Components. Urinary System. Formation of Urine. Functions of Kidney. Pathway of Urine. Kidney. Major functions of the kidneys include:

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1 Components Urinary System To Accompany: Anatomy and Physiology Text and Laboratory Workbook, Stephen G. Davenport, Copyright 2006, All Rights Reserved, no part of this publication can be used for any commercial purpose. Permission requests should be addressed to Stephen G. Davenport, Link Publishing, P.O. Box 15562, San Antonio, TX, The urinary system consists of the (1) kidneys, (2) ureters, (3) urinary bladder, and (4) urethra. Figure 27.1 Functions of Kidney Major functions of the kidneys include: (1) filter blood, (2) process the filtrate to form urine, (3) produce hormones and enzymes that regulate blood pressure and red blood cell production, and (4) regulate blood ph. Formation of Urine Three major processes are involved in the formation of urine, (1) filtration -filters the blood to produce a filtrate (2) reabsorption - selectively reabsorbs needed water and essential solutes from the filtrate (3) secretion - moves substances into the filtrate for removal as part of the urine. Processing of the tubular fluid produces the waste product called urine. Pathway of Urine Urine, which consists mostly of water and solutes, is transported from the kidneys by the ureters to the urinary bladder, which temporarily stores the urine. Urine is removed from the body when the bladder is emptied through the urethra. The discharge of urine is called micturition. Kidney The paired kidneys are located in the posterior aspect of the abdomen behind the peritoneum (retroperitoneal). Their superior border is about level with the superior border of the twelfth thoracic vertebra, and they are partially protected by the eleventh and twelfth ribs. Inferiorly, they are slightly above the inferior border of the third lumbar vertebra. The right kidney is slightly lower than the left. The average adult kidney is about four inches long, two to three inches wide, and a little more than one inch thick. 1

2 Kidney Figure 27.2 Illustration of the kidney (in frontal section) showing general structures. Anatomy of the Kidney Lab Activity 2 - Dissection External Anatomy Curvatures The kidney has two major curvatures, a large lateral curvature and a small medial curvature. The smaller medial curvature has an indented area (notch) called the hilus. Renal hilus The renal hilus is a notch on the medial surface of the kidney that serves as a passageway for the ureter, blood vessels, nerves, and lymphatics. The hilus opens into a central area of the kidney called the renal sinus Renal capsule The renal capsule is a thin, fibrous capsule that is attached to the surface of the kidney. Anatomy of the Kidney Lab Activity 2 - Dissection Anatomy of the Kidney Lab Activity 2 - Dissection Figure 27.3 After identification of the kidney s external anatomy, make a frontal section through the kidney. This photograph of the sectioned kidney shows the presence of injected red latex in the ureter, renal pelvis, and calyces. Usually, the red latex is NOT present in the kidney s internal cavities. Figure 27.4 Frontal section of the preserved sheep kidney showing the general structure of the kidney. Anatomy of the Kidney Lab Activity 2 - Dissection Anatomy of the Kidney Renal hilus The renal hilus is a notch on the medial surface of the kidney that serves as a passageway for the ureter, blood vessels, nerves, and lymphatics. Renal sinus The renal sinus is the cavity of the kidney that contains the renal pelvis and the calyces. The renal sinus serves as a passageway for the renal blood vessels, nerves, and lymphatics. Renal cortex The cortex is the outer region of the kidney. It is lighter in color and more granular than the inner region called the medulla. The cortex contains numerous units of capillaries, the glomeruli, which give the cortex its granular appearance. Figure 27.5 Frontal section of the dissected sheep kidney showing details of internal structures. 2

