Lesson XIV: The Urinary System

Transcription

1 Lesson XIV: The Urinary System The urinary system is comprised of the two kidneys, the two ureters that lead from each kidney and empty into the urinary bladder, from which extends the urethra that discharges the urine that is produced in the kidneys. The kidneys are the primary functional unit of the urinary system, the remainder of the organs that comprise the urinary tract functioning simply as storage or to conduct urine outside the body. The primary functions of the kidneys include: 1. Regulation of blood ion composition, including Na +, K +, Ca 2+, Cl -, and phosphate (HPO 4 2- ). 2. Regulation of blood ph by excreting H + and conserving HCO Regulating blood volume, but adjusting the amount of water in the blood plasma. 4. Regulating blood pressure by activation of the angiotensin-aldosterone pathway, which results in an increase in blood pressure. 5. Maintaining the osmolarity of the blood, or the optimal concentration of solutes dissolved in the blood, including proteins, glucose, and ions. 6. Producing hormones, including erythropoietin (which stimulates blood cell development) and the activation of calcitrol (vit. D) from cholecalciferol. 7. Regulating blood glucose level by engaging in gluconeogenesis 8. Excreting wastes, including ammonia, urea, bilirubin, creatine, uric acid, drugs and other toxins. I. Anatomy of the Urinary system The paired kidneys are reddish-colored kidney-bean shaped organs located just above the waist, between the peritoneum and the posterior wall of the abdomen, and are therefore retroperitoneal. The typical kidney in adults is about cm long and 3 cm thick, with a weight between g. Both kidneys display a deep vertical fissure called the renal hilus through which the uterer exits the kidney, along with blood and lymphatic vessels and nerves. Three layers of tissue surround the kidneys, the renal capsule, adipose capsule, and renal fascia. The renal 1

2 capsule is comprised of dense irregular connective tissue that protects the kidneys against trauma. The adipose capsule is a mass of metabolically active fatty tissue superficial to the renal capsule that also serves to protect and warm the kidney. The renal fascia is a thin layer of dense irregular connective tissue that anchors the kidney to the surrounding structures and to the abdominal wall. Internally the kidney consists of an outer renal cortex and inner renal medulla. Contained within the medulla are 8-18 cone-shaped renal pyramids, the base end of the pyramid facing the cortex and the tip of the pyramid, or renal papilla, pointing towards the renal hilus at the centre of the kidney. The renal cortex extends from the base of the pyramids to the renal capsule, and in between each pyramid (called a renal column), and functionally, is divided into an outer cortical zone and an inner juxtamedullary zone. The term renal lobe refers to a renal pyramid, the areas of the renal cortex just above it, and portions of the renal column between each pyramid. The functional unit of the kidneys are microscopic structures called nephrons, about one million in each kidney, contained in the renal lobes. The urine that is formed by the nephrons empties into papillary ducts that drain into minor and major calyces, each of the 8-18 minor calyces emptying into two or three major calyces. The urine contained in the major calyces is in turn conducted to a single large cavity called the renal pelvis, which empties into the ureter. Another structure in the kidney, called the renal sinus, is a space that is continuous with the hilus, containing a portion of the renal pelvis, the calyces, and branches of the renal blood vessels and nerves. Adipose tissue is found surrounded in each sinus to stabilize the structures therein. Blood supply to the kidneys Blood supply to the kidneys is via the left and right renal arteries that branch off from the thoracic aorta, and in turn, divide into segmental arteries that supply the various tissues of the kidney. At the bases of the renal pyramids the segmental arteries divide into interlobar arteries that wind between the renal medulla and renal columns, which in turn divide into arcuate arteries that divide into interlobular arteries that enter into the renal cortex and branch into afferent arterioles that supplies an individual nephron. Within the nephron each afferent arteriole is 2

