The Urinary System

Renal Corpuscle

150-250 µm in length and diameter. It is made up of the glomerular (Bowman's) capsule, a cup-shaped chamber that envelopes the capillary network called the glomerulus. Characteristics: Glomerulus, intraglomerular mesengial cells, and basement membrane, enclosed by the glomerular capsule, visceral layer (podocytes) and capsular outer layer (simple squamous epithelium) separated by capsular space. Primary Function: Filtration of Blood Plasma

Overview of the Urinary System

2 Kidneys: Produces Urine 2 Ureters: Muscular tubes that transports urine toward the urinary bladder. Urinary Bladder: Temporarily stores urine prior to urination. Urethra: Transports urine from the body and in males also transports semen. Female - 1.5 inches Male - 7 inches

Kidney Blood Supply and Innervation

20-25% of the cardiac output is delivered to kidney. Each kidney receives blood through a renal artery. This vessel originates along the lateral surface of the abdominal aorta near the level of the superior mesenteric artery. As it enters the renal sinus, the renal artery provides blood to the five segmental arteries. Segmental arteries further divide into a series of interlobar arteries. These arteries radiate outward through the renal columns between the renal pyramids. The interlobar arteries supply blood to the arcuate arteries, which arch along the boundary between the cortex and medulla of the kidney. Each arcuate artery gives rise to a number of cortical radiate arteries, also called interlobular arteries. They supply the cortical regions of the adjacent kidney lobes. Branching from each cortical radiate artery are numerous afferent arterioles. These vessels deliver blood to the capillaries supplying individual nephrons. After passing through the capillaries of the nephrons, blood enters a network of venules and small veins that converge on the cortical radiate veins, also called interlobular veins. The cortical radiate veins deliver blood to arcuate veins. These veins in turn empty into interlobar veins, which drain directly into the renal vein. There are no segmental veins. Renal nerves innervate the kidneys and ureters. Most of the nerve fibers involved are sympathetic postganglionic fibers from the celiac plexus and the inferior splanchnic nerves. A renal nerve enters each kidney at the hilum and follows the branches of the renal arteries to reach individual nephrons. The sympathetic innervation (1) adjusts rates of urine formation by changing blood flow at the nephron and (2) influences urine composition by stimulating release of the hormone renin.

How would a decrease in blood pressure affect the GFR?

A decrease in blood pressure would decrease the GFR by decreasing the blood hydrostatic pressure within the glomerulus.

Age Related Changes Affecting Kidney Function and Urination

A decrease in the Number of Functional Nephrons. The total number of kidney nephrons decreases by 30-40 percent between ages 25 and 85. A Reduction in the GFR. This reduction results from fewer glomeruli, cumulative damage to the filtration apparatus in the remaining glomeruli, and reduced renal blood flow. A Reduced Sensitivity to ADH. With age, the distal segments of the nephron and collecting system become less responsive to ADH. Water and sodium ions are reabsorbed at a reduced rate, and more sodium ions are lost in urine. Problems with the Urinary Reflexes. Three factors are involved in such problems: (1) The external sphincter loses muscle tone and become less effective at voluntarily retaining urine. This leads to incontinence, often involving a slow leakage of urine. (2) The ability to control urination can be lost due to a stroke, Alzheimer's disease, or other CNS problems affecting the cerebral cortex or hypothalamus. (3) In males, urinary retention may develop if the prostate enlarges and compresses the urethra, restricting the flow of urine.

In which region of the kidney is a glomerulus located?

A glomerulus is located in the renal cortex.

If ADH is high you will produce?

A small volume of concentrated urine High solutes

Juxtaglomerular Apparatus/Complex (JGA or JGC)

A special feature of the nephron and its blood supply. A modification of the distal convoluted tubule cells and afferent arteriole cells where they approach each other. The juxtaglomerular complex (JGC) is a structure that helps regulate blood pressure and filtrate formation. It consists of the macula densa, juxtaglomerular cells (granular cells), and extraglomerular mesangial cells. Macula Densa: A group of tall, closely packed epithelial cells in the distal tubular epithelium. Juxtaglomerular Cells: Modified smooth muscle cells in the wall of the afferent arteriole that secrete renin. These cells function as baroreceptors that monitor blood pressure in the afferent arteriole. Extraglomerular Mesangial Cells: Located in the triangular space between the afferent and efferent glomerular arterioles. These cells provide feedback control between the macula densa and the juxtaglomerular cells. Functions: JG Cells Secrete Erythropoietin Renin (Enzyme)

Aldosterone-sensitive portions of the distal convoluted tubule and collecting duct allow for the exchange of which ions?

Aldosterone-sensitive portions of the distal convoluted tubule and collecting duct allow for sodium ions to be reabsorbed in exchange for potassium ions to be passively lost.

What four factors shown result in an increased blood volume?

An increased blood volume results from increased Na+ retention, increased fluid consumption, increased fluid retention, and constriction of systemic veins.

Glomerulonephritis

An inflammation of the glomeruli that impairs filtration in the kidneys. The condition is often an immune complex disorder. It may develop after an infection involving Streptococcus bacteria. The kidneys are not the sites of infection, but as the immune system responds, the number of circulating antigen-antibody complexes rapidly increases. These complexes are small enough to pass through the basement membrane, but too large to fit between podocyte foot processes of the filtration membrane. The complexes clog up the filtration mechanism. As a result, filtrate production decreases. Any condition that leads to a massive immune response, including viral infections and autoimmune disorders, can cause glomerulonephritis.

Obstruction of a ureter by a kidney stone would interfere with the flow of urine between which two points?

An obstruction of a ureter would interfere with the passage of urine between the renal pelvis and the urinary bladder.

Autoregulation of the GFR

Autoregulation (local blood flow regulation) maintains an adequate GFR despite changes in local blood pressure and blood flow. Myogenic mechanisms—how arteries and arterioles react to an increase or decrease in blood pressure—play a role in the autoregulation of blood flow. Changes to the luminal diameters of afferent arterioles, efferent arterioles, and glomerular capillaries maintain GFR. The most important regulatory mechanisms stabilize the GFR when systemic blood pressure decreases. One of these mechanisms relies on the supporting intraglomerular mesangial cells. Actin-like filaments in these cells enable them to contract. In this way, these cells control capillary luminal diameter and the rate of glomerular capillary blood flow. The GFR also remains relatively constant when systemic blood pressure increases. An increase in renal blood pressure stretches the walls of afferent glomerular arterioles, and the smooth muscle cells respond by contracting. The reduction in the luminal diameter of afferent glomerular arterioles decreases glomerular blood flow and keeps the GFR within normal limits. If autoregulation is ineffective, the kidneys call for a wider response to increase the GFR

Kidney External Anatomy

Bean-shaped organs served by renal blood vessels, lymphatics, and nerves adjacent to the proximal region of the ureters. 4 inches in length, 2 inches wide, and 1 inch thick. Hilum: A prominent medial indentation, is the point of entry for the renal artery and renal nerves. The hilum is also the point of exit for the renal vein and the ureter.

Hydrostatic Pressure

Blood pressure is low in typical systemic capillaries. The reason is that capillary blood flows into the venous system, where resistance is fairly low. However, at the glomerulus, blood leaving the glomerular capillaries flows into an efferent glomerular arteriole, whose luminal diameter is smaller than that of the afferent glomerular arteriole. For this reason, the efferent arteriole offers considerable resistance. Relatively high pressures are needed to force blood into it. As a result, glomerular pressures are similar to those of small arteries. Glomerular hydrostatic pressure (GHP) averages about 50 mm Hg, instead of the 35 mm Hg typical of peripheral capillaries. Capsular hydrostatic pressure (CsHP) opposes glomerular hydrostatic pressure. The CsHP results from the resistance to flow along the nephron and the conducting system. (Before additional filtrate can enter the capsule, some of the filtrate already present must be forced into the PCT.) The CsHP averages about 15 mm Hg. The net hydrostatic pressure (NHP) is the difference between the glomerular hydrostatic pressure, which tends to push water and solutes out of the bloodstream, and the capsular hydrostatic pressure, which tends to push water and solutes into the bloodstream. We can calculate net hydrostatic pressure as follows: NHP = GHP - CsHP

Name the body systems that make up the excretory system.

Body systems that make up the body's excretory system include the urinary, integumentary, respiratory, and digestive systems.

Damage to which part of a nephron would interfere with the hormonal control of blood pressure?

Damage to the juxtaglomerular complex of a nephron would interfere with blood pressure regulation.

