Chapter 19 The Kidneys - Silverthorn 7e

How is micturition in someone who is toilet trained?

A person who has been toilet trained acquires a learned reflex that keeps the micturition reflex inhibited until he or she consciously desires to urinate. -The learned reflex involves additional sensory fibers in the bladder that signal the degree of fullness.--> Centers in the brainstem and cerebral cortex received that information --> override the basic micturition reflex by directly inhibiting the parasympathetic fibers and by reinforcing contraction of the external sphincter. -When an appropriate time to urinate arrives, those same centers remove the inhibition and facilitate the reflex by inhibiting contraction of the external sphincter.

What is a portal system?

A portal system consists of 2 capillary beds in series (one after the other).

Summary of filtration, reabsorption, secretion

Filtration takes place in the renal corpuscle as fluid moves from the capillaries of the glomerulus into Bowman's capsule. -Reabsorption and secretion occur along the remainder of the tubule, transferring materials between the lumen and the peritubular capillaries. -The quantity and composition of the substances being reabsorbed and secreted vary in different segments of the nephron. -Filtrate that remains in the lumen at the end of the nephron is excreted as urine

CONCEPT CHECK 2: What happens to the force of cardiac contraction if plasma Ca2+ levels decreased substantially?

Force of contraction decreases.

The colloid osmotic pressure (pi sign)

Inside glomerular capillaries is higher than that of the fluid in Bowman's capsule. -This pressure gradient is due to the presence of proteins in the plasma -averages 30 mm Hg and favors fluid movement back into the capillaries.

How is the interior of the kidney arranged?

Into 2 layers: -an outer cortex -inner medulla the layers are formed by the organized arrangement of microscopic tubules called nephrons. --About 80% of the nephrons in a kidney are almost completely contained within the cortex, but the other 20% called the juxtamedullary nephrons dip down into the medulla.

The internal sphincter

Is a continuation of the bladder wall and consists of smooth muscle. -Its normal tone keeps it contracted.

Secretion

Is the transfer of molecules from ECF into the lumen of the nephron. -Depends mostly on membrane transport systems. --The secretion of K+ and H + by the distal nephron is important in the homeostatic regulation of those ions. --Many organic compounds are secreted. These compounds include both metabolites produced in the body and substances brought into the body, or xenobiotics. -Enables a nephron to enhance excretion of a substance. --If a substance is filtered and not reabsorbed, it is excreted very efficiently. If, however the substance is filtered into the tubule not reabsorbed, and then more of it is secreted into the tubule from the peritubular capillaries, excretion is even more sufficient. -Is an active process because it requires moving substrates against their concentration gradients. --Most organic compounds are secreted across the proximal tubule epithelium into the lumen by indirect active transport.

If blood pressure is increased, does GFR increase too?

No GFR is constant over a wide range of blood pressures as long as mean arterial blood pressure remains between 80 mm Hg and 180 mm Hg. GFR averages 180 L/day.

GFR is controlled primarily by...

Regulation of blood flow through renal arterioles. -If the overall resistance of the renal arterioles increases, renal blood flow decreases, and blood is diverted to other organs. -The effect of increased resistance on GFR, depends on WHERE the resistance change takes place. -If the resistance increases in the AFFERENT arteriole, hydrostatic pressure decreases on the glomerular side of the constriction --> DECREASE in GFR. -If resistance increases in the EFFERENT arteriole --> blood "damns up" in front of the constriction, and hydrostatic pressure in the glomerular capillaries increases. --> Increased glomerular pressure --> INCREASES GFR. -The opposite changes occur with decreased resistance in the afferent or efferent arterioles. -Most regulation occurs at the afferent arteriole.

CONCEPT CHECK 10: if systemic blood pressure remains constant but that afferent arteriole of a nephron constricts, what happens to renal blood flow and GFR in that nephron?

Renal blood flow and GFR decrease.

The 3 filtration barriers of the renal corpuscle

Substances leaving the plasma must go through: 1) the glomerular capillary endothelium 2) a basal lamina (basement membrane) 3) the epithelium of Bowman's capsule

Paracellular Pathway

Substances pass through the cell cell junction between 2 adjacent cells.

Active Transport Of Na+

The active reabsorption of Na+ is the primary driving force for most renal reabsorption. -Filtrate entering the proximal tubule is similar in ion composition to plasma, with a higher Na+ concentration than is found in cells. -->Na+ and the filtrate can enter tubule cells passively by moving down its electrochemical gradient. -Once inside a tubule cell, Na+ is actively transported out across the basolateral membrane in exchange for K+ by the Na+-K+-ATPase. A basolateral K+ leak channel prevents K+ from accumulating in the cell.-->Na+ reabsorption across the tubule epithelium

The urethra in males exits through...