3 divided into a tangled mass of capillaries called a glomerulus, surrounded by a capsule, that reunites and exits the nephron as the efferent arteriole. This is the only example of a capillary bed that exists between two arterioles, rather than an arteriole and a venule. The efferent arterioles then divide into peritubular capillaries that wind around the tubular portions of the nephron. These reunite to form peritubular veins, interlobular veins, arcuate veins and then to the interlobular veins that run between renal pyramids. The filtered blood then exits the kidney via a single renal vein. Nervous supply to the kidneys The majority of the nerves that supply the kidney arise from the celiac ganglion that pass through the renal plexus into the kidneys along with the blood vessels through the renal hilus. Renal nerves are under the influence of the sympathetic nervous system, and when activated, promote the vasoconstriction of renal arteries. The nephron Each nephron consists of a renal corpuscle, where the blood plasma is filtered, and a renal tubule, into which the filtered fluid passes. The two components of the renal corpuscle are the glomerulus and the glomerular (Bowman's) capsule. The glomerulus has already been mentioned, derived from the efferent arteriole that services each nephron, arranged as a knot of blood capillaries in which the blood plasma is filtered through the surrounding glomerular (Bowman's) capsule. The collected fluid is then passed along to the renal tubule, which, in order of the fluid that passes through it, consists of the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT). The loop of Henle is a portion of the collecting tubules that travels down into the renal medulla, comprised of a descending limb that extends from the proximal convoluted tubule and an ascending limb that connects to the distal convoluted tubule. The distal convoluted tubules of several nephrons empty into a single collecting duct that in turn unite to empty into a papillary duct that drains into the minor calyces. Please refer to the diagram of the structure of a nephron in the accompanying text to visualize its anatomy. The majority of nephrons (80-85%) are called cortical nephrons, their renal corpuscles lying in the outer portion of the renal cortex, and have short loops of Henle that 3

4 penetrate into the outer regions of renal medulla. The remaining number of nephrons (15-20%) are called juxtamedullary nephrons, referring to the fact that their renal corpuscles lie close (juxta near to ) the renal medulla. These nephrons have very long loops of Henle that extend into the deepest regions of the medulla. Further, both the ascending and descending limbs of the loop of Henle in juxtamedullary nephrons have thin and thick portions that contain different cells types than the majority of nephrons. The long loops of Henle found in juxtamedullary nephrons allow for an ability to excrete very dilute or highly concentrated urine. Nephron histology The nephron is a microscopic structure, the wall of the glomerular capsule, renal tubule and collecting ducts comprised of a single layer of epithelial cells. The glomerular capsule consists of a parietal and visceral layer. The visceral layer is made up of modified simple squamous epithelial cells called podocytes. These cells have projections that wrap around the glomerular capillaries and form the inner wall of the capsule. The parietal layer of the glomerular capsule consists of squamous epithelium that forms the outer wall of the capsule. Between them is the capsular space, which receives the fluid filtered from the glomerular capillaries. The cells that line the renal tubule and collecting duct have a varied appearance. In the PCT the cells are simple cuboidal epithelial cells that have microvilli on their apical surface, increasing the surface area for absorption in much the same way as the epithelial cells that line the small intestine. The descending limb and the first portion of the (thin) ascending limb of the loop of Henle is composed of simple squamous epithelium. The (thick) ascending limb of the loop of Henle is comprised of simple cuboidal and columnar epithelium. Where the ascending limb of the loop of Henle comes into contact with the arterioles that enter into and leave the renal corpuscle, the columnar cells are crowded together to form the macula densa. Within the walls of the arterioles that lie next to the macular densa are smooth muscle fibers called juxtaglomerular cells. Along with the macula densa, the juxtaglomerular cells form the juxtaglomerular apparatus (JGA) that plays an important role in regulating the flow of blood into the glomerulus by narrowing or relaxing the arteriole. A short distance beyond the JGA is the DCT, comprised of simple cuboidal cells up to its last 4