Kidney Layers of Connective Tissue

Deep to Superficial: 1. Fibrous Capsule: A layer of collagen fibers that covers the outer surface of the entire organ. 2. Perinephric fat: A cushioning, thick layer of adipose tissue that surrounds the fibrous capsule 3. Renal Fascia: A dense, fibrous outer layer that anchors the kidney to surrounding structures. Collagen fibers extend outward from the fibrous capsule through the perinephric fat to this layer. Posteriorly, the renal fascia fuses with the deep fascia surrounding the muscles of the body wall. Anteriorly, the renal fascia forms a thick layer that fuses with the peritoneum.

What effect would a high-protein diet have on the composition of urine?

Digestion of a high-protein diet would lead to increased production of urea, a nitrogenous waste formed from the metabolism of amino acids during the breakdown of proteins. As a result, the urine would contain more urea, and urine volume might also increase as a result of the need to flush the excess urea.

Diuretics

Diuresis is the elimination of urine. Urination is an equivalent term in a general sense, but diuresis typically indicates the production of a large volume of urine. Diuretics are drugs that promote water loss in urine. The usual goal in diuretic therapy is to decrease blood volume, blood pressure, extracellular fluid volume, or all three. The ability to control renal water losses with relatively safe and effective diuretics has saved the lives of many people, especially those with high blood pressure or congestive heart failure. Diuretics have many mechanisms of action. However, all such drugs affect transport activities or water reabsorption along the nephron and collecting system. For example, consider the class of diuretics called thiazides. These drugs increase water loss by decreasing sodium and chloride ion transport in the proximal and distal convoluted tubules. Diuretic use for nonclinical reasons is on the rise. For example, some bodybuilders take large doses of diuretics to improve muscle definition temporarily. Some fashion models or horse jockeys do the same. Their goal is to reduce body weight for brief periods. This practice of "cosmetic dehydration" is extremely dangerous. It has caused several deaths due to electrolyte imbalance and consequent cardiac arrest.

Reabsorption in the Proximal Convoluted Tubule

Enter: Filtrate from Bowman's Capsule Osmolarity of filtrate entering PCT = 300 mOsm/L = same as plasma Lumen of PCT Filtrate Nutrients Glucose 100% Reabsorbed Amino Acids 100% Reabsorbed Body holds onto valuable energy source Sodium 70% Reabsorbed Water 70% Reabsorbed Obligatory H2O must follow Na++ Other ions K+ MgH HCO3- Phosphate Sulfate Osmolarity of filtrate exiting from PCT 300 mOsm/L The cells of the proximal convoluted tubule normally reabsorb 60-70 percent of the volume of the filtrate produced in the renal corpuscle. The tubular cells reabsorb organic nutrients, ions, water, and plasma proteins (if present) from the tubular fluid. The reabsorbed materials enter the peritubular fluid, diffuse into peritubular capillaries, and are quickly returned to the bloodstream. The epithelial cells can also secrete substances into the lumen of the renal tubule. The PCT has the following major functions: Reabsorption of Organic Nutrients. Under normal circumstances, before the tubular fluid enters the nephron loop, the PCT reabsorbs more than 99 percent of the glucose, amino acids, and other organic nutrients in the fluid. This reabsorption involves a combination of facilitated diffusion and cotransport. Active Reabsorption of Ions. The PCT actively transports several ions, including sodium, potassium, and bicarbonate ions, plus magnesium, phosphate, and sulfate ions. Although the ion pumps involved are individually regulated, they may be influenced by circulating ion or hormone levels. For example, angiotensin II stimulates sodium ion (Na+) reabsorption along the PCT. By absorbing carbon dioxide (CO2), the PCT indirectly recaptures about 90 percent of the bicarbonate ions (HCO3−) from tubular fluid. Bicarbonate ions are important in stabilizing blood pH. Reabsorption of Water. The reabsorptive processes have a direct effect on the solute concentrations inside and outside the tubules. The filtrate entering the PCT has the same osmotic concentration as that of the surrounding peritubular fluid. As reabsorption proceeds, the solute concentration of tubular fluid decreases, and that of peritubular fluid and adjacent capillaries increases. Water then moves out of the tubular fluid and into the peritubular fluid by osmosis. Along the PCT, this mechanism results in the reabsorption of about 108 liters of water each day. Passive Reabsorption of Ions. As active reabsorption of ions takes place and water leaves the tubular fluid by osmosis, the concentration of other solutes in the tubular fluid increases above that in the peritubular fluid. If the tubular cells are permeable to them, those solutes move across the tubular cells and into the peritubular fluid by passive diffusion. Urea, chloride ions, and lipid-soluble materials may diffuse out of the PCT in this way. Such diffusion further decreases the solute concentration of the tubular fluid and promotes additional water reabsorption by osmosis. Secretion. Active secretion of hydrogen ions first takes place along the PCT. Because the PCT and DCT secrete similar substances, and the DCT carries out comparatively little reabsorption, we will consider secretory mechanisms when we discuss the DCT.

Reabsorption and Secretion in the Loop of Henle

Enter: Filtrate from PCT Osmolarity of filtrate entering Descending Limb 300 mOSM /L Descending Limb Permeable to H2O Impermeable to Solutes (Na+) Osmolarity of filtrate at bottom of descending limb 1200 mOsm / L Hyperosmotic Ascending Limb Impermeable to H2O Na+ and Cl- are pumped out Osmolarity of filtrate at top of ascending limb 100 mOsm / L Hypo-osmotic Approximately 60-70 percent of the volume of filtrate produced by the glomerulus has been reabsorbed before the tubular fluid reaches the nephron loop. In the process, useful organic substrates and many mineral ions have been reclaimed. The nephron loop reabsorbs about half of the remaining water and two-thirds of the remaining sodium and chloride ions. Recall that the nephron loop is made up of ascending and descending limbs, with thin and thick segments. The ascending limb is impermeable to water, but passively and actively removes sodium and chloride ions from the tubular fluid. Its ascending thin limb is permeable to sodium ions, which diffuse into the surrounding peritubular fluid. The thick ascending limb (TAL) actively transports sodium and chloride ions out of the tubular fluid. The effect of this movement is most noticeable in the renal medulla, where the long ascending limbs of juxtamedullary nephrons create unusually high solute concentrations in the peritubular fluid. The entire descending limb is freely permeable to water, but not to solutes, such as sodium and chloride ions. The descending thin limb has functions similar to those of the PCT: It reabsorbs sodium and chloride ions out of the tubular fluid. Because of the ion concentration gradients created by the thick ascending limb, water moves out of the descending limb, helping to increase the solute concentration of the tubular fluid.

Reabsorption in the Distal Convoluted Tubule

Enter: Filtrate from ascending limb of loop of henle Osmolarity of filtrate entering DCT 100 mOSM / L Sodium Controlled by: aldosterone (adrenal cortex) Increase Na+ reabsorption in DCT + CD Water Controlled by antidiuretic hormone (ADH) (posterior pituitary) Increase H2O reabsorption Osmolarity of filtrate exiting DCT and CD: depends on hormones Only 15-20 percent of the initial filtrate volume reaches the DCT. The concentrations of electrolytes and metabolic wastes in the arriving tubular fluid no longer resemble the concentrations in blood plasma. Selective reabsorption or secretion, primarily along the DCT, makes the final adjustments in the solute composition and volume of the tubular fluid. The DCT is an important site for three vital processes: (1) the active secretion of ions, acids, drugs, and toxins into the tubule; (2) the selective reabsorption of sodium ions and calcium ions from tubular fluid; and (3) the selective reabsorption of water, which assists in concentrating the tubular fluid. Throughout most of the DCT, the tubular cells actively transport sodium ions (Na+) and chloride ions (Cl−) out of the tubular fluid. Tubular cells along the distal regions of the DCT also contain ion pumps that reabsorb tubular sodium ions in exchange for another cation (usually potassium ions [K+]). The hormone aldosterone, produced by the adrenal cortex, controls the sodium ion channels and the ion pump. This hormone stimulates the synthesis and incorporation of sodium ion channels and sodium ion pumps in plasma membranes along the DCT and collecting duct. The net result is a reduction in the number of sodium ions lost in urine. However, Na+ conservation is associated with K+ loss. Prolonged aldosterone stimulation can therefore produce hypokalemia, a dangerous reduction in the plasma potassium ion concentration. The secretion of aldosterone and its actions on the DCT and collecting system are opposed by the natriuretic peptide ANP. The DCT is also the primary site of calcium ion (Ca2+) reabsorption. The circulating level of parathyroid hormone regulates this process

Functions of the Urinary System

Excretion, the removal of metabolic waste products from body fluids, and elimination, the discharge of these wastes out of the body. Homeostatic regulation of the volume and solute concentration of blood by: -Regulating blood volume and blood pressure, by adjusting the volume of water lost in urine, secreting erythropoietin, and releasing renin. -Regulating plasma concentrations of sodium, potassium, chloride, and other ions by influencing the quantities in urine. The kidneys also control the calcium ion level through the synthesis of calcitriol. -Helping to stabilize blood pH, by controlling the concentrations of hydrogen and bicarbonate ions in urine. -Conserving valuable nutrients by preventing their loss in urine while removing metabolic wastes - especially the nitrogenous wastes urea and uric acid. -Assisting the liver in detoxifying poisons and, during starvation, the deamination of amino acids for their metabolic use by other tissues.