The shaft of the penis

Glomerular filtration rate (GFR)

The volume of fluid that filters into Bowman's capsule per unit time. -Average GFR is 125 mL/minute or 180 L/day (remember that total plasma volume is only about 3 L). -This rate means that the kidneys filter the entire plasma volume 60 times a day or 2.5 times every hour. -If most of the filtrate were not reabsorbed during its passage through the nephron, we would run out of plasma in only 24 minutes of filtration.

ureter

The hollow tubes through which urine leaves the body -there are 2 ureters leading from each kidney to the urinary bladder.

Glucose clearance

-Normally all glucose that filters is reabsorbed.

What is the function of the renal portal system?

(1)To filter fluid out of the blood and into the lumen of the nephron at the glomerular capillaries, then (2)to reabsorb fluid from the tubular lumen back into the blood at the peritubular capillaries. -The forces behind fluid movement in the renal portal system are similar to those that govern filtration of water and molecules out of systemic capillaries and other tissues.

Inulin clearance

- Is equal to GFR -infiltration and excretion of a substance are the same, then there is no net reabsorption or secretion and the clearance of a substance equals GFR.

The Bowman's capsule

-A hollow ball like structure that surrounds the glomerulus.-The endothelium of the glomerulus is fused into the epithelium of Bowman's capsule so that the fluid filtering out of the capillaries passes directly into the lumen of the tubule. -The combination of glomerulus and Bowman's capsule is called the "renal corpuscle".

The bladder

-A hollow organ whose walls contain well-developed layers of smooth muscle -In the bladder, urine is stored until released in the process known as urination, avoiding, or more formally, micturition. -The neck of the bladder is continuous with the urethra, a single tube through which the urine passes to reach the external environment. -The opening between the bladder and urethra is closed by 2 rings of muscle called sphincters (internal sphincter and external sphincter)

Macula densa (figure 19.7 B)

-A plaque of cells in the modified portion of the tubule epithelium.

Which hormones influence arteriolar resistance?

-Angiotensin II: a potent vasoconstrictor -Prostaglandins: vasodilators affect the filtration coefficient by acting on podocytes or mesangial cells. If the slits widen, more surface area is available for filtration and GFR increases. Contraction of mesangial cells changes the glomerular capillary surface area available for filtration.

3 pressures that influence glomerular filtration

-Capillary blood pressure -capillary colloid osmotic pressure -capsule fluid pressure

The epithelium of Bowman's capsule

-Consists of specialized cells called podocytes -have long cytoplasmic extensions called foot processes that extend from the main cell body. -Foot processes wrap around the glomerular capillaries and interlace with one another leaving narrow filtration slits closed by a semi-porous membrane. -The filtration slits contain several unique proteins, including nephrin and podocin --when nephrin and podocin are absent or abnormal, proteins leak across the glomerular filtration barrier into the urine

Urinary bladder

-Expands and fills with urine until, in a reflex called micturition or urination, the bladder contracts and expels urine through a single tube called the urethra.

The glomerular capillary endothelium

-Fenestrated capillaries with large pores that allow most components of the plasma to filter through the endothelium, but small enough to prevent blood cells from leaving the capillary. -The negatively charged proteins on the pore surfaces also help repel negatively charged plasma proteins.

What are the 3 basic processes that take place in the nephron?

-Filtration -Reabsorption -Secretion

Endocytosis: Plasma Proteins

-Filtration of plasma at the glomerulus normally leaves most plasma proteins in the blood, but some smaller proteins and peptides can pass through the filtration barrier. -Most filtered proteins are removed from filtrate in the proximal tubule with the result that normally only trace amounts of protein appear in urine. -Filtered proteins are too large to be reabsorbed by carriers through channels --> most enter proximal tubule cells by receptor-mediated endocytosis at the apical membrane. -->Proteins are digested in lysosomes --> results in amino acids --> transported across the basolateral membrane and absorbed into the blood. The renal digestion of small filtered proteins is actually a significant method by which peptide signal molecules can be removed from the circulation.

The basal lamina

-In a cellular layer of extracellular matrix that separates the capillary endothelium from the epithelium of Bowman's capsule. -Consists of negatively charged glycoproteins, collagen and other proteins. -Acts like a coarse sieve excluding most plasma proteins from the fluid that filters through it.

Tubuloglomerular Feedback

-Is a local control pathway -is a paracrine signaling mechanism through which changes in fluid flow through the loop of Henle influence GFR.