5 portion, where principal cells that have receptors for the hormones aldosterone and ADH are found, as well as intercalated cells that help to maintain blood ph. II. Renal physiology In the process of producing urine the nephrons and collecting ducts perform three primary functions: 1. Glomerular filtration: water and solutes in the blood plasma enter into the glomerulus and are forced out into the glomerular capsule and into the renal tubules through the glomerular capsule. Please note that blood cells are not filtered, only the plasma. 2. Tubular reabsorption: the filtrate now flowing through the renal tubules is acted upon by the epithelial cells that line the tubules, about 90% of the water and several solutes reabsorbed and passed along to the peritubular capillaries that wind around the tubules. 3. Tubular secretion: as water and certain solutes are being reabsorbed back into circulation, some cells in the tubules actively secreting waste products reabsorbed into the peritubular capillaries back into the tubules. The fluid that is not reabsorbed back into the blood is passed along to the collecting duct, then the papillary duct, minor and major calyces and then to the ureters for elimination. These three steps outlined above describe the basic activities involved in urine formation. The following is a more detailed description of these processes. Glomerular filtration The fluid that enters into the glomerular capsule is called the glomerular filtrate. On an average daily basis adults will process between 150 and 180 liters of filtrate, only 1-2 liters however actually being secreted as urine. The podocytes are cells that comprise the visceral layer of the glomerular capsule, and completely cover the glomerular capillaries, and together with the cells that form the wall of the capillary, form a filtration membrane. This membrane maintains three basic characteristics: 5

6 1. The glomerular capillaries have relatively large pores or fenestrations that are small enough for the passage of water and solutes but not blood cells. 2. The basal lamina that lies between the endothelium of the capillaries and the overlying podocytes is a mixture of collagen and proteoglycans in a glycoprotein matrix that prevents the filtration of larger plasma proteins. 3. The podocytes display foot-like projections called pedicels that wrap around the capillaries, leaving tiny openings between them called filtration slits, permitting the passage of only very small molecules. Certain plasma proteins, such as albumin, while being relatively small, are not able to pass through these slits. Diagnostic tests used to determine the status of kidney function are conducted to determine the presence of albumin in the urine, in which case it can be inferred that kidney function is impaired if albumin is detected in the urine in any significant quantities. The rate of glomerular filtration depends upon three pressure gradients: 1. The glomerular blood hydrostatic pressure (GBHP) is a measure of the blood pressure in the glomerular capillaries, promoting the movement of fluid into the glomerular capsule. This pressure is typically 55 mm Hg. 2. The capsular hydrostatic pressure (CHP) is a measure of the pressure of the filtrate already in the glomerular capsule that acts against the pressure in the capillaries. This pressure is about 15 mm Hg. 3. The blood colloid osmotic pressure (BCOP) is a measurement of the osmotic force exerted by proteins in the blood that opposes the movement of fluid into the glomerular capsule. This pressure is about 30 mm Hg. Thus, we must take into account these three factors in pressure to determine the net filtration pressure (NFP), calculated as follows: GBHP CHP BCOP = NFP i.e. 55 mg Hg 15 mm Hg 30 mm Hg = 10 mm Hg Thus the NFP is 10 mm Hg, the net pressure that causes the blood plasma to be filtered. 6