Regulation of the GFR

Filtration depends on adequate blood flow to the glomerulus and on the maintenance of normal filtration pressures. Three interacting levels of control ensure that the GFR remains within normal limits: (1) autoregulation occurring at the local level, (2) hormonal regulation initiated by the kidneys, and (3) autonomic regulation maintained primarily by the sympathetic division of the autonomic nervous system.

Identify the role the urinary system plays for all other body systems.

For all systems, the urinary system excretes wastes collected from blood and maintains normal body fluid pH and ion composition.

Renin

Hormone secreted by the kidney; it raises blood pressure by influencing vasoconstriction (narrowing of blood vessels). Factors that stimulate renin release: 1- Sympathetic Nervous System 2- Low Blood Volume 3- Low Blood Pressure Action of Renin: Angiotensinogen (Plasma Protein) > Renin > Angiotensin I Angiotensin 1 > Angiotensin Converting Enzyme (ACE) > Angiotensin II (Biologically Active) Actions of Angiotensin II: Vasoconstriction - Blood Pressure Increase Aldosterone Release - Hormone (Conserve Sodium)

Hydrogen Ion Secretion

Hydrogen ion (H+) secretion is also associated with the reabsorption of sodium ions (Na+). Both involve the generation of carbonic acid (H2CO3) catalyzed by the enzyme carbonic anhydrase. Hydrogen ions generated by the dissociation of the carbonic acid are secreted by sodium ion-linked countertransport in exchange for sodium ions in the tubular fluid. The bicarbonate ions (HCO3−) diffuse into the peritubular fluid and then into the bloodstream. There they help prevent changes in plasma pH. Hydrogen ion secretion acidifies the tubular fluid while increasing the pH of the blood. Hydrogen ion secretion speeds up when the pH of the blood decreases. This can happen in metabolic acidosis (referred to as lactic acidosis), which can develop after exhaustive muscle activity, or ketoacidosis, which can develop in starvation or diabetes mellitus. The combination of hydrogen ion removal and bicarbonate ion production by the kidneys plays an important role in the control of blood pH. Because one of the secretory pathways is aldosterone sensitive, aldosterone stimulates hydrogen ion secretion. Prolonged aldosterone stimulation can cause alkalosis, or abnormally high blood pH.

What effect would a decrease in the sodium ion concentration of filtrate have on the pH of tubular fluid?

If the concentration of Na+ in the filtrate decreased, fewer hydrogen ions could be secreted by the counter transport mechanism involving these two ions. As a result, the pH of the tubular fluid would increase.

Osmolarity

Important term in describing the composition of body fluids. Total concentration of all solute particles in a solution. Number of dissolved solutes / volume (liter) = milliosmols / liter (mOsm) Normal plasma osmolarity = 300 mOsm / liter Measuring Osmotic Concentration The osmotic concentration, or osmolarity, of a solution is the total number of solute particles in each liter p. 94. We usually express osmolarity in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L). If a liter of a fluid contains 1 mole of dissolved particles, the solute concentration is 1 Osm/L, or 1000 mOsm/L. Body fluids have an osmotic concentration of about 300 mOsm/L. For comparison, seawater is about 1000 mOsm/L and fresh water is about 5 mOsm/L. In a clinical setting, the osmolarity of urine is often reported as osmolality, expressed as milliosmoles per kilogram of water (mOsm/kg H2O). When discussing osmotic concentrations in body fluids, these terms are often used interchangeably (1 liter of water has a mass of 1 kilogram). Ion concentrations are often reported in milliequivalents per liter (mEq/L). Milliequivalents indicate the number of positive or negative charges in solution, rather than the number of solute particles. To convert mmol/L to mEq/L, multiply mmol/L by the number of charges on each ion. For example, each sodium ion has one charge only, so for Na+, 1 mmol/L = 1 mEq/L; for Ca2+, each ion has two charges, so 1 mmol/L = 2 mEq/L. The concentrations of large organic molecules are usually reported in grams, milligrams, or micrograms per unit volume of solution (typically, per deciliter, or dL).

Potassium Ion Secretion

In effect, tubular cells trade sodium ions (Na+) in the tubular fluid for excess potassium ions in body fluids. Potassium ions are removed from the peritubular fluid in exchange for sodium ions from the tubular fluid. These potassium ions diffuse into the lumen of the DCT through potassium ion leak channels in the apical surfaces of the tubular cells.

What effect would an increased amount of aldosterone have on the potassium ion concentration in urine?

Increased amounts of aldosterone, which promotes Na+ retention and K+ secretion by the kidneys, would increase the K+ concentration of urine.

Pyelogram

Is an image of the urinary system. It is obtained by taking an x-ray of the kidneys after a radiopaque dye has been administered intravenously. Such an image provides an orientation to the relative sizes and positions of the main structures. Note that the sizes of the minor and major calyces, the renal pelvis, the ureters, the urinary bladder, and the proximal portion of the urethra are somewhat variable. These regions are lined by a transitional epithelium that can tolerate cycles of distension and relaxation without damage.

Urinary Obstruction

Local blockages of the collecting ducts or ureters can result from casts—small blood clots, epithelial cells, lipids, or other materials—that form in the collecting ducts. Casts are commonly eliminated in urine. They are visible in microscopic analyses of urine samples. Renal calculi, or kidney stones, form within the urinary tract from calcium deposits, magnesium salts, or crystals of uric acid. The condition is called nephrolithiasis; nephros, kidney; lithos, stone). The blockage of the ureter by a stone or by other means (such as external compression) creates urinary obstruction. This problem is an emergency. In addition to causing extreme pain, it decreases or prevents filtration in the affected kidney by increasing the capsular hydrostatic pressure. Calculi are generally visible on an x-ray. If urinary peristalsis and fluid pressures cannot dislodge them, they must be either surgically removed or destroyed. One nonsurgical procedure involves disintegrating the stones with a lithotripter, a device originally developed from machines used to de-ice airplane wings. Lithotripters focus sound waves on the stones, breaking them into smaller fragments that can be passed in the urine. Another nonsurgical approach is the insertion of a catheter armed with a laser that can shatter calculi with intense light beams.

Kidney Location

Located behind (posterior) to peritoneal lining of abdominal cavity. Retroperitoneal. Superior Lumbar Region On either side of the vertebral column, between vertebrae T12 and L3. Left kidney lies slightly superior to the right kidney. The right kidney is slightly inferior due to the position of the right lobe of the liver. The superior surface of each kidney is capped by an adrenal gland. The kidneys and adrenal glands lie between the muscles of the posterior body wall and the parietal peritoneum, in a retroperitoneal position.

Autonomic Regulation of GFR

Most of the autonomic innervation of the kidneys consists of sympathetic postganglionic fibers. Sympathetic activation has a direct effect on the GFR. It produces powerful vasoconstriction of afferent glomerular arterioles, which decreases the GFR and slows the production of filtrate. In this way, the sympathetic activation triggered by an acute decrease in blood pressure or a heart attack overrides the local regulatory mechanisms that act to stabilize the GFR. As the crisis passes and sympathetic tone decreases, the filtration rate gradually returns to normal. When the sympathetic division alters regional patterns of blood circulation, blood flow to the kidneys is often affected. For example, the dilation of superficial dermal blood vessels in warm weather shunts blood away from the kidneys. As a result, glomerular filtration decreases temporarily. The effect becomes especially pronounced during strenuous exercise. As the blood flow to your skin and skeletal muscles increases, kidney perfusion gradually decreases.