The external sphincter

-Is a ring of skeletal muscle controlled by somatic motor neurons. -Tonic stimulation from the central nervous system maintains contraction of external sphincter except during urination.

Micturition

-Is a simple spinal reflex that is subject to both conscious and unconscious control from the higher brain centers. -As the bladder fills with urine and its walls expand, stretch receptors send signals via sensory neurons to the spinal cord.--> Information is integrated and transferred to 2 sets of neurons. -The stimulus of a full bladder excites parasympathetic neurons leading to the smooth muscle in the bladder wall.--> Smooth muscle contracts, increasing the pressure on the bladder contents --> somatic motor neurons leading to the external sphincter are inhibited --> contraction of the bladder in a wave that pushes urine downward towards the urethra --> pressure exerted by the urine forces the internal sphincter open while the external sphincter relaxes. --> Urine passes into the urethra and out of the body aided by gravity. -This simple micturition reflex occurs primarily in infants who have not yet been toilet trained.

Organic Anion Transporter (OAT)

-Is able to transport a wide variety of endogenous and exogenous anions, ranging from bile salts to benzoate (used as a preservative and soft drinks), salicylate from aspirin and the artificial sweetener saccharin. -Secretion of organic anions on the OAT is an example of tertiary active transport, where the use of energy from ATP is 2 steps removed from the OAT.

Urea clearance

-Is an example of net reabsorption. -If filtration is greater than excretion, there is net reabsorption

Penicillin clearance

-Is an example of net secretion. -If excretion is greater than filtration, there is net secretion.

Clearance of a solute

-Is the rate at which that solute disappears from the body by excretion or by metabolism. -Clearance is mL plasma cleared of X per minute -units for clearance are mL plasma and time. -Note that a comparison of clearance values tells you only the NET handling of the solute. It does not tell you if a molecule is both reabsorbed and secreted. -Clearance calculations are relatively simple because all you need to know are the urine excretion rates and the plasma concentrations for any solute of interest. If you also know either inulin or creatinine clearance, then you can determine the renal handling of any compound.

What is the urinary system composed of?

-Kidneys and accessory structures (ureter, bladder, urethra)

Glomerular mesangial cells

-Lie between and around the glomerular capillaries -have cytoplasmic bundles of actin like filaments that enable them to contract and alter blood flow through the capillaries. -Secrete cytokines associated with immune and inflammatory processes. -Disruption of these cell function have been linked to several disease processes in the kidney.

Granular cells (also known as juxtaglomerular cells or JG cells)

-Located on the adjacent wall of the afferent arteriole. -Made up of specialized smooth muscle cells. -Secrete RENIN, and enzyme involved in salt and water balance. -When NaCl delivery past the macula densa increases as a result of increased GFR, the macula densa cells send paracrine message to the afferent arteriole. The afferent arteriole constricts, increasing resistance and decreasing GFR.

Primary cilia

-Located on the apical surface of the renal tubules, faces the lumen. -Act as flow sensors as well as signal transducers.

What are the arteries and veins of the kidney?

-Renal arteries which branch off the abdominal aorta, supply the kidneys -renal veins carry blood from the kidneys to the inferior vena cava

Fluid pressure (P fluid)

-Since Bowman's capsule is an enclosed space (unlike the interstitial fluid)-->presence of fluid in the capsule creates hydrostatic fluid pressure (P fluid) that opposes fluid movement into the capsule. -Fluid filtering out of the capillaries must displace the fluid already in the capsule lumen. -Hydrostatic fluid pressure in the capsule averages 15 mm Hg opposing filtration

Transepithelial transport (also called trans cellular transport)

-Substances cross the apical and basolateral membranes of the tubule epithelial to reach the interstitial fluid. -For solutes that move by transepithelial transport, their concentration or electrochemical gradients determine their transport mechanisms. --Solutes moving down the gradient use open leak channels are facilitated diffusion carriers to cross the cell membrane. --Molecules that need to be pushed against her gradient are moved by either primary or indirect (usually secondary) active transport.-->Na+ is directly or indirectly involved in many instances of both passive and active transport.

Subconscious factors that can affect the micturition reflex

-The "bashful bladder": A condition in which a person is unable to urinate in the presence of other people despite the conscious intent to do so. -The sound of running water facilitates micturition and is often used to help patients urinate if the urethra is irritated from insertion of a catheter, a tube inserted into the bladder to drain it passively.

The tubular elements of the kidney

-The kidney tubule consists of a single layer of epithelial cells connected together near their apical surface. -The apical surfaces are folded into microvilli or other area-increasing folds. -The basal side of the polarized epithelium rests on a basement membrane or basal lamina. -The cell-cell junctions are mostly tight but some have selective permeability for ions.