3 Anatomy of the Kidney Renal medulla The medulla is the region located to the inside of the renal cortex. The medulla is formed by striated conical masses called renal pyramids. The striations are produced by the parallel arrangement of collecting ducts and loops of Henle. Renal pyramids The renal pyramids are conical areas that form the renal medulla. The pyramids are mostly composed of parallel tubules that give the pyramids a striated appearance. The bases of the pyramids border the cortex. The apices of the pyramids are called the renal papillae and project inward into the minor calyces. Renal columns The renal columns are areas of renal cortex that separate the pyramids. The renal columns mostly serve as routes for vessels and nerves. Anatomy of the Kidney Minor calyces The minor calyces are small cavities that encircle the renal papillae. Fluid (urine) that leaves the papillary ducts of the renal papillae enters the minor calyces and continues into the major calyces. Major calyces The major calyces are large cavities that receive several minor calyces. The major calyces (only two or three in the kidney) converge inward into the renal pelvis. Urine from the major calyces drains into the renal pelvis. Renal pelvis The renal pelvis is the cavity that is located at the convergence of the major calyces. It is continuous with the ureter. Ureter The ureter is the tube that transports urine from the kidney s renal pelvis to the urinary bladder. Nephrons The functional units of the kidney are called nephrons. The nephrons begin in the kidney s cortex and end with their convergence into collecting ducts. A nephron consists of two major regions, (1) a group of capillaries called the glomerulus and (2) a renal tubule. Renal Tubule The renal tubule consists of four divisions, the (1) glomerular (Bowman s) capsule, the (2) proximal convoluted tubule, the (3) loop of Henle (ascending and descending limbs), and the (4) distal convoluted tubule. The distal convoluted tubule unites with a collecting duct. Collecting Ducts Collecting ducts receive many distal convoluted tubules and converge to the papillary ducts. Papillary ducts, located in the renal papillae, empty into small cavities, the minor calyces. Two Types of Nephrons There are two major classifications of nephrons, the (1) cortical nephrons and the (2) juxtamedullary nephrons. Their classification is based mostly upon the location of the loop of Henle and their vascularization. 3

4 Cortical Nephrons Cortical nephrons are mostly located in the kidney s cortex, with a small portion of their loop of Henle descending into the renal medullary pyramids. The cortical nephron is associated with capillaries called peritubular capillaries, which branch from the efferent arteriole. The peritubular capillaries surround the convoluted tubules and the loop of Henle Juxtamedullary Nephrons Juxtamedullary nephrons begin near the kidney s medullary pyramids and their loop of Henle descends deep into the medullary pyramids. The nephron is associated with two capillaries, the peritubular and the vasa recta capillaries, both branching from the efferent arteriole. The peritubular capillaries are associated with the convoluted tubules and The vasa recta are associated with the loop of Henle. Location and Structure of Nephrons Location and Structure of Nephrons Figure 27.6 Illustration showing the location and structure of cortical and juxtamedullary nephrons. Figure 27.7 Illustration showing the general structure and vascularization of a juxtamedullary nephron. Renal corpuscle Anatomy of the Nephron The renal corpuscle consists of the glomerulus and its glomerular capsule. 4

5 Glomerulus The glomerulus consists of a group of capillaries housed within the glomerular capsule. The glomerular capillaries are fenestrated capillaries, capillaries that are formed from endothelial cells that have minute membrane covered pores that enhance filtration The glomerulus is covered by cells (podocytes) of the inner (visceral) layer of the glomerular (Bowman s) capsule and a basement membrane, located between the glomerulus and the podocytes. Glomerular (Bowman s) capsule The glomerular (Bowman s) capsule is the cup-shaped end of the nephron which surrounds the glomerular capillaries. It consists of two layers, the inner (1) visceral layer and the outer (2) parietal layer. A capsular space is located between the visceral and parietal layers. Visceral Layer of Capsule The inner visceral layer, which consists of specialized cells called podocytes, is located on the surfaces of the capillaries. The podocytes have small extensions call pedicles that form minute narrow openings, the filtration slits, which function in forming a portion of the filtration membrane. Filtration Membrane The filtration membrane is formed from the (1) fenestrated endothelium of the glomerular capillaries, the (2) filtration slits of the podocytes (visceral membrane), and the (3) basement membrane (fused basal laminas of visceral layer and endothelium) between the capillaries and the podocytes (visceral membrane). Parietal Layer and Capsular Space Parietal Layer The outer parietal layer is the boundary of the glomerular capsule and consists of simple squamous epithelium. Capsular Space Located between the visceral and parietal layers is the capsular space. The capsular space receives filtrate through the filtration membrane and leads to the proximal convoluted tubule. Glomerular (Bowman s) capsule Figure 27.8 The glomerular (Bowman s) capsule is formed from two layers, the outer parietal layer and the inner visceral layer. The visceral layer consists of specialized cells called podocytes. Podocytes form filtration slits that function as part of the filtration membrane. 5