Glomerular Filtration

Movement of substances from glomerus > bowman's capsule. Composition of filtrate entering bowman's capsule: No: Proteins + Blood Cells Yes: Waste (Urea) H2O Salts (Ions) Gases Glucose Amino Acids Vitamins Drugs Hormones Ketones To enter capsular space, substances in glomerulus must pass through: 1. Glomerular capillary wall (fenestrated) 2. Dense layer 3. Slits through bowman's capsule visceral epithelium (podocytes) In glomerular filtration, blood plasma is forced through the pores of the specialized filtration membrane, and solute molecules small enough to pass through those pores are carried along. The filtration membrane restricts the passage of larger solutes and suspended materials from the plasma, producing filtrate. The primary factor involved in glomerular filtration is basically the same one that regulates fluid and solute movement across capillaries throughout the body. This factor is the balance between hydrostatic pressure, or fluid pressure, and colloid osmotic pressure, or pressure due to materials in solution, on each side of the capillary walls.

Reabsorption & Secretion

Movement of substances from lumen of nephron to blood. The processes of reabsorption and secretion alter the composition of the filtrate produced by glomerular filtration. Tubular reabsorption returns nutrients, such as glucose, amino acids, water, and salt, from the tubular fluid to the blood. Tubular secretion is the reverse of reabsorption: it adds substances from the blood to the tubular fluid.

Nephrons

Nephrons from different locations within a kidney differ slightly in structure. Approximately 85 percent of all nephrons are cortical nephrons, located almost entirely within the superficial cortex of the kidney. In a cortical nephron, the nephron loop is relatively short, and the efferent arteriole delivers blood to a network of peritubular capillaries, which surround the entire renal tubule. These capillaries drain into small venules that carry blood to the cortical radiate veins. The remaining 15 percent of nephrons, termed juxtamedullary nephrons, have long nephron loops that extend deep into the renal pyramids of the medulla). The efferent arterioles of juxtamedullary nephrons connect to the vasa recta, long, straight capillaries that parallel the nephron loop.

Blood Supply to the Nephron

No gas exchange. Not typical capillaries. Capillary bed between 2 arterioles.

Urea

Normally, the maximum solute concentration of the peritubular fluid near the turn of the nephron loop is about 1200 mOsm/L. Sodium and chloride ions pumped out of the loop's thick ascending limb make up about two-thirds of that gradient (750 mOsm/L). The rest of the medullary osmotic gradient results from the presence of urea. The thick ascending limb of the nephron loop, the DCT, and the collecting ducts are all impermeable to urea. As water is reabsorbed, the concentration of urea gradually increases in the tubular fluid. When the tubular fluid reaches the papillary duct, it typically contains urea at a concentration of about 450 mOsm/L. The papillary ducts are permeable to urea, so the urea concentration in the deepest parts of the medulla also averages 450 mOsm/L.

Renal Tubule

PCT, Nephron Loop, DCT 50 mm (1.97 in.) in length. This tubule has two convoluted (coiled or twisted) segments—the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT). They are separated by a simple U-shaped tube, the nephron loop, also called the loop of Henle (HEN-lē). The convoluted segments lie in the cortex of the kidney, and the nephron loop dips at least partially into the medulla. As the filtrate travels along the renal tubule, it is now called tubular fluid, and it gradually changes in composition. This is because the main functions of the renal tubule are reabsorption and secretion. When a substance is reabsorbed, it is "reclaimed," eventually reentering the blood. When a substance is secreted, it enters the tubular fluid from the blood. The changes that take place in the tubular fluid, and the characteristics of the urine that result, are due to the activities under way in each segment of the nephron. Functions: 1 - Reabsorbing all the useful organic nutrients in the filtrate 2 - Reabsorbing more than 90 percent of the water in the filtrate 3 - Secreting into the tubule lumen any wastes that did not pass into the filtrate at the glomerulus.

Blood Supply in Juxtamedullary Nephrons

Peritubular capilaries branch to form long loops of capillaries parallel loop to henle.

Why don't plasma proteins pass into the capsular space under normal circumstances?

Plasma proteins are too large to pass through the pores of the glomerular capillaries; only the smallest plasma proteins can pass between the foot processes of the podocytes.

Urine Formation

Production of urine purpose: excretion of wastes (urea) regulate volume + composition of blood 3 Processes involved in forming urine: 1. Filtration: Movement of fluid + solutes from blood in glomerulus to lumen of bowman's capsule. In filtration, blood pressure forces water and solutes across the walls of the glomerular capillaries and into the capsular space. Solute molecules small enough to pass through the filtration membrane are carried by the surrounding water molecules. 2. Reabsorption: Movement of substances from lumen of nephron back to blood in peritubular capillaries. Reabsorption is the removal of water and solutes from the filtrate, and their movement across the tubular epithelium and into the peritubular fluid, the interstitial fluid surrounding the renal tubule. Reabsorption takes place after the filtrate has left the renal corpuscle. Most of the reabsorbed substances are nutrients the body can use. Filtration is based solely on particle size, but reabsorption is a selective process. Reabsorption involves either simple diffusion or carrier proteins in the tubular epithelium. The reabsorbed substances in the peritubular fluid eventually reenter the blood. Water reabsorption takes place passively, through osmosis. 3. Secretion: Movement of substances from peritubular capillaries to nephron. Secretion is the transport of solutes from the peritubular fluid, across the tubular epithelium, and into the tubular fluid. Secretion is necessary because filtration does not force all the dissolved substances out of the plasma. Tubular secretion, which removes substances from the blood, can further lower the plasma concentration of undesirable materials. It provides a backup process for filtration. Secretion is often the primary method of preparing substances, including many drugs, for excretion.

Reabsorbed Substances

Reabsorbed by the renal tubules Ions: Na+ Cl- K+ Ca2+ Mg2+ SO42- HCO3- Metabolites: Glucose Amino Acids Proteins Vitamins

Transport Maximum and the Renal Threshold

Recall that when an enzyme is saturated, further increases in substrate concentration have no effect on the rate of reaction. Likewise, when a carrier protein is saturated, further increases in substrate concentration have no effect on the rate of transport across the plasma membrane. For any substance, the concentration at saturation is called the transport maximum (Tm), or tubular maximum. The Tm reflects the number of available carrier proteins in the renal tubules. In healthy people, carrier proteins involved in tubular secretion seldom become saturated. However, carriers involved in tubular reabsorption are often at risk of saturation, especially during the absorptive state following a meal. When the carrier proteins are saturated, excesses of that substrate are excreted. Normally, any plasma proteins and nutrients, such as amino acids and glucose, are removed from the tubular fluid by cotransport or facilitated diffusion. If the concentrations of these nutrients rise in the tubular fluid, the rate of reabsorption increases until the carrier proteins are saturated. A concentration higher than the transport maximum exceeds the reabsorptive abilities of the nephron. In this case, some of the material will remain in the tubular fluid and appear in the urine. The transport maximum thus determines the renal threshold—the plasma concentration at which a specific substance or ion begins to appear in the urine. The renal threshold varies with the substance involved. The renal threshold for glucose is approximately 180 mg/dL. When the plasma glucose concentration exceeds 180 mg/dL, glucose appears in urine; this condition is called glucosuria or glycosuria. After you have eaten a meal rich in carbohydrates, your plasma glucose level may exceed the glucose Tm for a brief period. However, the liver quickly lowers the circulating glucose level, and very little glucose is lost in your urine. Chronically increased plasma and urinary glucose concentrations are abnormal. The renal threshold for amino acids is lower than that for glucose. Amino acids appear in urine when their plasma concentrations exceed 65 mg/dL. Plasma amino acid levels commonly exceed the renal threshold after you have eaten a protein-rich meal, causing some amino acids to appear in your urine. This condition is termed aminoaciduria. The Tm values for water-soluble vitamins are relatively low. As a result, you excrete excess quantities of these vitamins in urine. (This is typically the fate of water-soluble vitamins in daily supplements.)

Kidney Functional Unit: The Nephron

Renal Tubule Where urine is made. 2 inches long/1.25 million per kidney 2 Types: 1- Cortical Nephrons (Renal Cortex) Bowman's Capsule Short loops of Henle 2- Juxtamedullary Nephrons (Renal Medulla) Bowman's Capsule is next to medulla. Long loops of Henle Allow you to make high solutes Papillary Duct (Papillae of Pyramids) Minor Calyx Major Calyx Renal Pelvis Ureter Urinary Bladder Urethra Toilet In the kidneys, the functional units—the smallest structures that can carry out all the functions of the system—are the nephrons. Each nephron consists of a renal corpuscle and a renal tubule. The renal corpuscle is a spherical structure containing a capillary network that filters blood. The renal tubule, a long tubular passageway, begins at the renal corpuscle. Each renal tubule empties into the collecting system, a series of tubes that carry tubular fluid away from the nephron.