GFR is influenced by 2 factors

-The net filtration pressure -the filtration coefficient filtration pressure is determined primarily by renal blood flow and blood pressure. Filtration coefficient has 2 components: -the surface area of the glomerular capillaries available for filtration -the permeability of interface between the capillary and Bowman's capsule. *In this respect, glomerular filtration is similar to gas exchange at the alveoli where the rate of gas exchange depends on partial pressure differences, the surface area of the alveoli, and the permeability of the alveolar capillary diffusion barrier [page 565].

The juxtaglomerular apparatus

-The twisted configuration of the nephron as shown on figure 19.7 a causes the final portion of the thick ascending limb of the loop of Henle to pass between the afferent and efferent arterioles. --> The tubule and arteriolar walls are modified in the regions where they contact each other and together they form the JUXTAGLOMERULAR APPARATUS.

Passive Reabsorption: Urea

-Urea has no active transporters in the proximal tubule but can move through the epithelial junctions by diffusion if there is a urea concentration gradient. -Initially urea concentrations in the filtrate and ECF are equal. However, the active transport of Na+ and other solutes out of the proximal tubular lumen creates a urea concentration gradient by the following process: 1) when Na+ and other solutes are reabsorbed from the proximal tubule, the transfer of osmotic the active particles makes the ECF more concentrated than the filtrate remaining in the lumen. -->2) in response to the osmotic gradient, water moves by osmosis across the epithelium. Up to this point, no urea molecules have moved out of the lumen because there has been no urea concentration gradient. -->When water is reabsorbed, the concentration of urea in the lumen increases (the same amount of urea is contained in a smaller volume) -->a concentration gradient for urea is created -->urea moves out of the lumen into the ECF by transport through cells or by the paracellular pathway.

What is the purpose of filtering and reabsorption?

1) Many foreign substances are filtered into the tubule but not reabsorbed into the blood.Many nutrients such as glucose and citric acid cycle intermediates are filtered, but the proximal tubule very efficiently reabsorbed them. - the highly daily filtration rate helps clear each substances from the plasma very rapidly. -Once the substance filters into the lumen of Bowman's capsule, it is no longer part of the body's internal environment. Anything in the lumen is destined to leave the body in urine unless there is a tubule mechanism for reclaiming it. 2) Filtering ions in water into the tubule simplifies the regulation. -If a portion of filtrate that reaches the distal nephron is not needed to maintain homeostasis, it passes into the urine. With a high GFR, this excretion can occur rapidly. -If the ions in water are needed, they are reabsorbed.

Where does reabsorption take place?

1) Most of the reabsorption takes place in the proximal tubule 2) Smaller amount of reabsorption in the distal segments of the nephrons. -Regulated reabsorption in the distal nephron allows the kidneys to return ions and water to the plasma selectively, as needed to maintain homeostasis

What are the 2 ways hormones and the autonomic nervous system alter glomerular filtration?

1) changing resistance in the arterioles 2) altering the filtration coefficient

What are the functions of the kidney?

1) regulation of ECF volume and blood pressure 2) regulation of osmolarity 3) maintenance of ion balance 4) homeostatic regulation of pH 5) excretion of wastes 6) production of hormones

What are the 3 characteristics of mediated transport in the nephron?

1) saturation 2) specificity 3) competition

Reabsorption involves what 2 types of transport?

1) transepithelial transport 2) paracellular pathway Which route a solute takes depends on the permeability of the epithelial junctions and on the electrochemical gradient of the solute.

Normal creatinine clearance

88-128 mL/min for healthy women 97-137 mL/min for healthy men

The average volume of urine leaving the kidneys is...

1.5 L per day

Net driving force

10 mm Hg in the direction favoring filtration. -May not seem very high but when combined with the very leaky nature of the fenestrated glomerular capillaries, it results in rapid fluid filtration into the tubules

FIGURE 19.4 If 120 mL of plasma filter each minute and the filtration fraction is 20% what is a daily renal plasma flow?

120 mL/minute times 1440 minutes/day equals 172,800 mL/day 172.8 L equals 20% of plasma flow. Plasma flow equals 864 L/day

The amount of plasma that passes into the nephrons every day is...

180 L

What percentage of the cardiac output do the kidneys receive?

20-25% of the cardiac output even though the kidneys constitute only .4% of the total body weight -this high rate of blood flow through the kidneys is critical to renal function

How much volume can the bladder hold?

500 mL

What percentage of the fluid entering the tubules must be reabsorbed into blood as filtrate moves through the nephrons?

99% -each day, 180 L of filtered fluid pass from the glomerular capillaries into the tubules, yet only about 1.5 L are excreted in the urine.