6 Renal Tubule The renal tubule begins with the glomerular capsule and leads to the proximal convoluted tubule Proximal convoluted tubule The proximal convoluted tubule is the twisted portion of the nephron that leaves the glomerular capsule. It receives filtrate from the glomerular capsule. Loop of Henle The loop of Henle follows the proximal convoluted tubule. The loop of Henle makes a sharp U-turn, with its two portions the descending limb and the ascending limb lying parallel to each other. Distal convoluted tubule The twisted distal convoluted is the last portion of the nephron and leads into a collecting duct. Afferent and Efferent Arterioles Afferent Arteriole The afferent arteriole arises from the interlobular arteries and leads to the glomerular capillaries. It is larger in diameter than the efferent arteriole. Efferent Arteriole The efferent arteriole arises from the glomerular capillaries. The efferent arteriole is considerably smaller than the afferent arteriole which leads to the glomerular capillaries. The smaller diameter of the efferent arteriole results in increased blood pressure in the glomerular capillaries, which promotes capillary filtration. Peritubular capillaries The peritubular capillaries arise from the efferent arteriole and surround the renal tubules located in the cortex. They function in absorption of water and solutes recovered by the tubular cells and secretion of waste products into the tubules. The peritubular capillaries drain into the interlobular veins. Vasa recta The vasa recta are capillaries which arise from the efferent arteriole and paralleling the loops of Henle descend into the medulla and then ascend to unite with veins at the cortical-medullary junction. The vasa recta functions in maintaining the solute concentration (osmolarity gradient) of the renal medulla. Juxtaglomerular Apparatus The juxtaglomerular apparatus is formed by the association of the distal convoluted tubule with the arterioles of the glomerulus. The modified cells of the afferent arteriole are called juxtaglomerular cells The modified cells of the distal convoluted tubule are called the macula densa. The juxtaglomerular apparatus produces the hormone erythropoietin and the enzyme renin. Microscopic Observation of Kidney Lab Activity 4 6

7 Lab Activity 4 Microscopic Observation of the Kidney Lab Activity 4 Microscopic Observation of the Kidney Figure Scanning power photograph of a kidney preparation showing a cross (or horizontal) section. Figure A scanning power photograph of a section of the kidney. Lab Activity 4 Microscopic Observation of the Kidney Lab Activity 4 Microscopic Observation of the Kidney Figure Low power view of the cortex (and medulla). Figure High power photograph showing the structure of the renal corpuscle, the glomerular (Bowman s) capsule and the glomerulus. Lab Activity 4 Microscopic Observation of the Kidney Lab Activity 4 Microscopic Observation of the Kidney Figure High power photograph showing the structure of the glomerular (Bowman s) capsule, the glomerulus, and the proximal convoluted tubule. Figure Low power view of the medulla (and cortex). 7

8 Lab Activity 4 Microscopic Observation of the Kidney Juxtaglomerular Apparatus Figure A low power view of a medullary pyramid. The vasa recta are capillaries associated with the loops of Henle (and collecting ducts) of the juxtamedullary nephrons. Figure 27.9 The juxtaglomerular apparatus is formed by the association of the distal convoluted tubule with the arterioles of the glomerulus. Filtration Processes of Urine Formation Three processes are involved in the production of urine (1) filtration, (2) reabsorption, and (3) secretion. Filtration is the process of separation by passing through a filter. A filter separates substances based upon the size of the filter s pores (openings). The kidney s filtration membranes function as filters. Reabsorption Reabsorption is the process where substances are absorbed, again. The process of filtration allows some substances that were absorbed from the digestive tract or interstitial fluid into the blood, to leave the blood and to enter the nephron, ultimately to become a component of urine. Reabsorption is the process where instead of allowing the substances to form urine, the substances are absorbed back into the blood (mostly by the peritubular capillaries). Secretion Secretion is the process where substances are moved from the cells or blood. In the formation of urine, secretion involves the movement of substances from the blood of the peritubular capillaries and the tubular cells into the tubular fluid. The tubular fluid is excreted as urine. Excretion is the process of disposal of waste, such as urine (or sweat) from the body s organs or blood. 8