7 Glomerular filtration rate Now that we have calculated the net filtration pressure, we can calculate the rate at which blood plasma is filtered in the kidneys, called the glomerular filtration rate (GFR). The total GFR averages about 125 ml/min in men and 105 ml/min in women. The actual GFR however will vary depending upon different factors. For example, in the case of hemorrhage and an overall loss in blood volume, the GFR will be reduced dramatically in order to conserve blood volume. If the overall blood volume is too high howere, as in hypertension, the GFR does not appear to increase all that much, in all likelihood because a very high GFR would damage the filtration mechanisms and allow too many vital constituents of the blood plasma to be passed into urine. The GFR is maintained by a three primary factors: renal autoregulation, neural regulation, and hormonal regulation. Renal autoregulation of the GFR is actually comprised of two different mechanisms, the myogenic mechanism and tubuloglomerular feedback. The myogenic mechanism occurs when there is an increase in the blood pressure, which stretches the walls of the afferent arteriole that feeds into the glomerular capillaries. This stretching causes the afferent arteriole to contract thus reducing the flow of blood into the glomerulus. Conversely, if the blood pressure is too low the wall of the afferent arteriole relaxes, thereby increasing the flow of blood into the glomerular capillaries. The mechanism of tubuloglomerular feedback involves the activities of the JGA. When there is an increase in blood pressure the speed of the filtrate moving through the DCT is increased, resulting in a compromised ability of the cells that line the tubules to reabsorb water and important ions such as Na + and Cl -. This causes the macula densa to inhibit the release of nitric oxide (NO) in the adjacent juxtaglomerular cells. Nitric oxide has vasodilatory properties so the inhibition of NO causes the juxtaglomerular cells that wrap around the afferent arteriole that feeds into the glomerulus to constrict. This constriction causes a reduction in the amount of blood flowing into the glomerulus, and thus a slowing of the GFR. Conversely, if the flow of water and solutes in the DCT is slowed, the macular densa lessens its inhibitory activity upon the release of NO in the adjacent juxtaglomerular cells, allowing more blood to flow into the glomerulus, thereby increasing the GFR. 7

8 The neural regulation of the GFR is due to the presence of sympathetic nerve fibers that directly innervate the juxtaglomerular cells through the release of norepinepherine. When sympathetic stimulation predominates, such as in exercise or hemorrhage, the juxtaglomerular cells are stimulated to contract and thereby decreasing the GFR, reducing urine output and conserving blood volume. The hormonal regulation of the GFR is due to the opposing activities of angiotensin II and atrial natriuretic polypeptide. Angiotensin II is a potent vasoconstrictor that is derived from a precursor called angiotensinogen produced by the liver. When stretch receptors in the wall of the afferent arteriole are stimulated by an increase in blood pressure, or when sympathetic stimulation predominates, the juxtaglomerular cells release an enzyme called renin into the blood stream. This enzyme then acts upon angiotensinogen to catalyze it into angiotensin I. As angiotensin I is passed into the lungs in systemic circulation it is acted upon by another enzyme called angiotensin converting enzyme (ACE). This enzyme catalyzes angiotensin I into angiotensin II. Angiotensin II then acts upon the afferent arterioles to constrict, thereby decreasing the GFR. Atrial natriuretic polypeptide (ANP) acts as a vasodilatory agent to the afferent arterioles, released by the heart in response to a large increase in blood volume, thus increasing the GFR. Both these hormones also influence aspects of tubular reabsorption and secretion, along with antidiuretic hormone (ADH, or vasopressin), which will be discussed shortly. Tubular reabsoprtion and secretion The collective activity of the nephrons result in a huge amount of the blood plasma being filtered, such that in 30 minutes of activity the kidneys have filtered all the blood plasma in the body. The process of reabsorption is carried by the cells in the renal tubules by both active and passive processes. Most reabsorption occurs in the PCT, and portions of the renal tubule distal to the PCT fine tune the concentration of water and solutes in the urine to maintain homeostasis. The movement of solutes and water into the peritubular capillaries can occur in the junctions between the cells that line the renal tubules, called paracellular reabsorption, as well as through cells, called transcellular reabsorption. 8