FIGURE 19.2 2)From the diagram, calculate the percentage of filtered volume that leaves A) the loop of Henle B) the collecting duct

A) 18/180 = 10% 1.5/180 = 0.8%

FIGURE 19.2 1) In which segments of the nephron do the following processes take place: A) filtration B) reabsorption C) secretion D) excretion

A) Bowman's capsule B) proximal tubule, loop of Henle, distal tubule, collecting duct C) proximal tubule, distal tubule, collecting duct D) collecting duct

Principles governing the tubular reabsorption of solutes.

Active transport of Na+ from the tubular lumen to the extracellular fluid creates a trans-epithelial electrical gradient in which the lumen is more negative than the ECF. -Anions then follow the positively charged Na+ out of the lumen. -The removal of Na+ and anions from the lumen to ECF dilutes the luminal fluid and increases the concentration of the ECF. -So water leaves the tubule by osmosis. -The loss of volume from the lumen increases the concentration of solutes (including K+, Ca2+ and urea) left behind in the filtrate. The same amount of solute in a smaller volume equals higher solute concentration. -Once luminal solute concentrations are higher than solute concentrations in the ECF, the solutes diffuse out of the lumen if the epithelium of the tubule is permeable to them.

The loop of Henle

After the filtrate leave the proximal tubule's --> the loop of Henle -where the regulation of salt and water balance takes place under the control of several hormones. -Reabsorption and secretion (to a lesser extent) determine the final composition of the filtrate. -By the end of the collecting ducts, the filtrate has a volume of about 1.5 L per day and then osmolarity that can range from 50 mOsm to 1200 mOsm. --The final volume and osmolarity of urine depend on the bodies need to conserve or excrete water and solute.

CONCEPT CHECK 5: Name one way in which filtration and secretion are alike. Name one way in which they differ.

Alike: both represent movement from ECF into the lumen. Different: filtration is only into Bowman's capsule; secretion takes place in the proximal tubule, distal tubule and collecting duct.

The composition of the 180 L of fluid that filters into Bowman's capsule each day

Almost identical in composition to plasma -nearly isosmotic to plasma at about 300 mOsm -as This filtrate flows to the proximal tubule, about 70% of its volume is reabsorbed, leaving 54 liters in the lumen

The renal handling of a substance during its passage through the nephron

Amount excreted = amount filtered - amount reabsorbed + amount secreted NOTE: -Not every substance in the plasma is filtered. -Substances that are filtered may or may not be reabsorbed or secreted

The urethral opening in females is found...

Anterior to the openings of the vagina and anus -because of the shorter length of the female urethra and its proximity to bacteria leaving the large intestine, women are more prone than men to develop bacterial infections of the bladder and kidneys or urinary tract infections (UTI's)

Auto regulation of glomerular filtration rate is a local or long distance control process?

Autoregulation is a LOCAL control process in which the kidney maintains a relatively constant GFR in the face of normal fluctuations in blood pressure.

FIGURE 19.6 QUESTION: what happens to capillary blood pressure, GFR and RBF when the afferent arteriole dilates?

Capillary blood pressure, GFR, and renal blood flow all increase.

FIGURE 19.3 A person filters 720 mmol of K+ in a day and secretes 43 mmol. She secretes 79 mmol in her urine what happened to the rest of the K+ and how much was it

E = F - R + S 79 mmol/day = 720 - R +43 R = 684 mmol reabsorbed per day

CONCEPT CHECK 6: A water molecule enters the renal corpuscle from the blood and ends up in the urine. Name all the anatomical structures that the molecule passes through on its trip to the outside world.

Glomerulus --> Bowman's capsule --> proximal tubule -->loop of Henle --> distal tubule -->collecting duct --> renal pelvis -->ureter -->urinary bladder -->urethra

CONCEPT CHECK 3: If net filtration out of glomerular capillaries occurs, then you know that capillary hydrostatic pressure must be greater than OR less than OR equal to capillary colloid osmotic pressure.

Greater than

Nephrons

Hollow tubules that make up the bulk of the paired kidneys -modify the composition of the fluid as it passes through. -The modified fluid when it leaves the kidney is called urine.

Figure question 19.11: why is the hydrostatic pressure so much lower in the peritubular capillaries than in the glomerulus?

Hydrostatic pressure is lower because blood flowing out of the glomerulus loses pressure as it moves along the peritubular capillaries.

How does the reabsorbed fluid get into the capillary?