Secreted Substances

Secreted into tubular fluid by the proximal and distal convoluted tubules. Ions: K+ H+ Ca2+ PO43- Wastes: Creatinine Ammonia Metabolic Acids and Bases Miscellaneous: Neurotransmitters - ACh, NE, E, Dopamine Histamine Drugs - Penicillin, atropine, morphine

Hormonal Regulation of GFR

The GFR is regulated by the hormones of the renin- angiotensin-aldosterone system (RAAS) and the natriuretic peptides. he release of renin ultimately restricts water and salt loss in the urine by stimulating reabsorption by the nephron. There are three triggers for the release of renin by the juxtaglomerular complex (JGC): 1 - A decrease in blood pressure at the glomerulus as the result of a decrease in blood volume, a decrease in systemic pressures, or a blockage in the renal artery or its branches. 2 - Stimulation of juxtaglomerular cells by sympathetic innervation. 3 - A decrease in the osmotic concentration of the tubular fluid at the macula densa. These triggers are often interrelated. For example, a decrease in systemic blood pressure reduces the glomerular filtration rate, while baroreceptor reflexes cause sympathetic activation. Meanwhile, a decrease in the GFR slows the movement of tubular fluid along the nephron. As a result, the tubular fluid is in the ascending limb of the nephron loop longer, and the concentration of sodium and chloride ions in the tubular fluid reaching the macula densa and DCT becomes abnormally low. A decrease in GFR leads to the release of renin by the juxtaglomerular complex. Renin converts the inactive plasma protein angiotensinogen to angiotensin I. Angiotensin I is also inactive but is then converted to angiotensin II by angiotensin-converting enzyme (ACE). This conversion takes place primarily in the capillaries of the lungs. Angiotensin II acts at the nephron, adrenal glands, and in the CNS. Angiotensin II stimulates contraction of vascular smooth muscle in peripheral capillary beds, promotes aldosterone production, and stimulates the sympathetic nervous system to increase arterial pressures throughout the body. The combined effect is an increase in systemic blood pressure and blood volume and the restoration of normal GFR. If blood volume increases, the GFR increases automatically. This increase promotes fluid losses that help return blood volume to the normal level. If the increase in blood volume is severe, hormonal factors further increase the GFR and speed up fluid losses in the urine. The heart releases natriuretic peptides when increased blood volume or blood pressure stretches the walls of the heart. For example, the atria release atrial natriuretic peptide (ANP). Among its other effects, this hormone triggers the dilation of afferent glomerular arterioles and the constriction of efferent glomerular arterioles. This mechanism increases glomerular pressures and increases the GFR. ANP also decreases sodium reabsorption at the renal tubules. The net result is increased urine production and decreased blood volume and pressure.

Nephron Loop (Loop of Henle)

The PCT makes an acute bend that turns the renal tubule toward the renal medulla. This turn leads to the nephron loop. It is divided into a descending limb and an ascending limb. Fluid in the descending limb flows toward the renal pelvis. Fluid in the ascending limb flows toward the renal cortex. The limbs contain thin and thick segments: the descending thin limb (DTL), the ascending thin limb (ATL), and the thick ascending limb (TAL). The terms thick and thin refer to the height of the epithelium, not to the diameter of the lumen. Thick limbs have cuboidal epithelium. Thin limbs are lined with squamous epithelium. Squamous or cuboidal cells. 30 mm in length 15 µm Diameter - Descending Limb - Reabsorption of water from tubular fluid. 30 µm Diameter - Ascending Limb - Reabsorption of ions; assists in creation of a concentration gradient in the renal medulla.

The ability to control the urinary reflexes depends on your ability to control which muscle?

The ability to control the urinary reflexes depend on the ability to control the external urethral sphincter, a ring of skeletal muscle, which acts as a valve.

Colloid Osmotic Pressure

The blood colloid osmotic pressure (BCOP) is the osmotic pressure resulting from suspended proteins in the blood. Under normal conditions, very few plasma proteins enter the capsular space, so no opposing colloid osmotic pressure exists within the capsule. However, if the glomeruli are damaged by disease or injury, and plasma proteins begin passing into the capsular space, a capsular colloid osmotic pressure is created that promotes filtration and increases fluid losses in urine.

Secretion of the Distal Convoluted Tubule

The blood entering the peritubular capillaries still contains a number of potentially undesirable substances that did not cross the filtration membrane at the glomerulus. In most cases, the concentrations of these substances are too low to cause physiological problems. However, any ions or compounds in peritubular capillaries will diffuse into the peritubular fluid. If those concentrations become too high, the tubular cells may absorb these substances from the peritubular fluid and secrete them into the tubular fluid. The rate of potassium and hydrogen ion secretion increases or decreases in direct response to changes in their concentrations in peritubular fluid. The higher their concentrations in the peritubular fluid, the higher the rate of secretion. Potassium and hydrogen ions merit special attention, because their concentrations in body fluids must be maintained within narrow limits.

Reabsorption of the Collecting System

The collecting ducts receive tubular fluid from many nephrons and carry it toward the renal sinus. The normal amount of water and solute loss in the collecting system is hormonally regulated: By aldosterone, which controls sodium ion pumps along most of the DCT and the proximal portion of the collecting system. As we have noted, these actions are opposed by atrial natriuretic peptide (ANP). By ADH, which controls the permeability of the DCT and collecting system to water. The secretion of ADH is suppressed by ANP, and this—combined with the effects of atrial natriuretic peptide on aldosterone secretion and action—can dramatically increase urinary water losses. The collecting system also has other reabsorptive and secretory functions. Many of them are important to the control of body fluid pH. For example, Type A intercalated cells secrete hydrogen ions and reabsorb bicarbonate ions, while Type B intercalated cells secrete bicarbonate ions and reabsorb hydrogen ions. Principal cells reabsorb water and secrete potassium ions. The collecting system reabsorbs sodium ions, bicarbonate ions, and urea as follows: Sodium Ion Reabsorption. The collecting system contains aldosterone-sensitive ion pumps that exchange sodium ions (Na+) in tubular fluid for potassium ions (K+) in peritubular fluid. Bicarbonate Ion Reabsorption. Bicarbonate ions (HCO3-) are reabsorbed in exchange for chloride ions (Cl−) in the peritubular fluid. Urea Reabsorption. The concentration of urea in the tubular fluid entering the collecting duct is relatively high. The fluid entering the papillary duct generally has the same osmotic concentration as that of interstitial fluid of the medulla—about 1200 mOsm/L—but contains a much higher concentration of urea. As a result, urea tends to diffuse out of the tubular fluid and into the peritubular fluid in the deepest portion of the medulla.

Secretion at the Collecting System

The collecting system is important in controlling the pH of body fluids through the secretion of hydrogen or bicarbonate ions. If the pH of the peritubular fluid decreases, carrier proteins pump hydrogen ions into the tubular fluid and reabsorb bicarbonate ions that help restore normal pH. If the pH of the peritubular fluid rises (a much less common event), the collecting system secretes bicarbonate ions and pumps hydrogen ions into the peritubular fluid. The net result is that the body eliminates a buffer and gains hydrogen ions that lower the pH.

Medullary Osmotic Gradient

The concentration gradient created in the peritubular fluid of the medulla is called the medullary osmotic gradient. Active transport at the apical surface moves sodium, potassium, and chloride ions out of the tubular fluid. The carrier is called a Na+ − K+/2 Cl− transporter, because each cycle of the pump carries a sodium ion, a potassium ion, and two chloride ions into the tubular cell. Then cotransport carriers pump potassium and chloride ions into the peritubular fluid. However, potassium ions are removed from the peritubular fluid as the sodium-potassium exchange pump moves sodium ions out of the tubular cell. The potassium ions then diffuse back into the lumen of the tubule through potassium ion leak channels. The net result is that sodium and chloride ions enter the peritubular fluid of the renal medulla. The removal of sodium and chloride ions from the tubular fluid in the ascending limb raises the osmotic concentration of the peritubular fluid around the descending thin limb. Recall that the descending thin limb is permeable to water but not to solutes. As tubular fluid travels deeper into the medulla within the descending thin limb, osmosis moves water into the peritubular fluid. Solutes remain behind. As a result, the tubular fluid at the turn of the nephron loop has a higher osmotic concentration than it did at the start.

