by the autonomic innervation to alter vascular smooth muscle tone or through vasodilator metabolites like NO, CO2, and protons that can relax this smooth muscle

Production of New Bicarbonate in the Kidneys

Bicarb is the most important buffering system used by the kidneys; but there are two other systems: phosphate buffer and ammonia buffer

Indicator Dilution Principle

Based on the principle of the conservation of mass --The mass of an indicator in a syringe can be calculated by multiplying the volume of the syringe by the concentration of the indicator --Injecting all of the indicator into a beaker of liquid means that the entirety of the mass of the indicator is now diluted in the volume of the liquid in the beaker --Because there has been no change in the mass of the indicator, we can determine the volume in the beaker by dividing the known indicator mass by the new concentration in the beaker This setup gives the following equation: Volume = Indicator Mass/Concentration = Q/C The equation only works if a few criteria for the indicator are met --The indicator must disperse only in the compartment to be measured; if small amounts of it are able to leak into another compartment, the measurement won't be accurate --The indicator must disperse evenly throughout the compartment so that when a sample is taken to determine the new concentration, we can be confident that the sample concentration is the same for all fluid in the compartment --The indicator cannot be metabolized or excreted from the body within the sampling window; if either of these occurs, then a correction must be added to the calculation to account for the change in mass

Mechanisms of Urine Secretion

Basic Steps Step 1: filtration at the glomerulus Step 2 and 3: materials exchanged between renal tubular fluid and peritubular capillaries --Step 2: reabsorption of content (tubule to capillary) --Step 3: secretion of content (capillary to tubule) --Step 4: excretion The amount of a substance that is excreted can therefore be determined as follows: Excretion = Filtration - Reabsorption + Secretion Filtration is not very selective for specific molecules, with the exception of proteins --About 20% of renal plasma flow is filtered at the glomerulus at any given time Reabsorption is much more selective and typically encompasses electrolytes like sodium, potassium, and chloride as well as nutritional substances like glucose --All glucose that gets filtered in the renal tubules should be reabsorbed, leaving none in the excreted urine --Most waste products, with the exception of urea, are not reabsorbed Secretion varies greatly and is important for removing waste products like acid, as well as foreign substances including drugs

Fluid Buffering

Bicarbonate is a major buffer in the extracellular fluid The hydration of carbon dioxide with the help of carbonic anhydrase is the first part of the process The second part is the dissociation of carbonic acid into a proton and bicarb (almost instantaneous) --These can then be buffered by hemoglobin This buffering is only temporary--eventually, excess acids and bases must be eliminated from the body either via the lungs or kidneys The Henderson-Hasselbach equation can be used to calculate pH for the main buffer mechanism in the blood pH = 6.1 + log ([HCO3-]/aPCO2) --The a (alpha) is the solubility of CO2 at body temperature in plasma (0.03 millimolar/mmHg) --6.1 is the pK governing the CO2/bicarb equilibrium at body temperature

Body Fluid Compartments

Body fluid is divided into four main compartments --Plasma (capillaries) --Interstitial fluid --Intracellular fluid --Transcellular fluid (i.e. synovial fluid or CSF) Fluid that resides inside all cells in the body comprises the largest proportion of fluid in the body (~60%) --Of the remaining 40% of fluid in the extracellular environment, 75% is interstitial fluid and 20% is found in plasma Substances that pass from the plasma across the capillary endothelial membrane enter the interstitial fluid before crossing into the cells

Calcium Reabsorption in the Proximal Tubule

Calcium mostly moves paracellularly in the proximal tubule, but some will move transcellularly The proximal tubule is highly permeable to calcium, and there is a strong gradient to drive reabsorption Transcellular reabsorption involves passive calcium uptake through apical channels and active basolateral extrusion through the calcium ATPase or an electrogenic sodium-calcium exchanger

Glucose Titration

Carrier proteins have a finite rate of transport, which means that they can undergo saturation --The transport maximum is the point at which the solute concentration is high enough that all of the carrier proteins are occupied Renal threshold is when the solute concentration in the plasma rises to the point that the filtration of that solute is greater than the transport maximum of the carriers --In this case, some of the solutes start to spill over and appear in the urine --The theoretical value for glucose transport maximum is 300 mg/dL --But the true renal threshold for glucose for the entire kidney is around 160-180 mg/dL ---Not every molecule or nephron in the kidney functions at 100% efficiency ---Some glucose molecules in the filtrate will avoid a carrier molecule, even though carriers are not 100% saturated ---Some nephrons will also have lower threshold maximums than others The difference between renal threshold and the transport maximum is a parameter known as splay --Nephrons differ in glomerular size and the density/distribution of carrier proteins --This accounts for some nephrons getting a filtered load of glucose that is greater than their capacity to reabsorb it

Phosphate Buffer System

Carries away excess protons that are formed alongside the newly generated bicarb Protons bind to divalent phosphate to form the monovalent phosphate (H2PO4), which is not easily transported across the apical membrane --So, the kidneys will excreted this proton-bound phosphate For each proton that is secreted into the lumen to make H2PO4, the cell generates one new bicarbonate ion to move to the blood Typically, these phosphates in the lumen are ionically linked to sodium as well (particularly in the proximal tubule) --This sodium-proton exchange, which is very sensitive to intracellular pH, appears to be the most important mechanism for acid secretion in this buffering system

Detection of Fluid Volume

Changes to the extracellular fluid volume can be sensed in specific regions where volume sensors are located, like thoracic blood vessels The relationship between extracellular fluid volume and effective circulating volume are not always equivalent --Effective circulating volume is a functional volume that reflects the extent of tissue perfusion --ECF includes all fluid that is outside of a cell membrane including plasma, transcellular fluid, and interstitial fluid Within extensive edema, the total ECF volume is greatly increase --But the effective circulating volume has decreased as plasma fluid has been filtered out into the surrounding tissues --Because this excess fluid is not in the bloodstream, the usual mechanisms for sensing blood volume will not be activated; as far as the bloodstream is concerned, there is no fluid problem in the body

Urea

Chief waste product from protein metabolism and is excreted from the body primarily through the urine (also through stool and sweat) Urea is freely filtered at the glomerulus and is then both reabsorbed and secreted --Urea reabsorption happens at the beginning of the tubule and at the end (proximal tubule and medullary collecting ducts) --Urea secretion occurs in the middle (thin loop of Henle) There is a net reabsorption of urea along the nephron --So the amount of urea that is excreted in the urine is less than the amount that was filtered

Comparing Glucose to Inulin

Clearance of inulin is a measure of GFR because inulin is freely filtered, but is neither reabsorbed nor secreted by the tubule Since glucose is an essential nutrient, its clearance should always be well below the GFR --At filtered loads that are low, the clearance of glucose is 0, because all filtered glucose is reabsorbed (so none in the urine) --As the filtered load surpasses renal threshold, glucose excretion in the urine will increase as plasma glucose concentration rises --When the filtered load is extremely high, then glucose will behave like inulin and its clearance will progressively increase as carriers cannot transport all the glucose that is present in the tubular fluid

Two Types of Nephrons

Cortical --Positioned almost entirely in the cortex of the kidney, which is the site of filtration --The base of the renal tubule enters the outer medulla --About 85% of nephrons in the kidney are cortical nephrons Juxtamedullary --Have a specialized role in the production of concentrated urine --Filtration apparatus resides near the cortical-medullary boundary, and renal tubules dive into the lower depths of the medulla --Supplied by the vasa recta in place of typical peritubular capillaries --Make up about 15% of nephrons

Potassium Transport at the Late Distal and Cortical Collecting Tubules

Direction of potassium transport depends on the cell type 70% of cells here are principal cells, which secrete potassium into the lumen --Secretion is mediated by basolateral uptake of potassium through the Na/K pump followed by passive diffusion through apical potassium channels --Aldosterone's function to promote sodium uptake at the expense of potassium secretion into the lumen occurs because of its effect on the Na/K pump in the principal cells --If aldosterone is blocked, then the principal cell response to secrete excess potassium in response to elevated intake does not occur, and hyperkalemia can develop (can happen with Addison's disease) The remaining 30% of cells here are intercalated cells, some of which reabsorb potassium and some of which secrete it --The alpha intercalated cell transports potassium from the lumen into the interstitium, which is reabsorption ---Occurs through active apical uptake of the H-K pump followed by passive efflux across the basolateral membrane ---Because of the coupled transport of protons and potassium, acidosis causes an increase in the activity of the H-K pump which promotes acidification of the urine --The beta intercalated cells secrete potassium into the lumen (in alkalosis, the beta-intercalated cells secrete excess bicarbonate)

Osmotic Pressure

Distribution of fluid across cell membranes is determined by osmotic forces from electrolytes Distribution of fluid across capillaries is determined by hydrostatic and colloid osmotic forces PI = CRT, where C = concentration (Osm/L) R = gas constant (62.3 if PI is in mmHg) T = Kelvin (273 + *C; 310 for normal body temp) For normal values, equation becomes: PI = C * 19.3 mmHg (for C is mOsm/L)

Diabetes Insipidus

Dysfunction in the ADH mechanism --Can be designated as central or nephrogenic, depending on where the defect in ADH signaling lies Central --The problem is with the production and release of ADH --The kidneys are capable of responding to ADH in the blood, but the levels in the plasma are insufficient because not enough is released from the brain when needed Nephrogenic --The problem is that renal responsiveness to ADH is reduced --Here, there is enough ADH in the blood but the problem is getting the kidneys to recognize and respond normally to the presence of plasma ADH

Calculating Tubular Reabsorption and Secretion

For a substance that is excreted at a lower rate than its filtered, there must be net reabsorption occurring --Amount reabsorbed is filtered load minus the amount excreted into the urine For a solute that is filtered and exhibits net secretion in the tubular fluid, the amount that shows up in the urine will be greater than what is filtered --To determine the net secretion, we take the difference of the excretion and filtration values --If the value comes out negative, it means that no net secretion is occurring Filt s = GFR * Ps Ps: plasma conc. of s Excret s = Us * V Us: urine conc. of s V: urine flow rate