9 Processes of Urine Formation Filtration at the Glomerulus Filtration Membrane Figure Filtration of blood occurs at the glomerulus. The nephron functions in reabsorption and secretion. Filtration Filtration is the process of separation by passing through a filter. A filter separates substances based upon the size of the filter s pores (openings). The kidney s filtration membranes function as filters in the filtration of blood and are formed from the (1) fenestrated endothelium of the glomerular capillaries, the (2) filtration slits formed by the podocytes (visceral membrane of glomerular capsule), and the (3) basement membrane (fused basal laminas of visceral layer and endothelium) located between the capillaries and the podocytes (visceral membrane). Filtration Membrane The filtration membrane restricts the passage of the blood s formed elements (cells and platelets) and most of the plasma proteins. Only the smallest of substances such as water, ions, simple sugars, and amino acids are allowed to pass into the capsular space as filtrate. Filtration Membrane Filtration Membrane Figure Filtration occurs at the glomerulus, with the filtrate entering the glomerular capsular space. Figure The filtration membrane is formed from the (1) fenestrated endothelium of the glomerular capillaries, the (2) filtration slits formed by the podocytes (visceral membrane of glomerular capsule), and the (3) basement membrane (fused basal laminas of visceral layer and endothelium) located between the capillaries and the podocytes (visceral membrane). 9

10 Filtration Membrane Figure High power photograph that shows the details of the renal corpuscle (and juxtaglomerular apparatus). The filtration membrane is formed from the association of the glomerular capillaries, podocytes, and the basement membrane (not shown in this photograph). Filtration Pressure The driving force for filtration is glomerular hydrostatic pressure, HPg, (or glomerular capillary blood pressure). Forces opposing glomerular hydrostatic pressure are glomerular (blood) osmotic pressure, OPg, and capsular hydrostatic pressure, HPc. Net filtration pressure (NFP) is determined by subtracting the opposing forces, glomerular (blood) osmotic pressure plus capsular hydrostatic pressure, from glomerular hydrostatic pressure NFP = HPg - (OPg + HPc). Glomerular hydrostatic pressure Glomerular hydrostatic pressure is the pressure of blood within the glomerular capillaries, and is about 60 mm Hg. Glomerular blood pressure is relatively high because the efferent arteriole is smaller than the incoming afferent arteriole. Glomerular osmotic pressure Glomerular osmotic pressure is mostly produced by the blood s albumins (and other plasma proteins). Water pushed into the glomerular capsule by blood hydrostatic pressure (HPg) tends to diffuse back into blood (osmosis) as a result of its higher concentration of solutes (lower concentration of water). Glomerular osmotic pressure opposes glomerular hydrostatic pressure and is measured at about 28 mm Hg. Capsular hydrostatic pressure Capsular hydrostatic pressure is the pressure due to the presence of fluid (filtrate) in the glomerular capsule. Capsular hydrostatic pressure is mostly produced as a back-pressure due to resistance of the fluid to flow. Capsular hydrostatic pressure opposes glomerular hydrostatic pressure and is measured at about 15 mm Hg. Net Filtration Pressure Net filtration pressure is the net pressure at the filtration membrane and is determined by subtracting the forces opposing filtration (OPg + HPc) from the force promoting filtration, HPg. Applying approximate numerical values, NFP = 60 mm Hg - (28 mm Hg mm Hg), or NFP = 17 mm Hg. 10