9 In the case of paracellular reabsorption some portions of the renal tubule can reabsorb up to 50% of the water and solutes by osmosis without them having to go through a tubule cell. The remaining portion of solutes however need to be transported across the apical membrane of the tubule cells. The remaining portion of water follows the movement of these solutes by osmosis. The process of transcellular reabsorption utilizes a variety of transport mechanisms depending upon the solute. As mentioned, some solutes freely diffuse across tubule cells into the peritubular capillaries, whereas others need to be actively transported, some processes requiring an expenditure of ATP. Tubule cells contain a variety of transporters located both in the apical surface of the tubule cell, as well as in the basolateral surface of the tubule cell that discharges solutes into the peritubular capillary. The majority of transport mechanisms involve symporters, in which two or more different substances bind to membrane proteins and are transported across the membrane together, and antiporters, in which one substance outside the membrane is brought into the cell while simultaneously another substance is excreted out of the cell. Most of these transport mechanisms involve Na + in some way or another. Na + symporters function to reabsorb Na + along with other solutes, including K +, Cl -, Ca 2+, Mg 2+, HPO 4 2-, SO 4 2-, lactic acid, glucose, amino acids and other solutes. The accompanying text describes the diversity of symporters in the PCT and their specific functions. Na + antiporters function to exchange Na + in the lumen with intracellular H +. Cells can manufacture H + to fuel this antiporter by taking advantage of the presence of CO 2, as a by-product of normal cellular activities, or from CO 2 diffusing into tubule cells from either the peritubular capillaries or from within the tubule lumen itself. Within the tubule cell CO 2 combines with water to form carbonic acid (H 2 CO 3 ), which is then acted upon by the enzyme carbonic anhydrase that converts it to H + and HCO 3 -. H + is then excreted into the lumen of the tubule in exchange for Na + and HCO 3 - is transported into the peritubular capillary via facilitated diffusion. The H + excreted into the tubule lumen then combines with any HCO 3 - present in the lumen readily forming into carbonic acid (H 2 CO 3 ), which then 9

10 quickly disassociates into CO 2 and water. The water and CO 2 then diffuse back into the tubule cells, the CO 2 used again to form more H + ions which fuels the Na + /H + antiporters. The PCT reabsorbs about 65% of the filtered water and the vast majority of solutes, and thus when the filtrate enters into the loop of Henle the rate at which reabsorption occurs slows considerably. In the PCT the rate of absorption was about 80 ml/min, and within the loop of Henle it slows to between ml/min. Despite the fact that many of the solutes have been removed, the osmolarity of the filtrate, or the measure of the total number of solutes within it, is approximately the same as the blood because osmosis keeps pace with the reabsorption of solutes. Another way to say this is that the filtrate remains isotonic with the blood. A particular feature of the loop of Henle is that the tubule cells contained therein are relatively impermeable to water, and thus the reabsorption of water is not coupled to the reabsorption of filtered solutes. The descending limb of the loop of Henle absorbs about 15% of the water, whereas the ascending limb does not absorb any. Contained in the plasma membrane of tubule cells in the ascending loop of Henle there are special Na + /K + /Cl - symporters that reclaim two ions of Cl - along with one Na + and K + each. The presence of K + leakage channels in the membrane of the tubule cells allows K + to assist in fueling this symporter, with a net reabsorption of Na + and Cl -. Further, the movement of K + into the lumen leaves the intracellular fluid of the tubule cells slightly negative, facilitating the absorption of cations such as Na +, Ca 2+, Mg 2+ and K + itself. Ammonia (NH 3 ) is a toxic by-product of protein metabolism that is filtered from the blood in the glomerulus and is secreted by the peritubular capillaries into the renal tubules. Within the lumen of the renal tubules NH 3 readily combines with H + to form an ammonium ion (NH + 4 ). Most of the ammonia produced however is converted by hepatocytes into urea by combining CO 2 with NH - 3 to form CO(NH 2 ) 2 or urea, which is similarly filtered out at the glomerulus and secreted into the renal tubules by the peritubular capillaries. The tubule cells that line the PCT - can also synthesize NH 3 by deaminating glutamine, a process which also generates HCO This NH 3 can combine with H + to form NH + 4, and can be used in place of H + in Na + /H + - antiporters. The HCO 3 generated is then 10