Renal Clearance

For solutes that the kidney doesn't synthesize, degrade, or accumulate, the entry point is the renal artery and the exit is the renal vein or ureter --We can determine the amount of solute entering the kidney by multiplying the concentration of solute X in the renal artery by the renal plasma flow in the renal artery --The resulting arterial input of solute X will then either leave through the renal vein or the ureter for excretion Clearance measurements compare the rate at which the glomeruli filter a substance with the rate at which the kidneys excrete it into the urine --By measuring the difference between filtered and excreted amounts, we can estimate the net amount that is reabsorbed or secreted by the renal tubules --This gives insight into the filtration, reabsorption, and secretion processes of the kidney Clearance cannot tell us information about a specific nephron, because this calculation measures overall nephron function for all nephrons in parallel in the kidneys We can use the clearance of a solute to determine important parameters that define kidney function --By definition, the clearance of a solute is the virtual volume of plasma that would be completely cleared of a solute in a given amount of time --All solutes that are excreted into the urine originally come from blood plasma perfusing the kidneys --So the rate at which the kidneys excretes a solute into the urine equals the rate at which the solute disappears from plasma --Assumes that the kidney does not produce, consume, or store the solute

Sodium Reabsorption

For transcellular movement, sodium enters the cell passively from the tubular lumen because both the electrical and chemical gradients drive sodium into the cell --But the method of entry differs between tubule segments: sometimes sodium movement is coupled with another solute, sometimes sodium moves through channels alone --The extrusion of sodium on the basolateral membrane occurs usually by activate transported mediated by the Na/K pump Paracellular movement does not require channels or transporters --Tight junctions allow epithelial cells to permit flow of fluid and solutes between the lumen and interstitial fluid --The direction of movement can change though ---The S2 and S3 proximal segments in the proximal tubule, as well as the thick ascending limb, passively reabsorbed paracellularly ---Everywhere else, there is a negative net driving force that favors passive paracellular sodium diffusion from the interstitial fluid to the lumen (referred to as backleak)

Myogenic Mechanism

From Ohm's Law, it is known that a higher arterial pressure would increase blood flow if resistance was stable This elevated pressure stretches the renal blood vessels, which open stretch-activated nonselective cation channels in vascular smooth muscle cells Cations can enter the cell and the resultant depolarization leads to an influx of calcium Calcium stimulates contraction, thereby diminishing the vessel radius and increasing vascular resistance Vasoconstriction slows down RBF, thereby stabilizing it under the new, high arterial pressure

GFR Regulation Summary

Increasing Kf = increased GFR Increasing Bowman capsule hydrostatic pressure = decreased GFR Increasing glomerular colloid osmotic pressure = decreased GFR Increasing filtration fraction = increased glomerular colloid osmotic pressure = decreased GFR Increasing glomerular hydrostatic pressure = increased GFR Increasing afferent arteriolar resistance = decreased glomerular hydrostatic pressure = decreased GFR Increasing efferent arteriolar resistance = increased glomerular hydrostatic pressure = increased GFR

Ammonia Buffer System

Functions to trap protons in the lumen for excretion Most of the luminal ammonia is not filtered at the glomerulus --We shouldn't have a lot of ammonia floating around in the bloodstream --Instead, most ammonia diffuses into the lumen from tubule cells In the proximal tubule, the conversion of glutamine to alpha-ketoglutarate generates 2 ammonium ions, which dissociate into 2 ammonia and 2 protons --Glutamine enters from both the luminal and interstitial sides through sodium-coupled cotransporters --Glutamine is converted into ammonium and glutamate by the glutaminase enzyme --Glutamate is then split into another ammonium ion and alpha-ketoglutarate Ammonia diffuses cross a lipid membrane better than ammonium --When ammonia leaves the relatively alkaline proximal tubular or collecting duct cells, it buffer luminal protons and become trapped because of low membrane permeability to ammonium The metabolism for alpha-ketoglutarate generates the substrate for carbonic anhydrase to convert new bicarb ions to enter the blood --Alpha-ketoglutarate contributes to the gluconeogenesis within the proximal tubular cell, producing new bicarb in the process--this bicarb enters the interstitial fluid through the basolateral membrane In the thick ascending limb, much of the NH4+ that was secreted by the proximal tubule and thin descending limb is reabsorbed --The ammonium ion can replace potassium on the Na/K/2Cl cotransporter to cross the apical membrane; it then leaves the basolateral side as ammonia This pattern of ammonia and ammonium transporter keeps the ammonia within the medulla of the kidney --Reabsorption of ammonia from the thick ascending limb generates a gradient to promote its secretion into the then descending limb and medullary collecting ducts --Ammonium ions in the interstitial fluid will dissociate into protons and ammonia, which take up by the proximal tubule and descending limb, and becomes trapped as ammonium --Interstitial ammonia is also taken up by the medullary collecting ducts and actively secreted into the lumen, where it buffers protons to become trapped as well --This set up prevents toxic ammonia from returning to the circulation by bypassing cortical portions of the nephron were a lot of reabsorption of solutes takes place In the medullary collecting duct, ammonia can diffuse directly from the interstitium to the lumen transcellularly --Or the Na/K pump can carry ammonium back into the cells of the medullary collecting ducts (due to ammonium being substituted for potassium) --The protons that results from intracellular dissociation of ammonium ions are then actively secreted into the lumen and are titrated by the ammonia to form new ammonium which remains in the tubular fluid for excretion

Effect of Diuretics on Calcium Reabsorption

Furosemide: Decrease --Inhibits the Na/K/2Cl cotransporter in the thick ascending loop of Henle --This inhibition has the consequence of reducing the lumen-positive voltage in the segment --So, the driving force for passive paracellular calcium reabsorption decreases, meaning more calcium is excreted into the urine when a patient is taking furosemide Thiazides: Increase --Block the sodium-chloride cotransporter in the distal tubule --With less influx from the lumen, the flow of ions through the basolateral sodium-calcium exchanger will increase --Low intracellular sodium level means a greater driving force for sodium to enter cells through the exchanger --This elevates calcium extrusion through the basolateral membrane, thereby enhancing the gradient for passive calcium uptake from the lumen Amiloride: Increase --Inhibits apical sodium channels in the initial and cortical collecting tubules --This hyperpolarizes the apical membrane which increases calcium uptake from the lumen The caveat for thiazides and amiloride is that both of these effects require physiological parathyroid hormone levels to keep apical calcium channels open

Compensation

Generally speaking, if the lungs are the problem, the kidneys are the mechanism for compensating the altered pH When the problem is metabolic, the initial compensatory mechanism is to alter ventilation rates in the lungs until the kidneys can kick in for long-term change to restore pH

Glomerular Filtration Rate

Glomerular filtration of solutes and fluid out of the capillaries is driven by Starling forces --Hydrostatic pressure difference: difference between the intravascular pressure within the capillary and the extravascular pressure of the interstitial fluid (positive hydrostatic pressure drives water out of the capillary) --Colloid osmotic pressure difference: difference between intravascular colloid osmotic pressure caused by plasma proteins and proteoglycans (positive colloid osmotic pressure difference attracts water into the capillary lumen) --The filtrate that is produced in this process resembles plasma, except that it lacks the proteins that are present in plasma Under normal conditions, GFR is 125 mL/min or around 180 L/day --This high rate is necessary to expose all extracellular fluid to the scrutiny of the renal tubular epithelium Size and charge play a role in filtration --Generally, any solute with a molecular weight that is less than or equal to 5.5 kDa is readily filtered at the glomerulus --Solutes that are between 5.5-6.9 kDa will also be filtered, but at a slower rate --6.9 kDa is the cutoff point: solutes above this size are not filtered

Glucose

Glucose reabsorption should be absolute along the tubule The first 1/3 of the proximal tubule absorbs most of the filtered load of glucose --Nearly all of the rest is reabsorbed in the more distal parts of the proximal tubule If any glucose remains in the tubular fluid as it moves into the loop of Henle, that glucose will show up in the urine --Due to the fact that glucose transport is mediated by carrier proteins, which are only expressed in the proximal tubular epithelium --This carrier-mediated transport differs from salt transport, which depends only on the size of the gradient

Macula Densa (Tubuloglomerular) Feedback

Has two components that act together to control GFR: afferent arteriolar feedback and efferent feedback The juxtaglomerular apparatus is involved in feedback mechanisms that regulate renal function --The juxtaglomerular apparatus contains the mesangium, macula densa cells (thick ascending loop and early distal tubule), and the juxtaglomerular cells in the wall of the arterioles --The macula densa are specialized epithelial cells that may secreted signaling substances towards arterioles as part of a sensory feedback mechanism; due to dense Golgi apparatus When there is a drop in GFR, it means that fewer solutes are present in the distal nephron --Lower solute delivery means that the concentration of sodium chloride reaching the macula densa is decreases --This triggers the macula densa to initiate two responses: vasodilation of afferent arterioles and vasoconstriction of efferent arterioles Vasodilation of Afferent Arterioles --Decreases afferent arteriolar resistance which increases glomerular hydrostatic pressure and returns GFR to normal --As GFR returns to normal, the sodium chloride reaching the distal nephron returns to normal concentrations and shuts off this mechanism Vasoconstriction of Efferent Arterioles --When GFR is decreased, the macula densa signals the juxtaglomerular cells to release renin --Renin functions as an enzyme to increase the formation of angiotensin I, which is then converted to angiotensin II --Angiotensin II preferentially constricts efferent arterioles --This vasoconstriction creates a bottleneck of blood flow, which increases the glomerular hydrostatic pressure and returns GFR toward normal

Ultrafiltration Pressures at Glomerular and Peritubular Capillaries

If the ultrafiltration pressure at the glomerular capillaries is positive, then this means that there is net filtration of solutes and fluid out of the capillaries --This filtration elevates the colloid osmotic pressure of the blood that is entering the peritubular capillary network from around 25 to 35 mmHg --As a blood passes through the efferent resistor, intravascular hydrostatic pressure decreases from 47 to 20 mmHg --The increase in colloid osmotic pressure and the decrease in hydrostatic pressure yields a negative ultrafiltration pressure, which means that fluid is more likely to enter peritubular capillaries than leave it --This is why reabsorption of solutes and fluid is favored at the peritubular capillaries At the glomerulus, the capillaries and Bowman's space are connected through endothelial cells, a basement membrane, and podocyte epithelial cells --The peritubular capillaries, however, do not have direct communication with the tubule epithelium --Instead, the exchange of substances occurs between the capillary blood and renal tubules by way of intermediary interstitial fluid