Hydrostatic pressure that exists along the entire length of the peritubular capillaries is less than the colloid osmotic pressure, so the net pressure gradient favors reabsorption. -The peritubular capillaries have an average hydrostatic pressure of 10 mm Hg (in contrast to the glomerular capillaries were hydrostatic pressure averages 55 mm Hg. Colloid osmotic pressure, which favors movement of fluid into the capillaries is 30 mm Hg. As a result, the pressure gradient in peritubular capillaries is 20 mm Hg, favoring the absorption of fluid into the capillaries. Fluid that is reabsorbed passes from the peritubular capillaries to the venous circulation and returns to the heart.

What happens to the filtered fluid/filtrate after it goes into the nephron?

It passes into the lumen of the nephron (where it becomes part of the body's external environment, just as substances in the lumen of the intestinal tract are part of the external environment) and is destined for excretion/removal in the urine, unless it is reabsorbed into the body.

CONCEPT CHECK 4: If net reabsorption into peritubular capillaries occurs, then capillary hydrostatic pressure must be greater than OR less than or equal to the capillary colloid osmotic pressure.

Less than

CONCEPT CHECK 9: if a hypertensive person's blood pressure is 143/107 mm Hg, and mean arterial pressure equals diastolic pressure +1/3 the pulse pressure, what is this person's mean arterial pressure? What is this person's GFR according to figure 19.6 B?

MAP = 119; GFR = 180 L/day

Neural control of GFR is mediated by THE PARASYMPATHETIC or THE SYMPATHETIC neurons?

Neural control of GFR is mediated by sympathetic neurons that innervate both the afferent and efferent arterioles. -Sympathetic innervation of alpha receptors on vascular smooth muscle causes vasoconstriction. --If systemic blood pressure drops sharply as with hemorrhage or severe dehydration, sympathetically induced vasoconstriction of the arterioles decreases GFR and renal blood flow. This is an adaptive response that helps conserve fluid volume.

Where are the kidneys located?

On either side of the spine at the level of the 11th and 12th ribs, just below the waist. -Although they are below the diaphragm, they are technically outside of the abdominal cavity, sandwiched between the membranous peritoneum which lines the abdomen and the bones in the muscles of the back. -Because of their location behind the peritoneal cavity, the kidneys are sometimes described as being retroperitoneal. -The concave surface of each kidney faces the spine. -The renal blood vessels, nerves, lymphatics and ureters all emerge from the surface.

How is renal handling determined?

Once we know a person's GFR, we can determine how the kidney handles any soul you'd by measuring the solutes plasma concentration and its excretion rate.

CONCEPT CHECK 8: Why is osmotic pressure of plasma in efferent arterioles higher than that in afferent arterioles?

Osmotic pressure is higher in efferent arterioles due to same amount of protein in a smaller volume.

Hydrostatic pressure (PH)

PH of blood flowing through the glomerular capillaries forces fluid through the leaky endothelium. -Capillary blood pressure averages 55 mm Hg and favors filtration into Bowman's capsule. -Although pressure decreases as blood moves through the capillaries, it remains higher than the opposing pressures. -->Filtration takes place along nearly the entire length of the glomerular capillaries.

What is the significance of creatinine clearance?

Physicians use creatinine to estimate GFR. -Creatinine is a breakdown product of phosphocreatinine, an energy storage compound found primarily in muscles. It is constantly produced by the body and need not be administered. Normally the production and breakdown rates of phospho and he are relatively constant, and the plasma concentration of creatinine does not very much. -Although creatinine is always present in the plasma and is easy to measure, it is not the perfect molecule for estimating GFR because a small amount is secreted into the urine. However, the amount secreted is small enough that, in most people, creatinine clearance is routinely used to estimate GFR.

From the Bowman's capsule, filtered fluid flows into the...

Proximal tubule -->loop of Henle (a hairpin shaped segment that dips down toward the medulla then back up. The loop of Henle is divided into 2 limbs-the descending limb and the ascending limb) --> distal tubule --> the collecting duct (up to 8 nephrons drain into a single collecting duct). The distal tubule and its collecting duct form the distal nephron. Collecting ducts pass from the cortex through the medulla and drain into the renal pelvis --> the filtered and modified fluid is now called "urine" --> ureter --> excretion

Reabsorption occurs when...

