B. Anatomy
        The parts of the urinary system are the:
            kidneys which filter waste from the blood, excrete urine, and regulate the electrolyte balance;
            ureters which transport urine from the kidneys to the urinary bladder;
            urinary bladder which stores urine; and
            urethra transports urine from the urinary bladder to the outside of the body.

        1. Kidneys
            The bean-shaped kidneys are located against the posterior lumbar body wall (see figure 26.2a).  Because they are located between the parietal peritoneum and the lumbar musculature, they are not located in the abdominopelvic cavity but rather behind it (retroperitoneal).  The kidneys are level with the T-12 to L-3 segments of the vertebral column.  The right kidney is situated slightly lower than the left one (see figure 26.1). A suprarenal (adrenal) gland is located superior to each kidney.

            Each kidney is surrounded by three capsules:
                true capsule: the innermost capsule; a fibrous membrane
                perirenal fat: the middle capsule; a layer of fat
                renal fascia: the outermost capsule; a fibrous membrane; anchors kidney to the body musculature

            The lateral surface of each kidney is convex and the medial surface is concave (see figure 26.3b).  The medial depression is called the hilum.  The renal artery enters the kidney at the hilum.  The ureter and renal vein exit the kidney at the hilum.

            Internally, the kidney is separated into an outer cortex and an inner medulla (see figure 26.3b).  Each of these areas occupies approximately one-half of the mass of the kidney.  The medulla is subdivided into 8-12 triangular masses called pyramids.  The broad base of each pyramid is oriented toward the cortex and the apex (papilla) towards the hilum.  Cortical tissue, called renal columns, extends between the pyramids.

            Each papilla projects into a cavity called a calyx.  The calyces merge to form the pelvis.  The pelvis is an enlarged cavity located in the region of the hilum.  The calyces and pelvis actually compose the distal end of the ureter.  Therefore, the calyces and pelvis, although they are surrounded by the kidney, are not part of the kidney.  The ureter proper, which is a tube-like structure, is continuous with the pelvis and exits the kidney at the hilum.

            Oxygen-rich arterial blood enters the kidney in the renal artery (see figure 26.3b).  This artery penetrates the kidney at the hilum.  The artery then divides into 3-5 branches called the interlobar arteries.  The interlobar arteries enter the renal columns between the pyramids.  The interlobar arteries then branch into the arcuate arteries which arch over the bases of the pyramids.  Many interlobular arteries branch from each arcuate artery and penetrate the cortex (see figure 26.3b).  Many short afferent arterioles arise from each interlobular artery (see figure 26.5a,b).  Each afferent arteriole terminates in a cluster of capillaries called a glomerulus.  Instead of merging to form the usual venule, the glomerular capillaries merge to form another arteriole called an efferent arteriole.  Each efferent arteriole gives rise to a network of capillaries called the peritubular capillaries and vasa recta (see figure 26.5a,b).  After flowing through these latter capillaries, blood collects into the interlobular veins, then flows into arcuate veins, then into interlobar veins, and finally exits the kidney in the renal vein.

            The afferent and efferent arterioles are innervated by the sympathetic nervous system.  The sympathetic nerve regulates the diameter of these arterioles (vasoconstriction/vasodilation) and thus regulates the blood flow through the kidneys and influences the blood pressure.

            The cortex and medulla of the kidney contain the microscopic, functional units of the kidney called nephrons.  There are approximately one million nephrons in each kidney.  Each nephron is composed of two basic parts: the renal corpuscle and the renal tubule (see figure 26.4b and 26.5a,b).  The renal corpuscle, which is located in the cortex, is subdivided into two parts: the glomerulus and the renal (Bowman’s) capsule.  The glomerulus was mentionned previously as the cluster of capillaries located between the afferent and efferent arterioles.  Each glomerulus is partially surrounded by a cup-shaped structure called the renal capsule.  The renal capsule is continuous with the second portion of the nephron, the renal tubule.  The renal tubule is subdivided into three segments: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.  The proximal convoluted tubule, which is located in the cortex, is a twisted area continuous with the renal capsule.  The loop of Henle is a U-shaped portion of the tubule.  It is subdivided into a descending arm, so-called because it descends from the cortex into the medulla, and an ascending arm, so-called because it ascends from the medulla into the cortex.  The diameter of the descending arm is narrower than that of the ascending arm.  The distal convoluted tubule, which is located in the cortex, is a twisted area at the end of the renal tubule.  In the cortex, the distal convoluted tubules of several nephrons merge with a collecting duct.  The collecting duct is not part of the nephron.  The many collecting ducts extend from the cortex into the medullae (pyramids) and join with the calyces surrounding the papillae of the pyramids.

            Some distal convoluted tubules lie in contact with afferent arterioles. The juxtaglomerular apparatus is located at these points of contact.  Juxtaglomerular cells line the afferent arteriole and cells of the macula densa line the distal convoluted tubule.  The juxtaglomerular cells are baroreceptors that are sensitive to changes in blood pressure.  When blood pressure drops, these cells release renin which sets into play a series of reactions that lead to vasoconstriction and absorption of water from the urine into the blood.  The cells of the macula densa are chemoreceptors which are sensitive to changes in the solute concentration of the newly formed urine.  They release a chemical that regulates the diameter of the afferent arteriole thereby influencing blood flow along the arteriole.