Importance of Renal Autoregulation

If we start with a mean arterial pressure of 100 mmHg, GFR is 125 L/day of which 124 L is reabsorbed from the tubules --This leaves a urine volume of 1L produced per day If pressure were increased to 120 mmHg and we had poor autoregulation and no change in tubular reabsorption, a lot more urine would be produced per day (because filtration increases greatly while reabsorption stays the same) --Urine production would be 26 L/day With good autoregulation, even in the absence of change in tubular reabsorption, the change in urine production would not be so high (~6 L/day) When both mechanisms are functioning properly, the result is a very small increase in urine production per day (~1.2 L/day)

Balance Concept

In order to maintain homeostatic conditions, fluid and electrolytes must be in balance --Whatever is lost from the body must be replaced in kind Intake --Fluid: regulated by thirst mechanisms and dietary habits --Electrolytes: regulated by dietary habits Output --Fluid and electrolyte levels in the blood are regulated mainly by the kidneys, which provide an avenue for excreting anything that is in excess A 70kg adult with a normal diet tends to take in most fluids through ingestion with metabolism contributing about 10% of fluid intake --Fluid is then excreted in sweat, feces, and urine --There are also insensible water losses: water loss we are not generally aware of (transepidermal diffusion of water that passes through the skin and is lost be evaporation; evaporative water loss from the respiratory tract) If a person consumes a high salt diet, more fluids are ingested on a daily basis --To compensate, the kidneys adjust to excreted the excess fluid; but this adjustment takes time --Before the intake level is increased, intake and excretion levels are equal --Within 2-3 days of raising sodium intake, renal excretion increases by the same amount (balance between intake and output is re-established) --During those 2-3 days, there is a slight accumulation of sodium that raises ECF volume slightly --This triggers compensatory responses, including hormonal changes, that signals the kidney to increase sodium excretion Heavy exercise is another scenario that triggers an increase in fluid ingestion --But, strenuous exercise also causes an increase in ventilation rate and heat production through metabolism --So, insensible fluid loss through breathing and increased sweat account for a larger proportion of fluid output --With more fluid loss from other means, the kidney will adapt to reduce urine production so that fluid output matches intake

Acid Secretion

In the proximal tubule, thick ascending limb, and early distal tubule we get active secretion of protons into the renal tubule in exchange for sodium --Tubular reabsorption of bicarb occurs through the formation of carbonic acid and sodium reabsorption provides the energy for the active secretion of protons into the lumen In the intercalated cells of the late distal and collecting tubules, acid secretion occurs through primary active transport via a proton pump --For every proton that is secreted, one bicarb ion is absorbed and one chloride is passively secreted alongside the proton --This proton secretion occurs in two steps: 1. Dissolved CO2 in the cell combined with water to form carbonic acid 2. Carbonic acid then dissociates into a bicarb ion, which is absorbed in the blood, plus a proton, which is secreted into the tubule through the proton ATPase Proton secretion in the distal segments accounts for only about 5% of the total secretion of acid --But, this mechanism is very important for producing a maximally acidic urine

Calcium Reabsorption in the Thick Ascending Limb of the Loop of Henle

In the thick ascending limb, calcium reabsorption rates are governed by the amount of calcium in the plasma There is an intrinsic mechanism involving a calcium-sensing receptor located in the basolateral membrane --The expression of this receptor means that the kidney can independently respond to changes in extracellular calcium --When calcium concentrations in the interstitium are high, calcium reabsorption in the tubule is inhibited --This occurs through a G-protein coupled cascade Activating the inhibitory G protein decreases cAMP levels inside the cell --The Na/K/2Cl cotransporter is normally stimulated by cAMP; so reducing available cAMP decreases the activity of the cotransporter --The inhibitory G protein can also stimulated phospholipase A2 to increase arachidonic acid and 20-HETE, which both also inhibit the Na/K/2Cl cotransporter --Activating the GalphaQ pathway has the effect Inhibiting the Na/K/2Cl cotransporter causes the tubular lumen to become less positive --The signaling mechanism reduces both the electroneutral uptake of Na, K, and Cl (by inhibiting the transporter) and the apical secretion of potassium into the tubule --Normally, paracellular calcium absorption is because of the positive voltage that results from potassium cycling around the apical membrane --When this process shuts down, the luminal voltage becomes less positive, which means the electrical gradient that drives paracellular calcium movement is not as strong --So less calcium is reabsorbed and more calcium shows up in the urine

Factors that Control Aldosterone Secretion

Increase --Angiotensin II increases aldosterone through RAAS --Because potassium secretion in the tubule is also a consequence of aldosterone, hyperkalemia can increase aldosterone secretion --ACTH is released by the pituitary and acts on adrenal glands to stimulates the release of corticoids like cortisol and aldosterone Decrease --An increase in plasma sodium concentration will decrease aldosterone release because there is already plenty of sodium in the body --Likewise, ANP is released when there is excess sodium from the blood: the function of ANP, therefore, opposes the function of aldosterone with regard to sodium balance

Regulation of Glomerular Hydrostatic Pressure

Influenced by arterial pressure (effect is buffered by autoregulation), afferent arteriolar resistance, and efferent arteriolar resistance Afferent arteriolar resistance --If the resistance of the afferent arteriole were increased, then blood flow slows down, and glomerular capillary hydrostatic pressure is lowered --A drop in capillary hydrostatic pressure means a drop in GFR, because the pro-filtration variable has decreased Efferent arteriolar resistance --If the resistance of the efferent arteriole were to increase, then blood flow rate is reduced because blood cannot leave the glomerulus as quickly --This creates a bottleneck in the glomerular capillaries, which causes hydrostatic pressure to rise; this elevated pressure causes GFR to rise in response There is a direct relationship between GFR and efferent resistance as long as efferent resistance does not surpass 2-3 times the normal efferent arteriolar tone--at this point the effect of increasing glomerular capillary pressure dominates --However, at high resistances, GFR begins to fall as the effect of declining blood flow dominates --These opposing effects on capillary pressure and flow account for the biphasic dependence of GFR on efferent arteriolar resistance Reduced blood flow causes the filtration fraction to increase which causes the plasma colloid osmotic pressure to increase (which opposes filtration, so GFR goes down) --The filtration fraction is the volume of filtrate that forms from a given volume of plasma entering the glomeruli (FF = GFR/RPF; where RPF is renal plasma flow) --If a value for renal blood flow is given, the hematocrit will need to be used to cover it to renal plasma flow --Increasing the filtration fraction concentrates plasma proteins and raises glomerular colloid osmotic pressure; this reduces GFR

Principles of Osmotic Equilibria

Intracellular fluid makes up 60-70% of total body fluid, and the rest resides in extracellular fluid --Water moves across cell membranes easily, so the osmolarity of the ECF is equal to the osmolarity of the ICF When predicting how a particular solution will affect the volume of a cell, there are a few important terms --Molarity: a measure of the concentration of the solution (moles of solute per unit volume of solvent) --Osmolarity: another measure of the concentration of the solution (number of osmotically active particles of solute per unit of volume of solvent) ---The difference between molarity and osmolarity is whether or not the compound separates in solution --Osmolality: similar to osmolarity, except that the solvent is measure as mass instead of volume ---Because volume can vary with temperature and pressure, osmolality is a more accurate term for the relationship of solute to solvent

Sample Calculation: Osmolarity of a 3% NaCl solution

MW NaCl = 58.5 g/mol 3% = 3 g/100 mL = 30 g/L 30 g/L / 58.5 g/mol = .513 mol/L = 513 mmol/L For NaCl, 1 mmol = 2 mOsm 513 mmol/L * 2 mOsm/mmol = 1026 mOsm/L Sodium chloride dissociates in solution, so molarity and osmolarity values will be different The solution is clearly hyperosmotic to body fluid --Because the cell membrane is impermeable to NaCl, the solution is also hypertonic --A cell placed in this solution would shrink as water is drawn out of the cell

Sample Calculation: What is the osmolarity of a 5% glucose solution? Is the solution hyperosmotic, hypo-osmotic, or isosmotic?

MW glucose = 180 g/mol 5% = 5 g/100 mL = 50 g/L 50 g/L / 180 g/mol = .278 mol/L = 278 mOsm/L Osmolarity of body fluids is between 250-300 mOsm/L --The glucose solution is roughly isosmotic with the body fluid --If we place a cell into an isosmotic solution with only glucose as the solute, there will be an extremely large concentration gradient for glucose to enter the cell; as glucose cross the membrane water will follow ---So, the cell with swell and the solution is hypotonic Glucose, along with urea, are considered to be ineffective osmoles: osmoles that contribute to osmolarity but not to tonicity

Acidosis/Alkalosis

Metabolism is the major generator of nonvolatile acids, which leaves a net production of protons that must be excreted by the kidneys --Volatile acids: those acids that are secreted by the lungs (i.e. carbonic acid) --Nonvolatile acids: all other acids in the body, produce from sources other than CO2 The kidneys also generate bicarb to secrete into the blood to neutralize excess metabolic acids Physiological pH is around 7.40 Acidemia: arterial pH < 7.35 --Acidosis: physiological processes that cause acid accumulation or alkali loss Alkalemia: arterial pH > 7.45 --Alkalosis: physiological processes that cause alkali accumulation or acid loss There are three lines of defense against these pH changes --Chemical acid-base buffering systems in body fluids (acts immediately to trap acid until normal pH balance is reestablished) --Respiratory system (acts within a few minutes to remove carbon dioxide) --Kidneys (much slower, hours to days)

Reabsorption of Filtered Bicarbonate

Most reabsorption of bicarb occurs early in the nephron For each bicarb that is reabsorbed, one proton must be secreted into the tubular fluid --This is because every time a proton is formed from the hydration of CO2, a bicarb ion is formed as well By the time the tubular fluid reaches the medullary collecting duct, nearly all of the bicarb has been reabsorbed The basic mechanism is the same at all segments 1. A proton is transported into the lumen from the cell and titrates the filtered bicarb through the activity of carbonic anhydrases to produce CO2 and water, which then diffuse back into the cell 2. Once in the cell, CO2 and water are once again converted to regenerate the proton and bicarb 3. The proton is secreted into the lumen and the regenerated bicarb is moved into the interstitial space So, for every proton that is secreted into the lumen, one bicarb ion is lost from the lumen and one bicarb appears in the interstitial fluid --But these are not the same bicarb ions (bicarb is not reabsorbed as an intact ion)