11 Filtration Pressure Glomerular Filtration Maintenance of Glomerular Filtration Figure Net filtration pressure is determined by subtracting the forces opposing filtration (OPg + HPc) from the force promoting filtration, or applying approximate numerical values, NFP = 60 mm Hg - (28 mm Hg mm Hg), or NFP = 17 mm Hg. Glomerular Filtration The amount of filtrate produced by the kidneys per minute is called the glomerular filtration rate (GFR). Considering the filtration membrane, the most significant variable in the regulation of the filtration rate is glomerular (blood) hydrostatic pressure. Other factors that can change glomerular filtration rates, such as changes in the permeability of the filtration membrane, changes in filtration membrane surface area, and changes in the blood osmotic and capsular pressures may result from illness or disease. Maintenance of Glomerular Filtration Maintenance of a normal glomerular filtration rate results from the adjustment of glomerular blood pressure by directly influencing either the (1) afferent and efferent arterioles or (2) systemic blood pressure. Myogenic mechanism Myogenic means that the mechanism originates from the muscles, which in this case is the vascular smooth muscle of the afferent arteriole. The myogenic mechanism functions to maintain glomerular hydrostatic pressure because the smooth muscle of the afferent arteriole contracts in response to stretch. Increased blood pressure causes the contraction of the smooth muscle of the afferent arteriole, thus, reducing blood flow and pressure at the glomerular capillaries. If systemic blood pressure decreases, the smooth muscle of the afferent arteriole dilates, resulting in increased blood flow and pressure. Tubuloglomerular mechanism Tubuloglomerular means the mechanism originates from the interaction of the nephron (tube) and the glomerulus. Specifically, this interaction occurs at the juxtaglomerular apparatus, the association of the distal convoluted tubule with the arterioles of the glomerulus. The mechanism operates through the macula densa cells, a dense group of cells in the distal convoluted tubule of the juxtaglomerular apparatus. 11

12 Tubuloglomerular mechanism The macula densa cells respond to increased osmolarity (mostly due to increased sodium ions) by releasing a chemical mediator that targets the afferent arteriole and produces vasoconstriction. High osmolarity indicates that blood pressure is too high and tubular flow is too fast to allow adequate adjustment (decrease) of tubular fluid osmolarity. Vasoconstriction of the afferent arteriole decreases pressure, thus, flow and allows for increased reabsorption time. Tubuloglomerular mechanism Figure Increased osmolarity of the fluid in the distal convoluted tubule results in the macula densa cells releasing a chemical mediator that causes vasoconstriction of the afferent arteriole. Renin-Angiotensin Mechanism Renin is an enzyme released into the blood by the juxtaglomerular cells of the afferent arteriole, especially when the juxtaglomerular cells are subjected to reduced stretch, resulting from low systemic blood pressure. Renin functions to convert the inactive plasma enzyme angiotensinogen into angiotensin I. Angiotensin I is converted to angiotensin II by enzymes (mostly by enzymes in the endothelium of the lungs called angiotensin converting enzymes, or ACE). Renin-Angiotensin Mechanism Angiotensin II is a powerful vasoconstrictor and promotes an increase in systemic blood pressure. Angiotensin II also targets the cortex of the adrenal glands and results in the release of aldosterone. Aldosterone, the primary mineralocorticoid of the adrenal cortex, mostly targets the distal convoluted tubules and promotes the reabsorption of sodium ions from the tubular fluid. As sodium ions move back into the blood water osmotically follows, thus increasing blood volume. Increase blood volume results in increased systemic blood pressure. Renin-Angiotensin Mechanism Figure Decreased systemic blood pressure results in the release of renin from the juxtaglomerular cells. Sympathetic Nervous System Decreased systemic blood pressure results in stimulation of the vascular center resulting in systemic arteriole vasoconstriction and stimulation of the juxtaglomerular cells to release the enzyme renin. Vasoconstriction of systemic arterioles results in increased systemic blood pressure. Renin ultimately results in production of angiotensin II, which also promotes systemic arteriole vasoconstriction, and it promotes the release of aldosterone from the adrenal cortex. Aldosterone promotes increased sodium ion reabsorption and the osmotic reabsorption of water. Increased blood water volume results in increased systemic blood pressure. 12

13 Reabsorption Reabsorption Reabsorption is the process where substances are absorbed, again. The process of filtration allows some substances that were absorbed from the digestive tract or interstitial fluid into the blood, to leave the blood and to enter the nephron, ultimately to become a component of urine. Reabsorption is the process where instead of allowing the substances to form urine, the substances are absorbed back into the blood (mostly of the peritubular capillaries). Reabsorption at Proximal Convoluted Tubule Most of the tubular reabsorption occurs in the proximal convoluted tubules. All of the organic molecules such as glucose and amino acids, many ions, and water are reabsorbed into the peritubular capillaries. Reabsorption involves both passive and active processes. Reabsorption at Proximal Convoluted Tubule About 65% of the sodium ions are reabsorbed in the proximal convoluted tubule. As sodium moves back into the blood water osmotically follows (along its concentration gradient). The osmotic movement of water along its concentration gradient established by the movement of solutes (sodium ions) is called obligatory water reabsorption. Reabsorption at Proximal Convoluted Tubule Figure Most tubular reabsorption occurs in the proximal convoluted tubule. All organic molecules are reabsorbed along with most of the tubular water and ions. Reabsorption at the Loop of Henle The loop of Henle is the loop portion of the nephron located between the proximal and distal convoluted tubules. The loop of Henle consists first of a descending limb, which is followed by the ascending limb. The descending limb is permeable to water, which is reabsorbed either in the peritubular capillaries of the cortical nephron or by the vasa recta of the juxtamedullary nephron. The ascending limb is permeable to sodium and chloride ions, or NaCl. Sodium and chloride ions are reabsorbed into the peritubular capillaries of the cortical nephron and the vasa recta of the juxtamedullary nephron. 13