Collecting System

The distal convoluted tubule, the last segment of the nephron, opens into the collecting system. Many individual nephrons drain their tubular fluid into a nearby collecting duct. Each collecting duct begins in the cortex and descends into the medulla. Several collecting ducts then converge into a larger papillary duct, which in turn empties into a minor calyx. The epithelium lining the papillary duct is typically columnar. Two main types of cells are found in the collecting duct: intercalated cells and principal cells. Intercalated cells are cuboidal cells with microvilli. Type A intercalated cells and Type B intercalated cells make up the population of intercalated cells. Together, these cells regulate the acid-base balance in the blood. Principal cells are cuboidal cells that lack microvilli and reabsorb water and secrete potassium ions. The collecting system transports tubular fluid from the nephrons to the renal pelvis. Along the way, it also adjusts the fluid's composition and determines the final osmotic concentration and volume of urine, the final product. Collecting Duct Cuboidal cells 15 mm in length 50-100 µm diameter Reabsorption of water, sodium, ions; secretion or reabsorption of bicarbonate ions or hydrogen ions. Papillary Duct Columnar cells 5 mm in length 100-200 µm in diameter Conduction of tubular fluid to minor calyx; contributes to concentration gradient of the medulla.

Urination (Micturition)

The elimination of urine when the bladder is full. The urinary bladder contracts and forces urine through the urethra, which conducts the urine to the exterior.

List the factors that influence net filtration pressure.

The factors that influence net filtration pressure are net hydrostatic pressure and blood colloid osmotic pressure.

List the factors that influence the rate of filtrate formation.

The factors that influence the rate of filtrate formation are the filtration pressure across glomerular capillaries, plus interactions among the responses by autoregulation, hormonal regulation (endocrine), and autonomic regulation (neural).

Filtration Membrane

The fenestrated endothelium, the basement membrane, and the foot processes of podocytes form the filtration membrane. During filtration, which takes place in the renal corpuscle, blood pressure forces water and small dissolved solutes out of the glomerular capillaries through this membrane and into the capsular space. The larger solutes, especially plasma proteins, do not pass through. Filtration produces an essentially protein-free solution, known as a filtrate, that is otherwise similar to blood plasma. From the renal corpuscle, filtrate enters the renal tubule.

Kidney Internal Anatomy

The fibrous capsule covering the outer surface of the kidney also lines the renal sinus, an internal cavity within the kidney. The fibrous capsule is bound to the outer surfaces of the structures within the renal sinus. In this way, it stabilizes the positions of the ureter, renal blood vessels, and nerves. The kidney has an outer cortex and an inner medulla. Renal Cortex: The superficial region of the kidney, in contact with the fibrous capsule. The cortex is reddish-brown and granular. Produces urine. Renal Medulla: Consists of 6 to 18 distinct triangular structures called renal pyramids. The base of each pyramid touches the cortex. The tip of each pyramid—a region known as the renal papilla—projects into the renal sinus. Each pyramid has a series of fine grooves that converge at the papilla. Bands of cortical tissue called renal columns extend into the medulla and separate adjacent renal pyramids. The columns have a distinctly granular texture, similar to that of the cortex. Produces urine. Kidney Lobe: Consists of a renal pyramid, the overlying area of renal cortex, and adjacent tissues of the renal columns. Minor Calyx: Ducts within each renal papilla discharge urine into a cup-shaped drain called the Minor Calyx Major Calyx: Four or five minor calyces merge to form a major calyx, and two or three major calyces combine to form the renal pelvis, a large, funnel-shaped chamber. The renal pelvis fills most of the renal sinus and is connected to the ureter, which drains the kidney.

Proximal Convoluted Tubule (PCT)

The first segment of a renal tubule. Its entrance lies almost directly opposite the point where the afferent and efferent arterioles connect to the glomerulus. The lining of the PCT is a simple cuboidal epithelium whose apical surfaces have microvilli. Reabsorption of critical ions is the primary function of the PCT. Cuboidal cells with microvilli 14mm in length 60 µm in diameter Reabsorption of ions, organic molecules, vitamins, water; secretion of drugs, toxins, and acids

Incontinence

The inability to control urination voluntarily. Trauma to the internal or external urethral sphincter can contribute to incontinence in otherwise healthy adults. For example, some mothers develop stress urinary incontinence (SUI) if childbirth overstretches and damages the sphincter muscles. In this condition, increased intra-abdominal pressures—caused, for example, by a cough or sneeze—can overwhelm the sphincter muscles, causing urine to leak out. Incontinence can also develop in older people due to a general loss of muscle tone. Damage to the central nervous system, the spinal cord, or the nerve supply to the urinary bladder or external urethral sphincter can also produce incontinence. For example, incontinence commonly accompanies Alzheimer's disease or spinal cord damage. In most cases, the affected individual develops an automatic bladder. The urinary reflexes remain intact, but voluntary control of the external urethral sphincter is lost, so the person cannot prevent the reflexive emptying of the urinary bladder. Damage to the pelvic nerves of the sacral spinal cord can destroy the urinary reflexes entirely, because those nerves carry both afferent and efferent fibers of both reflex arcs. The urinary bladder then becomes greatly distended with urine. It remains filled to capacity while the excess urine flows into the urethra in an uncontrolled stream. The insertion of a catheter is often needed to facilitate the discharge of urine.

Countercurrent Multiplication

The kidneys couple two countercurrent mechanisms to establish the conditions necessary to regulate the volume and concentration of urine. Countercurrent refers to the fact that the exchange takes place between fluids moving in opposite directions in two adjacent segments of the same tube. These two mechanisms are a countercurrent multiplier (filtrate flow in the nephron loop) and a countercurrent exchanger (blood flow in the vasa recta). These mechanisms establish and maintain an increasing osmotic gradient from the renal cortex through the medulla. The descending thin limb and the thick ascending limb of the nephron loop lie very close together. They are separated only by the peritubular fluid, which surrounds all the nephrons. The exchange of substances between these segments of a nephron is called countercurrent multiplication. Tubular fluid in the descending limb flows toward the renal pelvis, while tubular fluid in the ascending limb flows toward the cortex. Multiplication refers to the fact that the effect of the exchange increases as movement of the fluid continues. Countercurrent multiplication performs two functions: It efficiently reabsorbs solutes and water before the tubular fluid reaches the DCT and collecting system. It establishes a concentration gradient in the peritubular fluid (called the medullary osmotic gradient) that permits the passive reabsorption of water from the tubular fluid in the collecting system. This allows the production of concentrated urine. The circulating level of antidiuretic hormone (ADH) regulates water reabsorption. Sodium ions (Na+) and chloride ions (Cl−) are pumped out of the thick ascending limb and into the peritubular fluid. This dilutes the tubular fluid. The pumping action increases the osmotic concentration in the peritubular fluid around the descending thin limb. This creates a small concentration difference between the tubular fluid and peritubular fluid in the renal medulla. The concentration difference results in an osmotic flow of water out of the descending thin limb and into the peritubular fluid. As a result, the solute concentration of the tubular fluid in the descending thin limb increases. he arrival of the highly concentrated tubular fluid in the thick ascending limb speeds up the transport of sodium and chloride ions into the peritubular fluid. Solute pumping out of the tubular fluid in the thick ascending limb leads to higher solute concentrations in the tubular fluid in the descending thin limb, which then brings about increased solute pumping in the thick ascending limb. Notice that this process is a simple positive feedback loop that multiplies the concentration difference between the hypotonic tubular fluid in the thick ascending limb and the hypertonic peritubular fluid in the renal medulla.

Net Filtration Pressure

The net filtration pressure (NFP) in the glomerulus is the difference between the net hydrostatic pressure and the blood colloid osmotic pressure acting across the glomerular capillaries. Under normal circumstances, we can summarize this relationship as NFP= NHP−BCOP This is the average pressure forcing water and dissolved substances out of the glomerular capillaries and into the capsular space. Problems that affect the net filtration pressure can seriously disrupt kidney function and cause a variety of clinical signs and symptoms.

Glomerular Capsule and Glomerulus

The outer wall of the renal corpuscle is formed by the glomerular capsule. This structure encapsulates the glomerular capillaries and is continuous with the initial segment of the renal tubule. The glomerulus consists of about 50 intertwined capillaries. Blood is delivered to the glomerulus by an afferent arteriole, and leaves the glomerulus in an efferent arteriole. Capsular Outer Layer: The outer wall of the capsule is made up of a simple squamous epithelium and is called the capsular outer layer. This layer ends at the visceral layer, which covers the glomerular capillaries. Capsular Space: Urinary space separates the capsular outer layer and visceral layer. The two layers are continuous where the glomerular capillaries are connected to the afferent arteriole and efferent arteriole. Therefore this space is continuous with the lumen of the renal tubule. The visceral layer of the glomerulus consists of large cells with complex processes, or "feet," that wrap around the dense layer, a specialized basement membrane of the glomerular capillaries. These visceral cells of the capsule are called podocytes. Their feet are known as foot processes, or pedicels. There are narrow gaps called filtration slits between adjacent foot processes. These slits are only 6-9 nm wide. Intraglomerular Mesangial Cells: located among the glomerular capillaries. They are specialized cells derived from smooth muscle. Functions of these contractile cells include structural support, filtration, and phagocytosis. Their contraction decreases the luminal diameter of the capillaries. Their phagocytic activities help clear debris from the filtration membrane.