Calcium

Nearly all filtered calcium is reabsorbed along the tubule Around two-thirds is reabsorbed in the proximal tubule Another 25% is reabsorbed in the thick ascending loop of Henle Less than 10% is reabsorbed in the distal tubule

Regulation of Effective Circulating Volume--Sympathetic ANS

Norepinephrine release from sympathetic neurons affects both renal blood vessels and the tubule itself High sympathetic stimulation reduces renal blood flow, which means that GFR and sodium excretion also decrease Even when sympathetic stimulation is low, it actives alpha-adrenergic receptors in the proximal tubule --This, in turn, stimulates sodium reabsorption independent of hemodynamic effects by increasing the activity of apical sodium-proton exchangers

Glucose Carriers

Nutrient transport occurs coupled with sodium movement, but glucose and amino acids do not utilize the same sodium-coupled transporters In the early proximal tubule, transcellular reabsorption occurs via the sodium-glucose cotransporter, SGLT2 --SGlT2 is a high capacity, low affinity carrier that moves one D-glucose for every sodium transporterd --On the basolateral side, sodium is pumped out by the Na/K pump and facilitated diffusion of glucose occurs by the GLUT2 channel, which also has low affinity for glucose --These low affinity channels work because in the early proximal tubule, there is plenty of glucose in the tubular fluid By the S3 segment in the late proximal tubule, around 98% of glucose has already been reabsorbed from the tubule --So here, the apical carrier is the SGLT1 protein, which transports 2 sodium with each glucose --SGLT1 can generate a much larger glucose gradient across the apical membrane because there is much more electrochemical energy per glucose --The basolateral side contains GLUT1 for facilitated diffusion of glucose into the interstitial space

Using Clearance to Determine GFR and RBF

PAH is an organic acid that is not made by the body but is readily filtered and will also be secreted into the renal tubule from the peritubular capillaries --So, the amount that is excreted is greater than the amount that is filtered --When PAH is infused intravenously at the proper rate, almost none of it is left in the renal vein after one passage through the kidneys --On a single passage, all of the plasma flowing through renal circulation is cleared of PAH --So, the rate of PAH clearance by the kidneys is equal to the rate at which the plasma is flowing through the kidneys --For this reason, PAH clearance can be used to determine overall plasma flow Inulin is a starch-like fructose polymer with a molecular weight of 5 kDa, which means that it is freely filtered at the glomerulus --So, the rate at which inulin is cleared from the plasma is equal to the filtration rate, making inulin clearance a good indicator of GFR Creatinine is an endogenous substance that can also be used to determine GFR --However, creatinine clearance won't give an exact measurement of GFR because while it is freely filtered at the glomerulus, it is also slightly secreted by the peritubular capillaries --So, creatinine clearance would tend to overestimate GFR by around 10% or so

Calcium Reabsorption in the Distal Tubule

Parathyroid hormone can modulate renal calcium transport --When parathyroid hormone is released into the blood, the result is an increase in calcium uptake by the GI tract and kidneys, as well as calcium release from the bones --The parathyroid gland utilizes a calcium sensing receptor to detect calcium levels in the blood --When there is an increase in calcium levels, the signaling cascade leads to an inhibition of PTH secretion--there will also be an increase in the expression of the vitamin D receptor, which makes the parathyroid gland more sensitive to vitamin D suppression of PTH synthesis --Activation of the calcium-sensing receptor inhibits cell growth of the gland as a whole --So, when there is plenty of calcium in the blood, less PTH shows up in the blood; when extracellular calcium levels drop, PTH secretion increases enough to restore calcium balance PTH works mainly through the distal tubule in the kidney --Transcellular apical uptake of calcium occurs through TRPV5 and TRPV6 calcium channels: activity of these channels can be blocked by acid in the lumen, and the presence of magnesium can compete with calcium uptake through these channels --TRPV5 expression and function is regulated by PTH --The distal tubule expresses both the calcium-sensing receptor and PTH receptor on the basolateral membrane --These two receptors have opposing effects on the apical TRPV5 calcium channel ---When circulating PTH levels are high, it means calcium must be low ---The resulting activation of the renal parathyroid hormone will stimulate an increase in the expression and function of TRPV5--this promotes calcium absorption from the tubule ---When calcium levels in the blood are sufficient, calcium binds to the calcium-sensing receptor to inhibit the TRPV5 channel, thereby reducing calcium reabsorption

Regulation of Effective Circulating Volume--Aldosterone

Part of the RAAS and functions to promote sodium retention in the kidneys Exerts its activities on the distal portions of the nephron --In the principal cells, aldosterone signaling causes an increase in reabsorption of sodium and potassium --In the intercalated cells, the effect is to increase acid secretion in the tubule The mechanism by which these actions occur is based on the change to gene expression induced when aldosterone binds to its receptor --The result is upregulation of apical epithelial sodium channels, apical potassium channels, basolateral Na/K pumps, and mitochondrial metabolism --When aldosterone binds to the mineralocorticoid receptors, the complex translocates to the nucleus and upregulates transcription (requires a few hours to synthesize aldosterone-induced proteins) Aldosterone is secreted by the adrenal gland cortex, particularly from the outermost region of the cortex --The central cortex secretes glucocorticoids, like cortisol, which regulate glucose metabolism and inhibit the inflammatory response --The innermost cortex secretes androgen hormones and the adrenal medulla stress hormones (catecholamines) Glucocorticoids are present in higher concentrations in the plasma than mineralocorticoids --Mineralocorticoid receptors don't distinguish very well between glucocorticoids and mineralocorticoids --So, the specificity of the mineralocorticoid receptor for aldosterone relies on the presence of the enzyme 11B-hydroyxsteroid dehydrogenase 2 in the principal cells (irreversibly converts cortisol to cortisone which won't bind mineralocorticoid receptors; won't metabolize aldosterone so provides specificity) --There are natural inhibits to 11B-HSD2, one of which comes from natura licorice ---When the enzyme is inhibited, cortisol remains in the form that can bind to mineralocorticoid receptors ---This cortisol-MR complex can still stimulates transcription, but is not subject to the mechanisms that regulate aldosterone secretion: this is known as apparent mineralocorticoid excess syndrome (causes abnormal retention of sodium in the body and leads to hypertension and hypokalemia)

General Fluid Levels

Plasma-- 3.0L Interstitial fluid-- 11.0L Extracellular fluid-- 3.0L + 11.0L = 14.0L Intracellular fluid-- 28.0 L These relative values change based on body type, sex, and age --Increasing obesity decreases total body water % --Increasing age decreases TBW%

Renal Handling of Urea

Proximal Tubule --At the beginning of the proximal tubule, the concentrations of urea in the lumen and plasma are the same --Paracellular fluid reabsorption tends to bring urea along for the ride by solvent drag ---Water movement tends to increase the luminal urea concentration so that a gradient is generated to drive urea reabsorption by diffusion as well ---The more fluid is reabsorbed, the more urea is reabsorbed ---About half the filtered urea is reabsorbed in this way Thin Loop of Henle --In juxtamedullary nephrons, the concentration of urea is lower in the lumen at the tip of the thin descending limb than in the interstitium --So, urea gets secreted into the tubule through facilitated diffusion by the urea transporter, UT-A2 --As tubular fluid turn to the thin ascending loop, urea secretion continues probably by facilitated diffusion Medullary Collecting Duct --By the time the fluid reaches the inner medullary collecting duct, we once again get urea reabsorption --This time it is through a transcellular route with facilitate diffusion occurring at both the apical and basolateral sides --The UT-A1 transporter is embedded in the apical membrane --The UT-A3 transporter is expressed in the basolateral membrane --An important distinction between these two channels is their response to ADH: ADH stimulates UT-A1 but not UT-A3; likely mediate by the phosphorylation of serine residues on UT-A1 which increases UT-A1 accumulation on the apical membrane

Potassium Transport at the Proximal Tubule and Thick Ascending Limb

Proximal Tubule --The transcellular pathways for potassium movement do not participate directly in potassium reabsorption --Potassium reabsorption is exclusively paracellular and occurs by two mechanisms ---Solvent drag: potassium reabsorption is highly dependent on net fluid reabsorption ---Electrodiffusion: occurs in the late proximal tubule due to the shift from a luminal-negative to a luminal-positive voltage as solutes are reabsorbed Thick Ascending Limb --Reabsorption occurs by both paracellular and transcellular pathways --Paracellular transport occurs due to the electrochemical gradient and lumen-positive voltage --Transcellular transport relies exclusively on the Na/K/2Cl cotransporter on the apical membrane ---If any of these three ions are removed, or if the transporter is blocked with furosemide, transcellular reabsorption of potassium stops ---Inhibiting cotransport also reduces the lumen-positive voltage, so paracellular movement would stop as well

Role of Loop of Henle in Urine Concentration

Reabsorption in the proximal tubule occurs isosmotically, so the osmolarity of tubular fluid entering the loop of Henle is roughly isosmotic with plasma The length of the loop of Henle is directly proportional to the concentrating ability of the nephron Interstitial osmolality progressively rises from the cortex to the medullary tip --This is due to the unique function of the descending and ascending limbs --The descending limb has a high permeability to water due to high expression of aquaporin-1 (this segment is impermeable to solutes, so fluid but not solutes are reabsorbed) --The ascending limb is impermeable to water and only exhibits reabsorption of solutes ---In the thin ascending limb, passive salt reabsorption occurs paracellularly through tight junctions due to electrochemical gradients ---In the thick ascending limb, the main mechanism is through active transcellular reabsorption through the Na/K/2Cl cotransporter ---The deposit of solutes into the medullary interstitium not only dilutes the tubular fluid, but also generates a solute concentration that is most concentrated at the inner medulla During water restriction or antidiuresis, the secretion of ADH increases water permeability of the distal nephron by altering the density of aquaporin 2 channels --This occurs in the apical membrane and ADH also enhances urea permeability which contributes to the osmotic gradient generated by the loop of Henle --Just like antidiuresis, Interstitial osmolality progressively rises from the cortex to the medulla tip under conditions of high water intake (diuresis) ---The biggest difference is that the maximum osmolality during diuresis is much lower at 500 mOsm/L which aids in the production of dilute urine