14 Reabsorption at the Loop of Henle Figure The loop of Henle consists of the descending limb followed by the ascending limb. The descending limb is permeable to water, and the ascending limb is permeable to sodium and chloride ions. In the juxtamedullary nephrons, water and salt are reabsorbed by the vasa recta. Reabsorption at the Distal Convoluted Tubule and Collecting Duct The distal convoluted follows the ascending limb of the loop of Henle. Reabsorption is mostly under hormonal control. Three major substances that are reabsorbed are water, sodium ions, and calcium ions. Water Reabsorption and Antidiuretic Hormone (ADH) Water reabsorption is mostly regulated by antidiuretic hormone, or ADH. ADH is released from the posterior pituitary gland in response to high blood osmolarity (or low water volume). ADH targets the distal convoluted tubule and the collecting duct and increases water reabsorption, thus blood volume and blood pressure are increased. Sodium Reabsorption and Renin- Angiotensin Mechanism Sodium reabsorption is mostly regulated by the hormone aldosterone and functions through the renin-angiotensin mechanism. A Aldosterone is the primary mineralocorticoid produced by the adrenal cortex. Increased aldosterone secretion is triggered by the release of the enzyme renin by the juxtaglomerular cells of the afferent arteriole in response to low blood pressure (reduced stretch of the juxtaglomerular cells). Sodium Reabsorption and Renin- Angiotensin Mechanism Aldosterone targets the distal convoluted tubule and the collecting duct and results in increased sodium reabsorption. As sodium is reabsorbed, water osmotically follows, thus, increasing blood volume and pressure. Calcium Ion Reabsorption and Parathyroid Hormone (PTH) Calcium ion reabsorption is influenced by parathyroid hormone, or PTH. Parathyroid hormone is produced by the parathyroid glands in response to low levels of blood ionic calcium. PTH increases calcium ion reabsorption in the distal convoluted tubule. 14

15 Reabsorption at the Distal Convoluted Tubule and Collecting Duct SECRETION Secretion is the process where substances are moved from the cells or blood. Figure Reabsorption at the distal convoluted tubule and the collecting duct. Three major substances that are reabsorbed are water, sodium ions, and calcium ions. Secretion In the formation of urine, secretion involves the movement of substances from the blood of the peritubular capillaries and the tubular cells into the fluid of tubules of the nephron and the collecting duct. Secretion Common substances that are secreted include hydrogen ions (H+), bicarbonate ions (HCO3-, and ammonium ions (NH4-). Most secretion occurs in the proximal convoluted tubule, with the distal convoluted tubule and collecting duct having a secondary role. Secretion Secretion mostly functions in the regulation of the body s ph and the removal of excess potassium. With increasing acidosis, hydrogen ions are increasingly secreted into the tubular fluid. With increasing alkalosis, bicarbonate ions (HCO3-) are increasingly secreted into the tubular fluid. Potassium secretion occurs mostly in the collecting duct and is under the influence of aldosterone; as sodium ions are reabsorbed, potassium ions are secreted. Secretion Figure Secretion functions in the balance of the body s ph and the removal of excessive or unnecessary substances. 15