What parts of the glomerulus are involved in filtration?

The parts of the glomerulus involved in filtration are the glomerular capillaries, the basement membrane, and the foot processes of the podocytes.

Carrier-Mediated Mechanisms

The processes of reabsorption and secretion by the kidneys involve a combination of diffusion, osmosis, leak channels (channel-mediated diffusion), and carrier-mediated transport. 4 Major Types of Carrier-Mediated Transport Facilitated Diffusion - A carrier protein transports a molecule across the plasma membrane without expending energy. Such transport always follows the concentration gradient for the ion or molecule transported. Active Transport - Driven by the hydrolysis of ATP to ADP on the inner plasma membrane surface. Exchange pumps and other carrier proteins are active along the renal tubules. Active transport can operate despite an opposing concentration gradient. Cotransport - Carrier protein activity is not directly linked to the hydrolysis of ATP. Instead, two substrates (ions, molecules, or both) cross the membrane while bound to the carrier protein. The movement of the substrates always follows the concentration gradient of at least one of the transported substances. Cotransport is used for the reabsorption of organic and inorganic compounds from the tubular fluid. Countertransport - Resembles cotransport, except that the two transported ions move in oppositedirections. Countertransport operates in the PCT, DCT, and collecting system.

What structure connects the proximal convoluted tubule to the distal convoluted tubule?

The proximal convoluted tubule is connected to the distal convoluted tubule by the nephron loop, which consists of the descending thin limb (DTL), ascending thin limb (ATL), and the thick ascending limb (TAL).

Which parts of a nephron are in the renal cortex?

The renal corpuscle, proximal convoluted tubule, distal convoluted tubule, and the proximal part of the nephron loop and collecting duct are all in the renal cortex. (In a cortical nephron, most of the nephron loop is in the renal cortex; in a juxtamedullary nephron, most of the nephron loop is in the renal medulla.)

Kidney Associated Structures

The superior surface of each kidney is capped by an adrenal gland. The kidneys and adrenal glands lie between the muscles of the posterior body wall and the parietal peritoneum, in a retroperitoneal position. The organs located entirely or partially retroperitoneally are the Suprarenal (adrenal) glands, Aorta and inferior vena cava, Duodenum, Pancreas, Ureters, Colon, Kidneys, Esophagus, and Rectum. Kidneys are held in position within the abdominal cavity by: 1 - The overlying peritoneum. 2- Contact with adjacent visceral organs 3 - Supporting connective tissue

Distal Convoluted Tubule (DCT)

The thick ascending limb (TAL) of the nephron loop ends where it forms a sharp angle near the renal corpuscle. The distal convoluted tubule (DCT), the third segment of the renal tubule, begins there. The initial portion of the DCT passes between the afferent and efferent arterioles. In sectional view, the DCT differs from the PCT in that the DCT has a smaller luminal diameter and its epithelial cells lack microvilli. The primary function of the DCT is to reabsorb water and selected ions, as well as active secretion of undesirable substances. Cuboidal cells with few if any microvilli 5mm in length 30-50 µm Diameter Reabsorption of sodium ions and calcium ions; secretion of acids, ammonia, drugs and toxins.

Identify the three distinct processes that form urine in the kidney.

The three distinct processes that form urine in the kidney are filtration, reabsorption, and secretion.

Ureters

The ureters are a pair of muscular tubes that extend from the kidneys to the urinary bladder—a distance of about 30 cm (12 in.). Each ureter begins at the funnel-shaped renal pelvis. The ureters extend inferiorly and medially, passing over the anterior surfaces of the psoas major. The ureters are retroperitoneal and are firmly attached to the posterior abdominal wall. The paths taken by the ureters in men and women are different, due to variations in the nature, size, and position of the reproductive organs. In males, the base of the urinary bladder lies between the rectum and the pubic symphysis. In females, the base of the urinary bladder sits inferior to the uterus and anterior to the vagina. The ureters penetrate the posterior wall of the urinary bladder without entering the peritoneal cavity. They pass through the bladder wall at an oblique angle. The ureteric orifices are slit-like rather than rounded. This shape helps prevent the backflow of urine toward the ureter and kidneys when the urinary bladder contracts. The wall of each ureter consists of three layer: 1 - An inner mucosa, made up of a transitional epithelium and the surrounding lamina propria 2 - A middle muscular layer made up of longitudinal and circular bands of smooth muscle. 3 - An outer connective tissue layer that is continuous with the fibrous capsule and peritoneum. About every 30 seconds, a peristaltic contraction begins at the renal pelvis. As it sweeps along the ureter, it forces urine toward the urinary bladder.

Urethra

The urethra extends from the neck of the urinary bladder and transports urine to the exterior of the body. The male urethra and the female urethra differ in length and in function. In males, the urethra extends from the neck of the urinary bladder to the tip of the penis. This distance may be 18-20 cm (7-8 in.). The male urethra is subdivided into three segments: the prostatic urethra, the membranous urethra, and the spongy urethra. The prostatic urethra (intermediate part of urethra) passes through the center of the prostate. The membranous urethra includes the short segment that penetrates the deep transverse perineal muscle of the pelvic floor. The spongy urethra extends from the distal border of the deep transverse perineal muscle to the external opening, or external urethral orifice, at the tip of the penis. In females, the urethra is very short. It extends 3-5 cm (1-2 in.) from the bladder to the vestibule. The external urethral orifice is near the anterior wall of the vagina. Both sexes have a skeletal muscular band, called the external urethral sphincter, that acts as a valve. The external urethral sphincter is under voluntary control through the perineal branch of the pudendal nerve. This sphincter has a resting muscle tone and must be voluntarily relaxed to permit urination. The urethral lining consists of a stratified epithelium that varies from transitional epithelium at the neck of the urinary bladder, to stratified columnar epithelium at the midpoint, to stratified squamous epithelium near the external urethral orifice. The lamina propria is thick and elastic. The mucosa is folded into longitudinal creases. Mucus-secreting cells are located in the epithelial pockets. In males, the epithelial mucous glands may form tubules that extend into the lamina propria. Connective tissues of the lamina propria anchor the urethra to surrounding structures. In females, the lamina propria contains an extensive network of veins. Concentric layers of smooth muscle surround the entire complex.

Urinary Reflexes

The urge to urinate generally appears when your urinary bladder contains about 200 mL of urine. Whether or not we urinate depends on an interplay between spinal reflexes and higher centers in the brain that provide conscious control over urination. Two spinal reflexes control urination (micturition): the urine storage reflex and the urine voiding reflex.

Urinary Bladder

The urinary bladder is a hollow, muscular organ that serves as temporary storage for urine. The dimensions of the urinary bladder vary with its state of distension. A full urinary bladder can contain as much as a liter of urine. A layer of peritoneum covers the superior surfaces of the urinary bladder. Several peritoneal folds assist in stabilizing its position. The median umbilical ligament extends from the anterior, superior border toward the umbilicus (navel). The lateral umbilical ligaments pass along the sides of the bladder to the umbilicus. These fibrous cords are the vestiges of the two umbilical arteries, which supplied blood to the placenta during embryonic and fetal development. The urinary bladder's posterior, inferior, and anterior surfaces lie outside the peritoneal cavity. In these areas, tough ligamentous bands anchor the urinary bladder to the pelvic and pubic bones. In sectional view, the mucosa lining the urinary bladder usually has folds called rugae that disappear as the bladder fills. The triangular smooth area bounded by the openings of the ureters and the entrance to the urethra makes up a region called the trigone of the urinary bladder. There, the mucosa is smooth and very thick. The trigone acts as a funnel that channels urine into the urethra when the urinary bladder contracts. The urethral entrance lies at the apex of the trigone, at the most inferior point in the urinary bladder. The region surrounding the urethral opening is known as the neck of the urinary bladder. It contains a muscular internal urethral sphincter. The smooth muscle fibers of this sphincter provide involuntary control over the discharge of urine from the bladder. The urinary bladder is innervated by postganglionic fibers from ganglia in the hypogastric plexus and by parasympathetic fibers from intramural ganglia that are controlled by branches of the pelvic nerves. The wall of the urinary bladder contains mucosa, submucosa, and muscular layers. The muscular layer consists of internal and external layers of longitudinal smooth muscle, with a circular layer between the two. Together, these layers form the powerful detrusor, a muscle of the urinary bladder. When the detrusor contracts, it compresses the urinary bladder and expels urine into the urethra.