Regulation of Effective Circulation Volume--Atrial Natriuretic Peptide (ANP)

Released by atrial cardiomyocytes in response to elevated BP caused by volume loading --Promotes natriuresis: excretion of sodium in the urine ANP causes renal vasodilation to drastically increase blood flow to the cortex and medulla of the kidneys --The increased cortical blood flow raises GFR and increases the filtered load of sodium in the proximal tubule and thick ascending limb --The increased medullary blood flow decreases the osmolality and reduces passive sodium reabsorption in the thin ascending limb --Both of these effects cause sodium load in the distal nephron to increase, which increases urinary excretion of sodium ANP directly inhibits sodium transport in the inner medullary collecting duct to promote excretion by keeping sodium in the tubular fluid of this segment Because of their opposing effects on ECF volume regulation, when ANP secretion is stimulated, the hormones involved in fluid retention are inhibited

Internal Potassium Regulation

Relies on the cell's ability to transport and maintain high concentration of potassium --If even a small amount of potassium is moved from the intracellular fluid to the extracellular, it can cause substantial change in the extracellular potassium concentration; could have severe consequences for neuromuscular and cardiac function When a load of potassium is introduced to plasma following ingestion, it moves transiently into cells for storage within the hour before the kidney eventually begins to process it for excretion --The purpose of this is to keep the plasma concentration of potassium from changing too much --Gives the kidneys time to filter and process the extra potassium load for excretion in the urine When intake increases, cells temporarily take up potassium until output can be increased to match the new intake level --Insulin, aldosterone, and B-adrenergic agonists (like epinephrine or albuterol) promote potassium transfer to the intracellular space through the Na/K pump (aldosterone also promotes placement of additional ROMK potassium channels in principal cells to boost permeability for secretion) --Patients deficient in RAAS or insulin tend to have lower tolerance for potassium loading (hyperkalemia often see in diabetics) Potassium can be released from intracellular stores in response to strenuous exercise, cell lysis, or indirectly by impairing sequestration (such as by diminishing Na/K pump activity) Changes to acid-base balance also affects potassium distribution --When intracellular pH becomes more acidic, there is a decrease in potassium binding to nondiffusable anions that are in the cell because the protons displace the potassium; makes potassium available to diffuse through the membrane --Acidosis also inhibits the Na/K pump and Na/K/2Cl cotransporter, which slows the rate of potassium entry into the cell --Alkalemia speeds up cellular potassium uptake and leads to plasma hypokalemia, probably as part of an acid-base disturbance

Role of Urea in Urine Concentration

Salts are not the only solutes that contribute to the hypertonic medullary interstitium, urea does as well Usually nephron segments from the thick ascending limb to the outer medullary collecting duct have a relatively low permeability to urea --But when ADH is present, the permeabilities to both water and urea substantially increase from the initial collecting tubule to the end of the nephron --The ADH-mediated increase in urea permeability is due to its effects on the apical UT-A1 channels The effect of these permeability differences is that urea is recycled in the inner medulla by reabsorbing from the medullary collecting duct and secreting it into the medullary region of the loop of Henle So urea accumulates in the interstitium and contributes to around half of the total osmolality in the deepest part of the inner medulla

PAH Titration

Since PAH operates via a transporter system that saturates, secretion as a transport maximum As the plasma PAH concentration increases, excretion initially rises much faster than filtration --Once a plasma value of around 20 mg/dL is reached, the secretory machinery is saturated and secretion hits a plateau --For this reason, PAH infusion is kept at low concentrations to ensure a complete secretion --So, to use PAH as a diagnostic agent for determining renal plasma flow, the concentration in the plasma must remain low As a general rule, if the clearance of a substances is greater than GFR, it indicates net secretion of that substance into the renal tubules (i.e. PAH) If the clearance of a solute is below GFR, then we get net reabsorption (i.e. glucose)

Sodium and Chloride Reabsorption in the Loop of Henle

Sodium --Reabsorption in the thin descending limb and thin ascending limb is almost entirely passive and paracellular --In the thick ascending limb, reabsorption occurs via the Na/K/2Cl cotransporter and Na/H exchange for uptake across the apical membrane --Sodium is then pumped across the basolateral membrane via the Na/K pump --There is a lumen-positive voltage that is maintained, in part, by the conductance of potassium channels: this provides a driving force for sodium diffusion across tight junctions and accounts for about half of sodium reabsorption in the thick limb Chloride --All in the thick limb --Also utilizes the Na/K/2Cl cotransporter which is electroneutral; driven by downhill sodium and chloride gradients --Chloride is then extruded on the basolateral side through chloride channels --Potassium will also be along for the ride with the Na/K/2Cl cotransporter: so, there are apical potassium channels through which a large fraction of potassium gets recycled back into the lumen --This cotransporter is the target of loop diuretics like furosemide: acts as a competitive inhibitor for the chloride binding site; the thick limb is the only part of the nephron were furosemide will work

Sodium and Chloride Reabsorption in the Proximal Tubule

Sodium --Sodium reabsorption across the apical membrane occurs by secondary active transport (through a sodium-nutrient cotransporter or a sodium-proton exchanger) --On the basolateral side, sodium is pumped out through the Na/K pump --There is also a sodium-bicarb cotransporter that facilitates movement of some sodium into the interstitial fluid --Paracellular reabsorption occurs in both directions ---Paracellular reabsorption through solvent drag means that sodium is pulled along with water as it is reabsorbed through tight junctions ---The backleak of sodium occurs because the voltage of the proximal tubular lumen is negative: this draws about 1/3 of the reabsorbed sodium back into the lumen Chloride --In the early proximal tubule, there is only paracellular reabsorption: solvent drag contributes a little bit; but mostly due to lumen-negative voltage: there is an electrical gradient for passive chloride movement from the lumen to the interstitial fluid (no concentration gradient; only electrical) --By the late proximal tubule, transcellular reabsorption kicks in ---Due to the expression of a chloride-formate exchanger ---The Na-H exchanger provides protons to neutralize the base in the lumen to sustain the gradient for the chloride-formate exchanger: formic acid diffuses back across the apical membrane and dissociates to release the proton, leaving the formate anion to power chloride movement ---On the basolateral membrane, chloride moves alongside potassium into the interstitial fluid --In the late proximal tubule, the lumen voltage is now positive which means that paracellular reabsorption is no longer favored--however, we do get paracellular chloride reabsorption due to a concentration gradient

Sodium and Chloride Reabsorption in the Late Distal Tubule and Collecting Ducts

Sodium --There are principal cells found in these segments (~70% of cells) --Sodium reabsorption by these cells is transcellular and occurs through apical epithelial sodium channels and the basolateral Na/K pump --There are also potassium channels on both membranes of these cells ---If there were only basolateral potassium channels to recycle potassium, then the electrogenic movement of sodium into the cell and potassium into the interstitial space would create a very negative transepithelial voltage ---The presence of apical potassium channels allows some potassium to return to the lumen to oppose this effect --The potassium-sparing diuretic, amiloride, blocks epithelial sodium channels in the distal nephron: amiloride does not produce as great a diuretic effect as loop diuretics, but allows for the retention of potassium Chloride --The other 30% of cells in these segments are intercalated cells (alpha and beta) --The beta intercalated cells reabsorb chloride through a transcellular process ---Involves chloride-bicarb exchange across the apical membrane and chloride channels in the basolateral membrane --The main difference between alpha and beta intercalated cells is their orientation: they contain similar channels, but the channels are expressed on opposite membranes --This is why chloride reabsorption specifically occurs in beta intercalated cells; alpha intercalated cells show transcellular chloride secretion into tubular lumen

Sodium

Sodium is the most important contributor to the osmolality of the extracellular fluid --Where sodium goes, water follows Daily excretion of sodium is only a very small amount of the sodium that is filtered at the glomerulus The majority of the filtered load, about 2/3, is reabsorbed in the proximal tubule --Another 25% if reabsorbed in the loop of Henle --Another 5% is reabsorbed in the distal tubule --The remaining 3% if reabsorbed in the collecting duct Usually, as sodium is reabsorbed, water is also --This does not happen, however, in the thick ascending limb of the loop of Henle which as a low permeability to water --Sodium reabsorption here is much faster than water reabsorption; as a result the sodium concentration of the tubular fluid entering the distal nephron decreases substantially--so, the thick ascending limb is referred to as the diluting segment of the nephron

Net Filtration Pressure at the Glomerulus

Starling forces --Glomerular hydrostatic pressure promotes filtration --Glomerular colloid osmotic pressure opposes filtration --Bowman's capsule hydrostatic pressure opposes filtration --Bowman's capsule colloid osmotic pressure is effectively absent: colloid osmotic pressure is mainly determined by proteins, and Bowman's space mostly lacks proteins Combining the Starling forces gives the net ultrafiltration pressure (Puf) Puf = (Pgc + PIbs) - (Pbs + PIgc) --If we multiply Puf by the ultrafiltration coefficient (Kf) the result is GFR --Kf is determined by the product of the hydraulic conductivity (permeability) of the capillary and the effective surface area available for filtration ---One way to alter Kf is through mesangial cells: these will respond to extrarenal hormones and produce vasoactive agents; contracting mesangial cells can change the glomerular capillary surface area which alters Kf and therefore GFR At the glomerulus, the net driving force favors ultrafiltration at any point along the glomerular capillaries --As fluid and solute are filtered out of the blood and into Bowman's space, the glomerular colloid osmotic pressure tends to increase along the capillary as fluid leaves the blood --Because the hydrostatic pressures of the two compartments remain relatively stable, the net filtration pressure drops as colloid osmotic pressure in the capillary steadily increases --These forces will balance to reach filtration equilibrium at some point along the glomerular capillary due to the increase in capillary colloid osmotic pressure

Sample Calculation: Effect of infusing 2.0L of 3% NaCl to a 70 kg person Assume: 1) no excretion of water or solutes 2) osmotic equilibrium 3) ECF = 20% body wt.; ICF = 40% body wt. 4) Plasma osmolarity = 280 mOsm/L