16 Production of the Medullary Osmotic Gradient An osmotic gradient is established in the kidney s medulla (renal pyramids) for the regulation of the osmotic movement of water Production of the Medullary Osmotic Gradient The process that establishes the osmotic gradient is called the countercurrent mechanism because of the opposing directions of tubular fluid flow in the descending and the ascending loops of Henle. The osmotic gradient ranges from about 300 mosm in the proximal convoluted tubule (cortex) to about 1200 mosm in the bend of the loop of Henle (medulla). The gradient is established because (1) sodium ions are actively transported out of the water impermeable ascending limb, (2) the descending limb is only permeable to water, and (3) glomerular filtration continually keeps fluid entering the nephron. Starting with the ascending limb Sodium ions are pumped out of the tubular fluid (Cl- follows) into the interstitial fluid. This reduces the solute (salt) concentration of the tubular fluid in the ascending limb. However, the solute concentration around the descending limb is increased. Descending limb Now, water osmotically leaves the descending limb increasing the solute (salt) concentration in the tubular fluid. The tubular fluid high is solute (salt) is pushed by incoming filtrate from the glomerulus into the ascending limb. Ascending limb Sodium is additionally pumped out of the tubular fluid into the interstitial fluid making the interstitial fluid solute (salt) more concentrated. The fluid in the ascending limb becomes less concentrated in solute (salt). Countercurrent Multiplication Mechanism The countercurrent mechanism is further described as the countercurrent multiplication mechanism because the solute concentration in the tubular fluid of the descending limb increases as filtrate moves through the lumen of the tubule. 16

17 Vasa Recta Figure Solute (salt) is actively transported out of the ascending limb, which increases the osmolarity of the interstitial fluid. Then, water osmotically moves from the descending limb, resulting in an increase in solute (salt) concentration of the descending limb s tubular fluid. This more solute concentrated tubular fluid moves into the ascending limb and is again used as a source for the transport of solute (salt) into the interstitial fluid, resulting in an increase (multiplication) of interstitial fluid solute (salt) concentration. Specialized capillaries, the vasa recta, are associated with the loop of Henle. The vasa recta is permeable to both salt and water and thus, is maintained at an equal salt and water concentration with the interstitial fluid. Because the vasa recta exchanges both salt and water to maintain the osmolarity of the interstitial fluid it is called the countercurrent exchanger. Final Regulation of urine Water Volume Figure The vasa recta are specialized capillaries associated with the loop of Henle of juxtamedullary nephrons. The vasa recta allows the maintenance of the highly concentrated medullary interstitial fluid by being permeable to both water and salt. Final Regulation of Urine Water Volume By the time tubular filtrate reaches the distal convoluted tubule two essential events have occurred, (1) the medullary interstitial fluid has a high osmotic gradient (ranging from 400 mosm to 1200 mosm) and (2) tubular fluid that enters the distal convoluted tubule has a low osmotic gradient (about 100 mosm). Water Elimination by Forming Dilute Urine When the body does not need to conserve water, excessive water is removed by the production of increased amounts of dilute urine. Dilute urine is produced by allowing the continued passage of the hypotonic (low solute concentration) tubular fluid into the minor calyces. As the tubular fluid passes into and through the collecting system little reabsorption of water occurs. Additionally, selective solutes may be actively reabsorbed, contributing to a further lowering the solute concentration of the tubular fluid (producing a more dilute urine). 17

18 Water Elimination by Forming Dilute Urine Figure Mechanism for the production of dilute urine. Water Conservation by Forming Concentrated Urine When the body needs to conserve water (osmoreceptors in the hypothalamus detect increased osmolarity of blood), antidiuretic hormone (ADH) is released by the posterior pituitary gland. ADH is a powerful vasoconstrictor and increases systemic blood pressure by causing vasoconstriction of arterioles. ADH also targets the distal convoluted tubule and the collecting duct causing both to increase permeability to water. Water diffuses from the tubules (hypotonic) into the interstitial fluid (hypertonic) where it is reabsorbed by blood capillaries. Thus, conservation of water increases blood volume and blood pressure. Water Conservation by Forming Concentrated Urine Depending upon the needs of the body, various concentrations of tubular fluid are produced depending upon the amount of ADH present, which is dependent upon the release of ADH at the posterior pituitary. The reabsorption of water is called facultative reabsorption as the amount of water reabsorption is contingent upon the amount of ADH present. Water Conservation by Forming Concentrated Urine Figure Mechanism for the production of concentrated urine. URETERS The ureters are the tubes that transport urine from the kidneys to the urinary bladder. Ureters Like the kidneys and the urinary bladder the ureters are retroperitoneal. Histologically, the walls of the ureter consist of three layers, (1) an inner mucosa. The epithelium of the mucosa is transitional epithelium (2) a middle muscularis. The muscularis consists of two layers of smooth muscle, an inner longitudinal layer and an outer circular layer. The stretch of the muscularis by urine functions as the primary control for initiation and production of peristaltic waves that move urine toward and into the urinary bladder. (3) an outer adventitia. 18