Body Systems Involved in Waste Excretion

The urinary system excretes wastes produced by other body systems, but it is not the only organ system involved in excretion. Indeed, the urinary, integumentary, respiratory, and digestive systems are together regarded as an anatomically diverse excretory system whose components perform all the excretory functions that affect the composition of body fluids: Integumentary System. Water losses and electrolyte losses in sensible perspiration can affect the volume and composition of the plasma. The effects are most apparent when losses are extreme, such as during peak sweat production. Small amounts of metabolic wastes, including urea, also are eliminated in perspiration. Respiratory System. The lungs remove the carbon dioxide generated by cells. Small amounts of other compounds, such as acetone and water, evaporate into the alveoli and are eliminated when you exhale. Digestive System. The liver excretes small amounts of metabolic waste products in bile. You lose a variable amount of water in feces.

No Transport Mechanism

Urea Water Urobilinogen Bilirubin

Metabolic Wastes

Urea: The most abundant organic waste. You generate approximately 21 g of urea each day, most of it through the breakdown of amino acids. Creatinine: Skeletal muscle tissue generates creatinine through the breakdown of creatine phosphate, a high-energy compound that plays an important role in muscle contraction. Your body generates about 1.8 g of creatinine each day. Virtually all of it is excreted in urine. Uric Acid: A waste formed during the recycling of the nitrogenous bases from RNA molecules. You produce approximately 480 mg of uric acid each day. These wastes are dissolved in the bloodstream. They can be eliminated only when dissolved in urine. For this reason, their removal involves an unavoidable water loss. The kidneys are usually capable of producing urine with an osmotic concentration more than four times that of plasma. If the kidneys were unable to concentrate the filtrate produced by filtration, fluid losses would lead to fatal dehydration in a matter of hours. The kidneys also ensure that the fluid that is lost does not contain potentially useful substances that are present in blood plasma, such as sugars or amino acids. These valuable materials must be reabsorbed and retained for use by other tissues.

Urinary Tract

Ureters, Urinary Bladder, Urethra Transports, stores, and eliminates urine.

Urinalysis

Urinalysis is the chemical and physical analysis of a urine sample. It is an important diagnostic tool, even in high-technology medicine. A standard urinalysis includes an assessment of the color and appearance of urine. These two characteristics can be determined without specialized equipment, but quantitative analytical tests are also used. Normal urine is a clear, sterile liquid. Its yellow color comes from the pigment urobilin. The kidneys generate this pigment from the urobilinogens produced by intestinal bacteria and absorbed in the colon. The characteristic odor of urine is due to the evaporation of small molecules, such as ammonia. Other substances not normally present, such as acetone or other ketone bodies, can also impart a distinctive smell.

Urine Composition

Urine is the fluid and dissolved substances excreted by the kidney. The analysis of that urine by physical, chemical, and microscopic means is called urinalysis. pH: 4.5 - 8 (Average = 6) Specific Gravity: 1.003 - 1.030 Osmotic Concentration (Osmolarity): 855 - 1335 mOsm/L Water Content: 93% - 97% Volume: 700 - 2000 mL/day Color: Pale Yellow Clarity: Clear Odor: Varies with Composition Bacterial Content: None (Sterile) The composition and concentration of urine are two related but distinct properties. The composition of urine reflects the filtration, reabsorption, and secretion activities of the nephrons. Some substances (such as urea) are neither actively excreted nor reabsorbed along the nephron. In contrast, organic nutrients are completely reabsorbed. Other substances, such as creatinine, are missed by filtration but are actively secreted into the tubular fluid. Filtration, reabsorption, and secretion determine the kinds and amounts of substances excreted in urine. The concentration of these substances in a given urine sample depends on the osmotic movement of water across the walls of the tubules and collecting ducts. Because the composition and concentration of urine vary independently, you can produce a small volume of concentrated urine or a large volume of dilute urine and still excrete the same amount of dissolved substances. For this reason, health care professionals who are interested in a detailed assessment of renal function commonly analyze the urine produced over a 24-hour period rather than a single urine sample.

Urine Storage Reflex

Urine storage occurs by spinal reflexes and the pontine storage center. When urine is being stored, afferent impulses from stretch receptors in the urinary bladder stimulate sympathetic outflow to the detrusor and internal urethral sphincter, inhibiting contraction of the detrusor and stimulating contraction of the internal urethral sphincter. The pontine storage center inhibits urination by decreasing parasympathetic activity and increasing somatic motor nerve activity at the external urethral sphincter. Urine storage (continence) occurs as urine is unable to pass through the urethra.

Urine Voiding Reflex

Urine voiding occurs by spinal reflexes and the pontine micturition center. The voiding reflex begins when afferent impulses from stretch receptors in the bladder stimulate interneurons that relay sensations to the pontine micturition center. This center initiates sacral spinal reflexes that (1) stimulate increased parasympathetic activity (detrusor contracts and internal urethral sphincter relaxes) (2) decrease sympathetic activity (detrusor contracts and internal urethral sphincter relaxes), and (3) decrease efferent somatic motor nerve activity (external urethral sphincter relaxes). Voiding (urination) occurs as urine passes through the urethra.

Regulation of Urine Volume and Osmotic Concentration

Urine volume and osmotic concentration are regulated through the control of water reabsorption. Water is reabsorbed by osmosis along the proximal convoluted tubule and the descending limb of the nephron loop. The water permeabilities of these regions cannot be adjusted. As a result, water reabsorption takes place wherever the osmotic concentration of the peritubular fluid is greater than that of the tubular fluid. The ascending limb of the nephron loop is impermeable to water. In the distal convoluted tubule and collecting system, 1-2 percent of the volume of water in the original filtrate is recovered during sodium ion reabsorption. All these water movements represent obligatory water reabsorption because they cannot be prevented. This reabsorption usually recovers 85 percent of the volume of filtrate. The volume of water lost in urine depends on how much of the remaining water in the tubular fluid is reabsorbed along the DCT and collecting system. (This remaining water represents 15 percent of the filtrate volume, or approximately 27 liters per day.) The amount reabsorbed can be precisely controlled by a process called facultative water reabsorption. Precise control is possible because these segments are relatively impermeable to water except when ADH is present. This hormone causes special water channels, or aquaporins, to be inserted into the apical plasma membranes. These water channels dramatically enhance the rate of osmotic water movement. The higher the circulating level of ADH, the greater the number of water channels, and the greater the water permeability of these segments.

Glomerular Filtration Rate (GFR)

Volume of filtrate formed by all nephrons of both kidneys per minute. GFR = 125 mL/min Urine Formation Rate = 1mL/min Each kidney contains about 6 m2—some 64 square feet—of filtration surface, and the GFR averages an astounding 125 mL per minute. This means that nearly 10 percent of the fluid delivered to the kidneys by the renal arteries leaves the bloodstream and enters the capsular spaces. In the course of a single day, the glomeruli generate about 180 liters (48 gal) of filtrate, approximately 70 times the total plasma volume. The glomerular filtration rate depends on the net filtration pressure across the glomerular capillaries. Any factor that alters the net filtration pressure also alters the GFR and affects kidney function. One of the most significant factors would be a decrease in renal blood pressure. If blood pressure at the glomeruli decreases by 20 percent (from 50 mm Hg to 40 mm Hg), kidney filtration stops, because the net filtration pressure falls to 0 mm Hg. For this reason, the kidneys are sensitive to changes in blood pressure that have little or no effect on other organs. Hemorrhaging, shock, and dehydration are relatively common clinical conditions that can cause a dangerous decrease in the GFR and lead to acute renal failure (loss of kidney function)

What occurs when the plasma concentration of a substance exceeds its tubular maximum?

When the plasma concentration of a substance exceeds its tubular maximum, the excess is not reabsorbed, so it is excreted in the urine.

How would a lack of juxtamedullary nephrons affect the volume and osmotic concentration of urine?

Without juxtamedullary nephrons, a steep osmotic concentration gradient could not exist in the medulla, and the kidneys would be unable to form concentrated urine.