Step 1: Initial Conditions --We can use the percentage of body weight to determine the volumes of the ECF and ICF ---ECF = 0.2 * 70 ---ICF = 0.4 * 70 --Since we now have the volume and concentration of each compartment (280 mOsm/L), we can calculate the total milliosmoles in each (ECF = 280 x 14 = 3920; ICF = 280 x 28 = 7840) Step 2: How many mOsm of NaCl are added? 3% NaCl solution = 1026 mOsm/L 2.0 L * 1026 mOsm/L = 2052 mOsm Step 3: After Osmotic Equilibrium --The first stop for salt and fluid is the plasma, which is where the infusion is added --So, add 2052 mOsm to ECF and also to total fluid --NaCl does not cross into the cell, so ICF volume remains unchanged after equilibrium --Now that we have a new number of mOsm after infusion, total fluid volume increases by 2L --Divide total mOsm by total volume to get new concentration of total fluid; concentrations in all compartments will be equal --Use the new osmolarity to calculated the volume change In this example, the initial values were 14L for ECF and 28L for ICF --Infusing 2L of 3% NaCl solution resulted in a gain of 5L to ECF --The 2L from the infusion stayed in the ECF plus an additional 3L of fluid that left the ICF to enter the ECF (accounts for the loss of 3L from the ICF)

Neurohormonal (Extrinsic) Control of GFR/RBF

Sympathetic Stimulation --Could occur with severe hemorrhage --Triggers the release of catecholamines which will cause arteriolar vasoconstriction --Vasoconstriction slows down blood flow through renal circulation, and because the afferent side is also affected, GFR will decrease Angiotensin II --Part of the RAAS; engaged when fluid retention by the body is required --Angiotensin II has vasoconstrictor effects on arterioles --In the kidneys, angiotensin II preferentially constricts the efferent arterioles which raises glomerular hydrostatic pressure and lowers renal blood flow; at the peritubular capillaries, a decrease in RBF causes an increase in sodium and water reabsorption --Afferent arterioles are not really affected by angiotensin II because the vasoconstrictive effects are counteracted by the vasodilation effects of nitric oxide and prostaglandins --Even though efferent arteriole resistance increases glomerular hydrostatic pressure, angiotensin II does not cause a change in GFR --The increased formation of angiotensin II usually occurs following decreased arterial pressure or volume depletion--both of these scenarios would normally decrease GFR, so in these instances angiotensin II helps prevent such a decrease Prostaglandins --Cause arteriolar vasodilation --This is usually important only when there are other disturbances that tend to lower GFR (i.e, cirrhosis or heart failure) --Leads to increased GFR and RBF --If prostaglandin synthesis were blocked (mechanism for NSAIDs) then we get a decrease in GFR Nitric Oxide/EDRF --Derived from L-arginine by eNOS enzymes --Induces vasodilation and protects against excessive vasoconstriction: leads to an increased GFR and RBF Endothelins --Peptides that constrict blood vessels and raise blood pressure --When they are overexpressed, they contribute to high blood pressure and heart disease; so, endothelin antagonists could counteract elevated pressure that occurs in some conditions --Cause opposing responses in vascular smooth muscle, depending on the receptors engaged ---NO is derived from endothelial cells and this occurs when endothelins bind to ETB receptors; NO easily diffuses into vascular smooth muscle cells to promote relaxation/vasodilation ---Endothelin receptors are also present on the vascular smooth muscle cells themselves; the ETA receptor is the dominant receptor under normal conditions; when endothelin binds there is an increase in IP3 production and subsequent calcium release that causes smooth muscle contraction --There is a transient vasodilation because ETB receptors effects are initiated first followed by the longer lasting ETA vasoconstriction effects; the end result is vasoconstriction and increased vessel resistance; causes both GFR and RBF to decrease

Regulation of Effective Circulating Volume (ADH/Arginine Vasopressin)

Synthesized by magnocellular neurons in the hypothalamus and released by the posterior pituitary Functions to promote the movement (reabsorption) of water specifically across the renal epithelium in distal portions of the nephron --Acting through Gs proteins, ADH signaling increases the levels of cAMP to increase water retention and produce urine with a high osmolality Sodium reabsorption is stimulated by the opening of more sodium channels in collecting tubules and the stimulation of apical Na/K/2Cl cotransporters along with K+ channels in the thick ascending limb of the loop of Henle Permeability of the nephron to water differs as fluid moves through each segment along with the mechanism of water movement --The mechanism is the proximal tubules and thin descending loop is mediated by aquaporin-1 ---These regions are highly permeable to water because of the abundant presence of these aquaporin-1 channels on both the apical and basolateral membranes ---These are not sensitive to ADH signaling ADH dramatically increases water permeabilities in the distal nephron --Due to the insertion of aquaporin-2 channels in the apical membrane --Gs proteins activated by ADH binding causes cAMP to be formed and PKA to be activated --PKA will phosphorylate proteins involved in vesicle trafficking and fusion with the apical membrane, to insert aquaporin-2 --So, ADH modulates the density of aquaporin-2 channels in the apical membrane, not the activity of individual channels --The aquaporin 3 and 4 channels in the basolateral membrane of medullary collecting ducts are not sensitive to ADH When the osmolarity of the ECF becomes elevated, osmoreceptors in the hypothalamus sense the change and stimulate the production of ADH --ADH will be secreted from the pituitary and will act on the renal tubule to shift the nephron function to promote increased fluid reabsorption Central diabetes insipidus is to due to insufficient ADH production --Leads to an increase in plasma osmolarity, excessive thirst, and can cause hypernatremia (due to elevated sodium concentration after water loss) Inappropriate ADH syndrome occurs when the body secretes too much ADH --Causes a dilution of the plasma, which decreases the osmolarity and can cause hyponatremia due to excessive fluid retention

Obligatory Urine Volume

The ability of the nephron to vary the concentration of urine is incredibly useful, but there are limits Each day, a certain amount of solutes and waste products must be removed from the body through excretion --There is a minimum volume of urine that is required to accomplish this goal, known as obligatory urine volume Obligatory urine volume is calculated by dividing the number of milliosmoles of solute that must be excreted by the maximum urine osmolarity that the kidneys can generate When the kidneys are not functioning at their optimum level, this obligatory urine volume can change in response to the kidneys' changing ability to concentrate urine So, when the ability of the kidneys to concentrate the tubular fluid is disrupted, a consequence is an increase in the overall volume of urine produced each day --This is because the body still needs to rid itself of solutes to retain electrolyte balance, but it can no longer do so as efficiently

External Potassium Regulation

The amount of dietary intake of potassium is greater than the entire potassium content of the ECF --So, to maintain the potassium level within a tolerable range, potassium excretion rate must match ingestion rate Because GFR remains fairly constant, potassium excretion rates are controlled by modulating the rates of tubular reabsorption and secretion --Much of this potassium transport occurs in the late distal tubules and collecting duct (particularly in principal cells) Around 10% of filtered potassium remains in the tubule as it enters the distal nephron and what happens here is largely dependent on intake --When dietary intake is low, all distal portions reabsorb potassium from the filtered load, so around 1-3% appear in the urine (not so good as restricting potassium ion loss, so in prolonged deficiency it can lead to hypokalemia) --At normal to high potassium intake levels, the distal nephron will adapt to promote secretion of potassium: the distal potassium secretory system accounts for most of the urinary potassium excretion When potassium intake is high, the kidney traps extra potassium in the medulla --Juxtamedullary nephrons secrete potassium passively into the thin descending limb of the loop of Henle which means that tubular potassium load at the papilla tip may surpass the filtered load --Potassium reabsorption occurs at the thin and thick ascending limbs, which deposit potassium into the medullary interstitium --This reabsorption occurs to a greater extent than the secretion in the descending limbs (so there is net reabsorption along the loop) --Distal nephron secretion occurs in the collecting tubules and the final reabsorption occurs in medullary collecting ducts --The high concentration of potassium in the medullary interstitium helps to minimize passive potassium loss from the tubular fluid, which is also high in potassium thereby promoting potassium excretion from the body

Sodium and Chloride Reabsorption in the Early Distal Tubule

The early distal tubule is characterized almost exclusively by transcellular sodium reabsorption --Through the electroneutral sodium-chloride cotransporter and then the Na/K pump on the basolateral side --Here, chloride is moving alongside sodium into the epithelial cells and is extruded through basolateral chloride channels --The sodium-chloride cotransporter is in the same family as the Na/K/2Cl cotransporter, but does not move potassium (high sensitive to thiazide diuretics: produce less diuresis than loop diuretics, but are effective at removing excess sodium) Early distal tubule is not very permeable to water This is where the macula densa resides, so the salt concentration here influences the signaling feedback to change the GFR Account for around 5% of filtered NaCl load

Production of Dilute vs. Concentrated Urine

The juxtamedullary nephrons in the kidney have a specialized role in the production of concentrated urine --They can do so due to their ability to create a hyperosmolar medulla by way of countercurrent multiplication --Cortical nephrons are limited in this capacity Producing dilute urine involves the pumping of solutes out of the tubular fluid in segments that are impermeable to water Producing concentrated urine relies on osmosis as the driving force to concentrate the tubule contents as there are no pumps for active transport of water The proximal tubule reabsorbs around 2/3 of the filtered fluid isosmotically, regardless of the final urine osmolality For everywhere else, the tubular fluid differs based on the fluid status of the person --The loop of Henle and initial portion of the distal tubule reabsorb solutes but not water --So tubular fluid entering the distal tubule is hypo-osmotic to the blood plasma The most distal nephron segments determine whether the final urine is dilute or concentrated --These include four segments: the initial and cortical tubule and the outer/inner medullary collecting ducts --Water reabsorption in these segments is largely regulated by ADH ---When ADH levels are low, the water permeability of the distal nephron remains low: but the solutes are still reabsorbed from the tubule, leaving water in the tubular lumen to produce a dilute urine ---In the presence of ADH, water permeability of the distal nephron rises markedly to reabsorb more water from the tubule and produce a concentrated urine