11 absorbed into the blood to restore the buffering system of the blood. Reabsorption in the DCT is now even slower than it was in the PCT, reducing to a mere trickle of 25 ml/min compared to the 80 ml/min that occurs within the PCT. The DCT contains Na + /Cl - symporters that promote the absorbtion of these ions, and contains cells that are sensitive to parathyroid hormone that promotes the uptake of any remaining Ca 2+ in the filtrate. About 15% of the remaining water in the filtrate is also absorbed at this point. By the time the filtrate reaches the end of the DCT about 90-95% of all filtered solutes have been reabsorbed. Principal cells in the DCT and the collecting tubule absorb any remaining Na + while secreting K +, and intercalated cells reabsorb K + and HCO - 3 and excrete H +. Hormonal regulation of reabsorption and secretion There are four hormones that influence the activities of tubular reabsorption and secretion: angiotensin II, aldosterone, anti-diuretic hormone (ADH), and atrial natriuretic peptide (ANP). Apart from promoting the constriction of the afferent arterioles that service the glomerulus and therefore decrease the GFR, angiotensin II enhances the reabsorption of Na +, Cl - and water in the PCT. Angiotensin II also promotes the secretion of the adrenal cortex to release aldosterone that causes the principal cells to reabsorb more Na + and Cl -, and excrete more K +. Anti-diuretic hormone (ADH) is released by the posterior pituitary gland in response to a reduction in water volume in blood plasma and the interstitium, detected by osmoreceptors in the hypothalamus. ADH is then secreted by the pituitary to cause principal cells in the DCT and collecting tubules to reabsorb more water. When the osmolarity of the plasma and interstitium decreases, i.e. it has more water, a negative feedback loop promotes a reduction in ADH secretion. The last hormone discussed that influences tubular reabsorption and secretion is atrial natriuretic peptide (ANP). ANP can inhibit the reabsorption of Na +, and also actively inhibits the secretion of aldosterone and ADH, 11

12 thereby promoting the excretion of more Na +, which results in a greater volume of water being excreted in the urine, and thus a reduction in blood volume. Urine Urine is the liquid that is produced through the activities of tubular reabsorption and secretion. In some case the urine can be dilute, when the blood volume is high and the secretion of angiotensin II, aldosterone and ADH are inhibited, and atrial natriuretic peptide is more active. Conversely, when blood volume is low, angiotensin II, aldosterone and ADH are secreted in greater volumes, and atrial natriuretic peptide is inhibited. The urine is eliminated in volumes of between 1-2 liters daily, but this can vary significantly between individuals and other factors such as exercise, perspiration, body temperature, water intake, blood pressure, diet and the influence of drugs. It is normally straw colored but can vary with naturally occurring pigments found in the diet, from herbal supplements, vitamins or from drugs, as well as from blood. The yellow color of urine is derived from urochrome, a pigment produced from the breakdown of bile, and from urobilin, derived from the breakdown of hemoglobin. A darker urine that is otherwise normal is more concentrated whereas a lighter colored urine is more dilute. The urine is transparent when voided but may appear turbid from the presence of blood, calculi, mucus or semen: if collected and left to stand the urine will become cloudy on its own. The odor is mildly aromatic when voided but has an ammonia smell if left standing: other factors such as the consumption of asparagus can create the formation of compounds that can alter the normal odor. The ph of urine ranges between 4.6 to 8, and becomes more alkaline with a diet rich in carbohydrates, and more acidic with high protein diets. Urine transport, storage and elimination The urine that is produced by the nephron is carried by the collecting ducts to the papillary ducts, which in turn empties into the minor calyces, then into the major calyces 12