19 The Urinary Bladder The urinary bladder functions as a temporary storage site for urine. The Urinary Bladder Histologically, the wall of the urinary bladder consists of four layers, (1) an inner mucosa. The epithelium of the mucosa is transitional epithelium (2) a submucosa. (3) a middle muscularis. The muscularis consists of three layers of smooth muscle and is called the detrusor muscle. (4) an outer adventitia.. Internally, an area called the trigone is identified as the triangular region marked by three sites, the entrance of each ureter and the origin of the urethra. A muscular sphincter, the internal urethral sphincter, provides for involuntary flow of urine into the urethra. URETHRA Figure Low power photograph of the urinary bladder (fetal, human). The wall of the urinary bladder is divided into four primary regions, the mucosa, submucosa, muscularis externa, and serosa. Bundles of smooth muscle form the detrusor muscle, the muscle that controls the emptying of the urinary bladder. The urethra is the tube that serves as the pathway for the exit of urine from the urinary bladder. Urethra The urethra is the tube that serves as the pathway for the exit of urine from the urinary bladder. In the male, the urethra also serves as the passageway for semen. Two sphincters are associated with the flow of urine, the internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter is a continuation and modification of the detrusor muscle and is located at the origin of the urethra. The internal urethral sphincter is under involuntary control. The external urethral sphincter surrounds the urethra as it passes through the floor of the pelvic cavity at the urogenital diaphragm. The external urethral sphincter is under voluntary control. Urethra Compared with the urethra of the male (about 8 inches long), the urethra of the female is short (about 1.5 inches long). The urethra of the male is divided into three regions, the (1) prostatic urethra, the (2) membranous urethra, and the (3) spongy, or penile, urethra. 19

20 Micturition MICTURITION Micturition is the term that describes the discharge of urine. Two other commonly used terms for the discharge of urine are urination and voiding. Micturition is controlled through a two stage cycle called the micturition reflex. The micturition reflex is divided into the filling (storage) phase and the emptying phase. Micturition Micturition is the term that describes the discharge of urine. Two other commonly used terms for the discharge of urine are urination and voiding. Micturition is controlled through a two stage cycle called the micturition reflex. The micturition reflex is divided into the filling (storage) phase and the emptying phase. Filling Stage During the filling (storage) phase, stretch receptors in the wall of the urinary bladder send sensory information (by visceral afferent fibers of pelvic nerves) into the sacral region of the spinal cord. A reflex circuit is activated to somatic motor fibers that results in contraction of the external urethral sphincter. A circuit is also activated with sympathetic fibers resulting in contraction of the internal urethral sphincter and inhibition of the bladder s detrusor muscle. Afferent (sensory) information is directed to the cerebral cortex producing conscious awareness of the need for micturition when the bladder fills to about 200 ml. Emptying Phase During the emptying phase, the micturition center in the pons is activated. Stretch receptors in the wall of the urinary bladder send sensory information (by visceral afferent fibers of pelvic nerves) into the sacral region of the spinal cord. The sensory information is conveyed to the micturition center of the pons which establishes parasympathetic control of micturition. Parasympathetic outflow results in relaxation of the internal urethral sphincter and contraction of the bladder s detrusor muscle (sympathetic stimulation is inhibited). The somatic motor fibers are inhibited resulting in relaxation of the external urethral sphincter. Emptying Phase Voluntarily withholding micturition interrupts the emptying phase by the relaxation of the bladder s detrusor muscle. The bladder is allowed to continue the filling (storage) phase until about another 200 ml. of urine is stored. After which, cortical awareness becomes more intense for allowing the initiation of the emptying phase. The reflex arc cycle can continue until the bladder reaches maximal filling and the internal urethral sphincter is forced open, followed by the reflexive opening of the external urethral sphincter which allows involuntary micturition. 20