Kidney Blood Circulation

The kidneys receive about 20% of the cardiac output, which is necessary for the kidneys to filter blood contents Renal circulation contains two sets of capillary beds in series: the glomerular capillaries and the peritubular capillaries Blood flowing through a single capillary bed will pass through an arteriole, then a capillary, and then into a venule --Since there are two capillary beds in series, there are actually two arterioles Renal circulation has a high resistance afferent arteriole that sends blood into a high pressure glomerular capillary network --From there, blood passes into a high resistance efferent arteriole and then to the low pressure capillary network around the renal tubules called the peritubular capillaries --A benefit of having these resistance vessels on either side of the glomerular capillary is that they can exert tight control over the blood pressure inside the glomerulus --Arteriolar tone is under the control of sympathetic nerves and chemical mediators Glomerular capillaries primarily function to filter blood Peritubular capillaries serve to reabsorb fluid and substances from renal tubules into the blood --There is a subpopulation of small curved arterioles that descend into renal papillae to form hairpin-shaped vessels called vasa recta --The vasa recta are effectively the peritubular capillaries for juxtamedullary nephrons Around 90% of blood entering the kidney perfuses the superficial glomeruli in the cortex --Only about 10% perfuses the juxtamedullary glomeruli in the medulla

Renal Tubules

The lining of the proximal tubule has a brush border to increase the surface area and allow more transporters for absorption --This is the only segment where sugars and amino acids are reabsorbed --If sugars and amino acids get past the proximal tubule, they'll appear in the urine Both the proximal and distal tubules have a lot of mitochondria which are important in producing the ATP required to fuel active transport

Nephrons

The nephron is the functional unit of the kidney and each kidney contains about a million nephrons--epithelial cells attached to a basal membrane The glomerulus is the cluster of blood vessels where the plasma filtrate originates--located within Bowman's capsule Bowman's capsule is made up of epithelial cells that surround the glomerulus and contains Bowman's space which is contiguous with the tubular lumen --This is where the blood vessels and renal tubule meet Filtrate flows through the renal tubules, beginning at the proximal tubule, which leads to the loop of Henle, then the distal tubule, connecting tubule, initial collecting tubule and cortical collecting tubule --Finally, filtrate enters the medullary collecting duct which receives filtrate from multiple nephrons The region of the nephron where filtration occurs is called the renal corpuscle

Sodium Reabsorption and Oxygen Usage by the Kidney

The oxygen and nutrients that are normally delivered to the kidneys greatly exceed their metabolic needs The kidneys consume 7-10% of the body's total oxygen consumption --One reason for this is that nearly all sodium transport in the kidneys relies on the Na/K pump --So, there is a high demand for ATP by oxidative phosphorylation to fuel sodium reabsorption

Countercurrent Multiplication

The process by which the kidney establishes a longitudinal osmotic gradient, by iterating, or multiplying, a single effect to create a large overall gradient Upon entering the loop for the first time, tubular fluid has a concentration of 300 mOsm/L which is isosmotic with interstitial fluid --In the ascending portion of the loop, the goal is to create a 200 mOsm/L difference between interstitial and tubular fluid ---The only way to do this is to pump out salts which results in interstitial fluid at 400 mOsm/L and tubular fluid at 200mOsm/L --At the descending limb, now, the filtrate needs to equilibrate ---Water leaves passively until filtrate in the descending limb equals the interstitial fluid osmolarity (which will not change) ---Now the interstitial fluid and descending limb are both at 400 mOsm/L --As new 300 mOsm/L fluid enters the loop, everything advances and now the descending limb has 300 mOsm/L and 400 mOsm/L fluid and the ascending limb has 400 mOsm/L and 200 mOsm/L fluid ---The same process as above will occur again to generate a 200 mOsm/L difference at the thick ascending limb and then to equilibrate with the descending limb The process repeats continuously --The more times it repeats, the greater the final osmolarity at the tip of the loop of Henle and therefore the interstitial environment within the medulla --The net effect of this countercurrent multiplier system is to trap solutes in the renal medulla This effect sets up the necessary transepithelial gradients that maintain the body's fluid and electrolyte balance by enabling the concentration of all tubular fluid passing through the common medullary collecting ducts --What this means is that the specialized concentrating ability of the juxtamedullary nephrons serve to establish the ideal environment for concentrating all urine produced by all nephrons since all tubular fluid eventually drains into the common medullary collecting ducts and are subjected to the medullary osmotic gradient

Water Permeability in the Nephron

The proximal nephron segments have the highest permeability due to the abundant presence of aquaporin 1 on apical and basolateral membranes The middle segments have very low water permeability, which is why tubular fluid tends to become diluted as it flows through these segments The distal parts of the nephron can have high or low water permeabilities, depending on the presence of ADH The osmolarity of tubular fluid in the distal segments ranges from hypoosmotic to hyperosmotic, depending on the needs of the body ADH regulates this fluid balance in conjunction with thirst mechanisms --Osmoreceptors in the hypothalamus can sense when the extracellular fluid osmolarity increases and can trigger two responses: stimulation of thirst to promote intake of fluid into the body and release of ADH to promote fluid retention --Both of these mechanisms increase the volume of fluid in the body to bring extracellular osmolarity back down to normal ADH secretion and thirst are not only responsive to osmolarity --A drop in blood volume or pressure can also stimulate these effects --But the effect of volume changes versus osmolarity will affect ADH release in different ways ---Plasma ADH levels increase sharply once volume depletion hits a 15% change or greater ---There is a tight linear relationship between ADH levels and an increase in osmolarity

Effect of Water on Solute Reabsorption

The rate at which a solute crosses the tubular epithelium is not the only determinant of its concentration at various points along the tubule Water movement will also change solute concentration --If water is reabsorbed more so than the solute, then the solute will become more concentrated (i.e. creatinine, inulin) --If not, the solute becomes less concentrated as fluid moves along the tubule (i.e. glucose, amino acids)

Renal Corpuscle

The region where filtration occurs Consists of three parts: vascular elements (i.e. glomerular capillaries), Bowman's space and capsule, and the mesangium The mesangium consists of mesangial cells and matrix that support the capillary loops --Mesangial cells are smooth muscle-like cells that contain actin and myosin and therefore contractile --Contraction of these cells regulates the size of the glomerular capillary lumen and can therefore play a role in regulating the glomerular filtration rate The mesangium is continuous with the juxtaglomerular apparatus, which contains specialized smooth muscle cells called juxtaglomerular cells (found mainly in the afferent arteriolar wall) --These function to produce, store, and release renin --The juxtaglomerular apparatus, therefore, is part of a complex feedback mechanism that regulates renal blood flow and filtration rate (also indirectly modulates sodium balance and systemic blood pressure)

Kidneys and the Urinary System

The renal system consists of two kidneys and one urinary bladder The kidneys sit behind the peritoneum on each side of the vertebral column, extending from the 12th thoracic vertebra to the 3rd lumbar vertebra --Tasked primarily with the regulation of water and ions in the blood --The excretion function is secondary There are two basic layers to the kidneys --Cortex: granular outer layer; the granularity of the cortex is due to the presence of glomeruli with microscopic tufts of capillaries and a large number of highly convoluted epithelial tubules --Medulla: darker inner region; lacks glomeruli and consists of a parallel arrangement of tubules and small blood vessels Blood is filtered in the cortex and the filtrate that is produced then moves down through the pyramids of the medulla where it is further processed before waste is excreted --There are anywhere from 8-18 pyramids in the medullary region --The pyramid bases lie at the cortical-medullary border, and the tips are in the renal pelvis --Urine flows to the renal sinus through tiny perforations in the pyramid tip through the calices and renal pelvis until it passes into the ureter The ureters serve to conduct waste from the kidneys to the bladder where urine is stored before micturition --Micturition: excretion caused by smooth muscle contraction

Acid Secretion Mechanisms

The sodium-proton exchanger in the early segments in responsible for the largest fraction of net proton secretion Apical proton secretion also occurs by the ATP-driven electrogenic proton pump --This pump establishes a steep transepithelial proton gradient, much steeper than can be generated by the Na-H exchanger --This proton pump is found in all segments, but is most highly concentrated in the alpha-intercalated cells of the cortical collecting tubule and medullary collecting duct cells Another acid secretion mechanism in the late nephron segments is through the electroneutral H-K pump --The intercalated cells play a key role in distal potassium reabsorption, mainly in exchange for protons through the H-K pump

Local (Intrinsic) Control of GFR/RBF

The stability of blood flow is important for ensuring that renal tubules receive a relatively stable filtered load of solutes over a wide range of pressures --Also assures that a sudden increase in renal artery pressure won't damage the fragile glomerular capillaries The myogenic response and tubuloglomerular feedback mechanism are the main methods of autoregulation --Work together to keep GFR and RBF constant --Autoregulation is independent of renal nerves and circulating hormones The kidneys' autoregulation of RBF relies on its ability to respond to a rise in renal arterial pressure with a proportional increase in resistance of afferent arterioles, and vice versa --If the renal artery pressure were to drop from 100 to 80 mmHg, the initial consequence would be a drop in GFR and RBF; dilation of the afferent arterioles in response would work quickly to restore GFR and RBF --If the renal artery pressure were to be increased to 120 mmHg, blood flow and GFR would increase; constriction of afferent arterioles would slow down blood flow and reduce filtration back to normal rates GFR and RBF are generally stable over a wide range of arterial pressures --Elevated arterial pressure above normal does not really affect renal blood flow and GFR --Efferent arteriole and venous resistance is pretty stable at typical arterial pressures --Afferent resistance, however, greatly increases as arterial pressure rises: so autoregulation is mainly regulated by afferent arteriolar resistance

Vasa Recta

The vasa recta serve as the peritubular capillaries for juxtamedullary nephrons and are considered countercurrent exchangers The biggest difference between the mechanism in the loop of Henle and the mechanism in the vasa recta is the permeabilities of their respective loops --The loop of Henle is a countercurrent multiplier because the descending limb is permeable to water only and the ascending limb is permeable to solute only --The vasa recta is a countercurrent exchanger because both the descending and ascending limbs are permeable to both solutes and water (this is a capillary so there are no regional permeability differences) The countercurrent exchange does not create the osmotic gradient, but it does mirror what's going on int he medullary interstitium and helps to preserve the gradient that was established by the loop of Henle The shape of the vasa recta also mirrors the shape of the loop of Henle which is very important for maintaining the hypertonic medullary interstitium --If the vasa recta were straight, its contents would increase in osmolarity as blood flows through it into the inner medulla --This set-up would effectively remove all the work of the countercurrent multiplier as blood leaving the medulla would carry away many of those interstitial solutes --The shape of the vasa recta ensures that solutes in the medulla remain trapped and less is washed away as blood leaves the kidneys