Proximal tubule cells transport solutes out of the lumen and water follows by osmosis -filtrate leaving the proximal tubule has the same osmolarity as filtrate that entered -the primary function of the proximal tubule is iso-osmotic reabsorption of solutes and water

Saturation

Refers to the maximum rate of transport that occurs when all available carriers are occupied (are saturated with) substrate -at substrate concentrations below the saturation point, transport rate is directly related to substrate concentration. -At substrate concentrations equal to or above the saturation point, transport occurs at a maximum rate. The transport rate at saturation is the transport maximum (Tm) example: glucose -at normal plasma glucose concentrations, all glucose that enters the nephron is reabsorbed before it reaches the end of the proximal tubule. The tubule epithelium is well supplied with carriers to capture glucose as the filtrate flows past. -If blood glucose concentrations become excessive (as in diabetes mellitus)--> glucose is filtered faster than the carriers can be absorb it -->the carriers become saturated and are unable to reabsorb all the glucose that flows through the tubule -->some glucose escapes reabsorption and is excreted in the urine.

Secretion versus excretion

Secretion: to separate something from its source -secreted solutes are moved from plasma to tubule lumen Excretion refers to the removal of a substance from the body. -Besides the kidneys, other organs that carry out excretory processes include the lungs (CO2) and intestines (undigested food, bilirubin).

Secondary Active Transport:Symport With Na+

Sodium Link secondary active transport in the nephron is responsible for the reabsorption of many substances including glucose, amino acids, ions and various organic metabolites. Figure 19.8 C shows one example of sodium dependent glucose reabsorption across the proximal tubule. The apical membrane contains the Na+-glucose transporter (SGLT) that brings glucose into the cytoplasm against it concentration gradient by harnessing the energy of Na+ moving down its electrochemical gradient. On the basolateral side of the cell, Na+ is pumped out by the Na+-K+ ATPase, while glucose diffuses out with the aid of the facilitated diffusion GL UT transporter. -The same basic pattern holds for many other molecules absorbed by Na+ dependent transport: an apical symport protein and a basolateral facilitated diffusion carrier or ion exchanger. -Other molecules that are reabsorbed by similar mechanisms include: amino acids, lactate, citric acid cycle intermediates such as citrate and Alpha ketoglutarate, and ions such as phosphate and sulfate. -A few of apical transporters use H + in place of Na+.

The plasma concentration at which glucose 1st appears in the urine is called...

The RENAL THRESHOLD for glucose -remember that excretion equals filtration - reabsorption (E = F - R). When plasma glucose concentrations are low enough that 100% of the filtered glucose is reabsorbed, no glucose is excreted. -Once the carriers reach saturation, glucose excretion begins. -Glucose excreted = glucose filtered - glucose reabsorbed --under normal conditions, all filtered glucose is reabsorbed. In other words filtration is equal to reabsorption.

How do the kidneys regulate osmolarity?

The body integrates kidney function with behavioral drive such as thirst to maintain blood osmolarity at a value close to 290 mOsm.

CONCEPT CHECK 7: What would happen to the body if filtration continued at a normal rate but reabsorption drop to half the normal rate?

The body would run out of plasma in under an hour.

What hormones do the kidneys produce?

The kidneys are not endocrine glands but play a part in 3 endocrine pathways: 1) kidneys synthesize erythropoietin, the cytokine/hormone that regulates red blood cell synthesis. 2) kidneys release renin-an enzyme that regulates the production of hormones involved in sodium balance and blood pressure. 3) kidneys help convert vitamin D3 into a hormone that regulates Ca2+ balance.

How do the kidneys maintain ion balance?

The kidneys keep concentrations of key ions within a normal range by balancing dietary intake with urinary loss. -Na+ is the major ion involved in the regulation of ECF volume and osmolarity. -K+ and Ca2+ concentrations are also closely regulated.

What waste products do the kidneys remove?

The kidneys remove metabolic waste products and xenobiotics or foreign substances such as drugs and environmental toxins: -creatinine from muscle metabolism -nitrogenous substances: urea and uric acid -a metabolite of hemoglobin called urobilinogen which gives your in its characteristic yellow color -hormones -examples of foreign substances the kidneys actively remove: artificial sweeteners (saccharin, benzoate, potassium benzoate from diet soft drink)

How do the kidneys regulate ECF volume and blood pressure?

The kidneys work with the cardiovascular system to ensure that blood pressure and tissue perfusion remain within acceptable range. -When ECF volume decreases, BP also decreases when ECF volume and BP fall too low, the body cannot maintain adequate blood flow to the brain and essential organs.

What is the vasa recta?

The long peritubular capillaries, in the juxtamedullary nephrons, that dip into the medulla

CONCEPT CHECK 1: Ion regulation is a key feature of kidney function. What happens to the resting membrane potential of a neuron if extracellular K+ levels decrease?

The membrane potential hyper polarizes (becomes more negative)

Filtration

The movement of fluid from blood into the lumen of the nephron. -Filtration takes place only in the renal corpuscle where the walls of glomerular capillaries and Bowman's capsule are modified to allow bulk flow of fluid.