13 that in turn unite to empty into the renal pelvis. From the renal pelvis the urine drains into the ureters and then into the urinary bladder where it is stored until it is discharged by the urethra. Ureters The ureters transport the urine collected in the renal pelvis of each kidney and transports it to the urinary bladder. They are about cm long, with thick walls but are relatively narrow, varying between 1 to 10 mm at various points. The walls of the ureters are comprised of three tissue coats, the internal surface being a mucosa comprised of transitional epithelium. The mucus secreted by goblet cells in the mucosa acts to protect the epithelial cells from the urine, which may have a very different ph and concentration of solutes than what is found in the cytosol of epithelial cells. The mucosa is bound to an overlying layer of lamina propria comprised of areolar connective tissue with varying amounts of collagen, elastic fibers and lymphatic tissue. Lying over top of the lamina propria is an intermediate coat of an inner longitudinal arrangement and outer circular layer of smooth muscle fibers called the muscularis. Lying over top of the muscularis is the adventitia, which is a layer of areolar connective tissue containing blood vessels, lymphatic vessels and nervous tissue that supplies the muscularis. The ureters, like the kidneys, are retroperitoneal, and at their base curve medially to attach and empty into the posterior wall of the urinary bladder. The movement of urine through the ureters is largely controlled by peristaltic muscular contractions, but gravity and hydrostatic pressure also contribute. As urine is produced and fills up the urinary bladder the opening of the ureters into the bladder are compressed, which prevents the back flow of urine into the ureters. Urinary bladder The urinary bladder is a hollow, muscular organ located in the pelvic cavity, lying posterior to the pubic symphysis, held in place by the peritoneum. When the bladder is empty it essentially collapses upon itself, whereas when it contains urine it becomes progressively more spherical in shape. The capacity of the bladder is between ml, slightly less in women because of the uterus which takes up space in the abdominopelvic cavity. 13

14 The floor of the urinary bladder is a triangular area called the trigone, a portion of the bladder mucosa that does not collapse because it is firmly bound to the underlying muscularis. The two posterior corners of the trigone is where the two ureters empty into the bladder, and the anterior corner is where the urethra begins. The deepest layer of the bladder wall is the mucosa, composed of transitional epithelium and an underlying lamina propria. Much like the stomach, when the bladder is empty the mucosa folds upon itself into rugae. Surrounding the mucosa is the intermediate muscularis or the detrusor muscle, consisting of three layers of smooth muscle fibers, an inner and outer longitudinal layer, and a medial circular layer. Surrounding the opening to the urethra circular muscle fibers form the internal urethral sphincter. Below this lies the external urethral sphincter which is comprised of the urogenital diaphragm, which is skeletal muscle and therefore under conscious control. The most superficial layer on the inferior and posterior surfaces of the urinary bladder is the adventitia, whereas the superior surface is covered by the serosa of the visceral perotineum. The voiding of urine from the bladder is induced by the micturition reflex, which is initiated by the distension of the bladder wall as the bladder fills with urine. When the baldder contains about ml of urine stretch receptors in the bladder wall send impulses to the sacral spinal cord where they trigger a spinal reflex that results in the stimulation of parasympathetic impulses along nerves that innervate the bladder wall and internal urinary sphincter, causing the detrusor muscle to contract and the internal urinary sphincter to relax. The actual voiding of urine is under conscious control and regulated by the external urinary sphincter. Urethra The urethra is a small tube extending from the urethral orifice in the anterior portion of the urinary bladder. In females the urethra lies directly posterior to the pubic symphysis, and extends for about 4 cm to the external urethral orifice located between the clitoris and vaginal opening. The female urethra is comprised of a deep mucosa, which in turn is comprised of epithelium and a lamina propria, and an overlying superficial muscularis. In males the urethra extends from the internal urethral 14

15 orifice, passing first through the prostate and then into the penis, traveling a distance of between cm. The histology of the male urethra is similarly comprised of a mucosa and overlying muscularis, but contains openings to several accessory structures including the prostate, seminal vesicles, and bulbourethral glands that all provide the different components of semen, as well as the vas deferens which delivers semen outside the body along with these other secretions during ejaculation. A detailed overview of the male and female genitourinary tract is discussed in Lesson XV: The Reproductive system. 15