Urea Transporters

There are different forms of urea transporters, which a formed by different splicing patterns of the UT-A gene UT-A1 is the longest isoform --It is basically UT-A2 and UT-A3 combined Due to this setup, ADH can only stimulate UT-A1 --ADH will bind to its receptor on the basolateral membrane and activates the Galpha subunit which activates adenylyl cyclase to produce cAMP for PKA stimulation --There are likely two consequences of activating PKA (protein kinase A) ---The first is to increase the insertion of vesicles containing UT-A1 into the apical membrane ---The second (probably more likely) is direct phosphorylation of UT-A1 proteins to increase the channel opening probability So when the body senses the need to increase fluid reabsorption, there is a change to urea reabsorption in the inner medullary collecting duct to promote the production of a more concentrated urine

Renal Control of Extracellular Proton Concentration

There are three approaches to regulating the proton concentration in the ECF 1. Secrete acid into the tubular fluid, which acidifies the urine 2. Reabsorb filtered bicarb --Secretion of acid is critical for this approach because bicarb must react with a proton to form carbonic acid before it can be reabsorbed --In conditions of alkalosis, the kidney's won't reabsorb all of the filtered bicarb, which increases the amount excreted in the urine to lower the extracellular pH back to normal 3. Production of new bicarb

Renal Handling of PAH

There is a lot more PAH that is excreted in the urine than was filtered at the glomerulus --This is because PAH is filtered, but is also secreted into the renal tubule Clearance is defined as the virtual volume that is completely cleared of solute per unit time --Because PAH is completely cleared from the blood in a single passage through renal circulation, PAH clearance estimates will give values that are approximately equal to renal plasma flow Late Proximal Tubule --Transcellular secretion of PAH against a large electrochemical gradient --The Na/K pump establishes the sodium gradient to drive alpha-ketoglutarate into the cell (metabolic product of glutamine and glutamate breakdown) --With alpha-ketoglutarate higher inside the cell, its downhill movement out of the cell fuels the uphill movement of PAH across one of two transporters: OAT1 (high affinity) and OAT3 (low affinity) --Once in the cell, PAH is secreted across the apical membrane and into the lumen by electrogenic facilitated diffusion or by a PAH-anion exchanger --Certain anion drugs (like probenecid) compete at PAH-anion exchangers to inhibit PAH secretion from the blood to the lumen

Calculating Renal Clearance

To determine the plasma volume that would be totally cleared of solute X in a given time, we need to know 3 parameters: urine concentration of X, urine flow rate and plasma concentration of X Cx = Ux(V)/Px The plasma concentration here is renal artery plasma concentration--we are calculating clearance which means we are calculating the volume of plasma from which all solute has been removed --So, the renal vein concentration should be 0 p-aminohuppurate (PAH) is a substance that is completely cleared by the kidney of a single pass through renal circulation --This means that whatever amount of PAH isn't filtered at the glomerulus is then secreted by peritubular capillaries so that none remains in the renal vein We can use the clearance calculation to determine the glomerular filtration rate (GFR) as well --But, GFR equals the clearance rate only if the solute freely filters at Bowman's capsule in the glomerulus, and the tubules do not synthesize, degrade, or accumulate the solute --We also have to ensure that the substance is not net reabsorbed or net secreted

Determinants of Renal Blood Flow (RBF)

To sustain a high GFR, renal blood flow must also be high, which is why around 20% of cardiac output goes to renal circulation RBF = dP/R --dP: difference between renal artery pressure and renal vein pressure --R: total renal vascular resistance (Ra + Re + Rv); sum of all resistances in kidney vasculature Arterioles are the resistance vessels of the circulatory system --Renal circulation has two arterioles in series on either side of the glomerular capillaries --A significant pressure drop occurs as blood passes through each arteriole; because of this setup the glomerular capillary pressure is relatively high and the peritubular capillary hydrostatic pressure is relatively low --By selectively constricting or relaxing the afferent and efferent arterioles, there can be highly sensitive control of the hydrostatic pressure in the intervening glomerular capillary, which influences glomerular filtration

Indicators to Measure Specific Compartments

Total Body Water --Tritiated water (3H2O) and heavy water (D2O): mix with total body water within a few hours of being injected into the blood --Antipyrine: very lipid soluble molecule that is small and can rapidly penetrate cell membranes; distributes uniformly throughout the intracellular and extracellular compartments in proportion to water content Extracellular Fluid --Need a substance that will disperse throughout the ECF compartments within 30-60 minutes following injection, but does not enter cells --Examples: radiolabeled sodium (22Na), inulin, thiosulfate Intracellular Fluid: total body water - extracellular fluid Plasma Volume --Must ensure that the indicator cannot cross the capillary endothelium, but will remain in the vascular system following inejction --Radioactive chromium (51Cr) or radiolabeled albumin (125I-albumin) bind tightly to RBCs which ensures that they will remain in the bloodstream --To determine plasma volume, we cannot just measure the volume of whole blood, because whole blood contains other components ---plasma volume = blood volume * (1-HCT)

Pathways for Solute Movement

Transcellular --A substance moves through the epithelial cells --The rate of transport will depend on the electrochemical gradients, ion channels, and transporters at the apical and basolateral membranes Paracellular --A substance moves between the epithelial cells through tight junctions --The rate of transport depends on the transepithelial electrochemical driving forces and the permeability properties of the tight junctions that govern ion movement

Tonicity

Used to describe a solute relative to another solution separated by a barrier (usually a semi-permeable membrane) The question to ask when determining tonicity is which solutes are able to cross the semi-permeable membrane --When a solute moves across a membrane, water will follow it --So, tonicity is mainly concerned with the solutes that are not able to cross the membrane and therefore exert an osmotic effect to move water Tonicity is not a direct measure of concentration: just because a solution is isosmotic with a cell, doesn't mean that the solution is also isotonic Isotonicity: when a cell is placed in an isotonic solution, there is no change in its volume Hypertonicity: when a cell is placed into a hypertonic solution, water moves from the cell into the hypertonic solution; therefore reduces the cell volume Hypotonicity: when a cell is placed into a hypotonic solution, water enters the cell causing it to swell

Renal Handling of Water and Solutes

Water --Of the water that is filtered, the majority is reabsorbed so that only a small amount is excreted Sodium --Same scenario as water, with only a small amount being excreted each day Glucose --All filtered glucose is reabsorbed so that none shows up in the urine Creatinine --Freely filtered, but not reabsorbed --Because whatever is filtered will pretty much be what shows up in the urine, the rate that the plasma is cleared of creatinine must be the same as the rate at which it was filtered at the glomerulus --So, clearance of creatinine is a good measure of GFR

Respiratory Responses

When plasma pH is low, the respiratory system responds quickly to restore a normal pH by shifting the buffering equation to the left (favors production of CO2 rather than carbonic acid) --If there is too much acid, the respiratory system will ventilate more to remove CO2 --As more CO2 is expired, the PCO2 of blood decreases, which feeds back to decrease the amount of acid in the blood If there is too little acid, the respiratory system will ventilate less to trap CO2 in the body --Causes partial pressure of CO2 to increase which will lower the pH back toward its set point Acidosis (PCO2 > 45 mmHg) --Causes 1. Inadequate alveolar ventilation 2. Overproduction of CO2 3. Increase in CO2 intake (normally negligible in ambient air) Alkalosis (PCO2 < 35 mmHg) --Causes 1. Head injury, stroke, pain, fear 2. Hyperventilation due to peripheral chemoreceptor stimulation --Pulmonary causes: embolism and edema Although the increased dead space caused by a pulmonary embolism impairs efficient elimination of CO2, the body responds by stimulating medullary chemoreceptors to increase ventilation which would lower arterial PCO2 to normal or below normal

Responses to Low Fluid Volume

When the effective circulating volume is low, there are 4 distinct signals that can be generated 1. the renin-angiotensin-aldosterone system (RAAS) 2. increased sympathetic neural activity to reduce renal blood flow and therefore renal sodium excretion 3. posterior pituitary increases ADH secretion to promote renal retention of water (only in cases of large volume decreases) 4. the hormonal effect of decreasing ANP release to reduce sodium and therefore fluid excretion

Osmotic Diuresis

When there is an increase in osmotically active particles in the tubule fluid, the result is osmotic diuresis --Increased urination due to the presence of substances that cannot be reabsorbed and therefore contribute to the osmolarity of the tubular fluid This is the process by which polyuria is caused in diabetes insipidus --Polyuria is when there is abnormally large volume of dilute urine being produced --In the case of diabetes insipidus, this is due to excess glucose in the tubular fluid that was not reabsorbed Osmotic diuresis is not always a bad thing --Can be used as part of a treatment regimen by inducing it --Clinicals will use osmotic diuretics, like mannitol, to release intracranial pressure by drawing water out of the brain and into the vasculature to be excreted by the kidneys --This works because there is an isosmotic reabsorption of sodium and chloride, but not mannitol ---This causes the concentrations of mannitol to rise and sodium chloride to fall without really changing the overall osmolality of the tubular fluid ---Once luminal sodium levels have dropped sufficiently, the paracellular backleak of sodium from peritubular capillaries mostly balances out the active sodium reabsorption ---The net reabsorption of salt and water becomes 0, so water stays in the tubule resulting in diuresis

Carbonic Anhydrases

Zinc-containing enzymes that catalyze the hydration of CO2 Carbonic anhydrase I: found in abundance in RBCs and is essential for the ability of RBCs to carry oxygen Carbonic anhydrase IV: a glycophosphatidylinositol (GPI) linked membrane-bound enzyme that is exposed to the tubular fluid contents of the proximal tubule and thick ascending limb --In the apical membrane, CA IV facilitates the reabsorption of large amounts of bicarb --It can also be expressed on the basolateral membrane and may facilitate the exit of bicarb from the cell in some way (not fully understood) --Acetazolamide mainly blocks this isoform Carbonic anhydrase II: a soluble form that is found in the cytoplasm of cells of the proximal tubule, thick ascending limb, and intercalated cells of the distal tubule and collecting duct --Not expressed in principal cells --Catalyzes the intracellular hydration of CO2 which is important in the retrieval of filtered bicarb and the production of new bicarb ions