What is the functional unit (the smallest structure that can perform all the functions of an organ) of the kidney?

The nephron -each of the 1 million nephrons in a kidney is divided into sections, and each section is closely associated with specialized blood vessels. (Figure 19.1 F, G, H, I)

How do the kidneys maintain homeostatic regulation of pH?

The pH of plasma is normally kept within a narrow range (7.35-7.45) -if ECF becomes too acidic, the kidneys remove H+ and conserve HCO3- which act as a buffer -if ECF becomes too alkaline, the kidneys remove HCO3 - and conserve H + -the kidneys do not correct pH disturbances as rapidly as the lungs do.

Filtration fraction

The percentage of total plasma volume that filters into the tubule -all of the plasma in the capillary moves into Bowman's capsule. -Only 1/5 of the plasma that flows through the kidneys filters into the nephrons -the remaining 4/5 of the plasma, along with most plasma proteins and blood cells, flows into the peritubular capillaries.

Figure 19.9 Graph Question: what is the transport rate at the following plasma substrate concentrations: 3 mg/milliliters, 5 mg/milliliters, 8 mg/milliliters? At what plasma substrate concentration is a transport rate 2 mg/minute?

The transport rate at 3 mg/mL is 3 mg/minute; at 5 and 8 mg/mL, it is 4 mg/minute. The transport rate is 2 mg/minute at a plasma concentration of 2 mg/mL.

What is an important function of GFR autoregulation?

To protect the filtration barriers from high blood pressures that might damage them. -The MYOGENIC RESPONSE -TUBULOGLOMERULAR FEEDBACK

What is the filtrate called once it leaves the collecting ducts?

Urine -once filtrate leaves the collecting ducts, I can no longer be modified, and its composition does not change. -The filtrate, now called urine, flows into the renal pelvis and then down the ureter to the bladder with the help of rhythmic smooth muscle contractions.

Excretion

Urine output as a result of all the processes that take place in the kidney. -By the time the fluid reaches the end of the nephron, it bears little resemblance to the filtrate that started in Bowman's capsule (glucose, amino acids, and useful metabolites are gone, having been reabsorbed into the blood and organic wastes are more concentrated. The concentration of ions and water in the year and are highly variable, depending on the state of the body. -Although excretion tells us what the body is eliminating, excretion by itself cannot tell us the details of renal function.

What is the myogenic response to increased blood pressure?

VASOCONSTRICTION -the myogenic response of afferent arterioles is similar to autoregulation in other systemic arterioles: -when smooth muscle in the arteriole wall stretches because of increased blood pressure -->stretch sensitive ion channels open and the muscle cells to depolarize.--> Opens voltage gated Ca2+ channels--> vascular smooth muscle contracts -->vasoconstriction -->blood flow through the arteriole diminishes-->decrease filtration pressure in the glomerulus

What is the myogenic response to decreased blood pressure?

VASODILATION -if blood pressure decreases, the tonic level of arteriolar contraction disappears --> arteriole becomes maximally dilated. -->GFR decreases -HOWEVER vasodilation is not as effective at maintaining GFR as vasoconstriction because normally the afferent arteriole is fairly relaxed. -When mean blood pressure drops below 80 mm Hg -->GFR decreases --this decrease is adaptive in the sense that if less plasma is filtered, the potential for fluid loss in the urine is decreased. In other words, a decrease in GFR helps the body conserve blood volume.

CONCEPT CHECK 11: a person with cirrhosis of the liver has lower than normal levels of plasma proteins and consequently a higher than normal GFR. Explain why a decrease in plasma protein concentration causes an increase in GFR.

With fewer plasma proteins, the plasma has lower than normal colloid osmotic pressure opposing GFR so GFR increases.

How does blood flow in the renal portal system?

blood flows from renal arteries into an afferrent arteriole --> the glomerulus (the 1st capillary bed, a ball like network) -->efferrent arteriole -->peritubular capillaries (2nd set of capillaries) that surround the tubule.

Myogenic Response

is the intrinsic ability of vascular smooth muscle to respond to pressure changes. -the myogenic response of afferent arterioles is similar to autoregulation in other systemic arterioles.

After the filtrate leaves Bowman's capsule...

it is modified by reabsorption and secretion. -Reabsorption is a process of moving substances in the filtrate from the lumen of the tubule back into the blood flowing through peritubular capillaries. -Secretion selectively removes molecules from the blood and adds them to the filtrate in the tubular lumen. --Although secretion and glomerular filtration both moves substances from the blood into the tubule, secretion is a more selective process that usually uses membrane proteins to move molecules across the tubule epithelium.