        2. Ureters
            The ureters extend from the kidneys to the urinary bladder.  Embryologically, the development of the ureters begins at the urinary bladder.  As the ureters develop, they grow toward the kidney and terminate in the kidney as the pelvis and calyces.  For this reason, the bladder end of the ureter is the proximal end and the kidney end is the distal end.  Each ureter is approximately 25-30 cm long and 4-5 mm in diameter.  The wall is divided into three layers: an inner mucous membrane, a middle layer of smooth muscle, and an outer layer of connective tissue.

            The muscle in the wall of the ureter exhibits wave-like contractions called peristalsis which move the urine from the kidney to the bladder.  Kidney stones, if present, tend to lodge in constricted areas of the ureters.

            The ureters enter the lower, posterior portion of the bladder at a triangular area called the trigone.
 
        3. Urinary bladder
            The urinary bladder is located posterior to the pelvic symphysis.  Superiorly, it is covered by the peritoneum.  The wall of the bladder has three layers: an inner mucous membrane, a submucosa consisting of connective tissue, a layer of smooth muscle called the detrusor muscle, and an outer thin membrane called the serosa.  When the urinary bladder is empty, the inner surface of the mucosa is folded except at the trigone.  The trigone is triangular in shape.  The two ureters join the bladder at two of the points of the trigone and the ureter exits the bladder at the third point.  As the bladder fills with urine, the folds of the mucosa stretched out and disappear.

            The emptying of the bladder, called micturition, is controlled by the parasympathetic nervous system.  I will describe micturition later.

        4. Urethra
            The urethra extends from the urinary bladder to the urethral orifice on the surface of the body.  In females, the urethra is approximately 4 cm long; in males, it is approximately 8 in long.  In both sexes, the urethra transports urine during micturition.  In the male, it also transports semen during ejaculation.  Except at micturition, the urethra is kept closed by the internal and external urethral sphincters.  The internal urethral sphincter is a ring of the detrusor muscle located at the union between the bladder and the urethra.  The external sphincter is a ring of skeletal muscle located close to the urethral orifice.

    C. Physiology

        1. Urine formation
            The normal daily volume of urine is 1-1 1/2 L.  When dehydrated, a person excretes a lower volume of urine.  High fluid intake and some diseases can lead to high urine volume.  Regardless of the exact urine volume, a normal person excretes 60 g of solute in the urine daily.  The normal pH of the urine is approximately 6.0 and the normal specific gravity is 1.003-1.030.  A dilute urine has a lower specific gravity than a concentrated urine.  Urine contains water and a variety of solutes.  In general, the solutes are waste materials and materials occuring in excess in the blood.

            How does the kidney produce urine?  There are two stages of urine production: the glomerular stage and the tubular stage.  I will very briefly describe these processes.  Let me remind you that the Urinary Physiology Interactive CD-ROM in the ISL provides a very thorough presentation of these topics.

             a. Glomerular stage
                 As blood flows through the kidneys, it flows from afferent arterioles into glomeruli.  As blood passes along the glomerular capillaries, the blood exerts pressure against the walls of the capillaries.  Remember, the endothelial cells lining the capillaries have fenestrations through which materials exit and enter the blood.  There are also gaps between the endothelial cells through which materials exit and enter the blood.  The pressure of the blood against the walls of the glomerular capillaries forces substances in the blood that are smaller than the fenestrations and gaps to pass out of the capillaries (see figure 26.8, 26.9).  This process is called filtration.  The substances that pass through the capillary wall are said to filter through the wall.  About the only materials that are too big to filter through the capillary walls are the plasma proteins.  Because the filtrate is hypotonic to the blood in the glomerular capillaries, some filtered substances are absorbed (attracted) back into the glomerular capillaries.  Where does the filtrate go from here?  The filtrate passes through the wall of the renal capsule into the space in the interior of the capsule.

                What determines how much of a substance will filter from the blood into the renal capsule?  1)  Blood pressure influences the amount of filtration.  The higher one’s blood pressure is, the greater the filtration will be.  2)  The concentration of substances in the blood influences how much of it will filter out.  The greater the concentration of a substance in the blood, the more of it there will be in the filtrate.  3)  Permeability of the capillaries influences how much solute appears in the filtrate.  Increased permeability allows more solutes and water to filter out of the blood.

                Approximately 1.2-1.3 L of blood flows through the two kidneys each minute.  Approximately 125 mL of filtrate total is formed each minute.  Over a 24 hour period, 180 L of filtrate are formed.  But we do not excrete 180 L of urine each day; on the average, we excrete 1-1 1/2 L per day.  What happens to the other 188 1/2- 189 L?  It is reabsorbed during the tubular stage.

            b. Tubular stage
                During the tubular stage of the formation of urine, the renal tubule and collecting duct 1) remove some of the water and solute from the filtrate and place them back in the blood (reabsorption) and 2) secrete additional solutes into the filtrate (secretion).  Both reabsorption and secretion are passive and active events.  A passive event is one in which a substance is reabsorbed or secreted along a concentration gradient; in other words, the substance moves from an area where it is in high concentration to an area where it is in low concentration.  A passive movement does not require the expenditure of energy.  An active event is one in which a substance is reabsorbed or secreted against a concentration gradient.  An active event requires the expenditure of energy.

                1.) Reabsorption
                    Most of the reabsorption of the filtrate is performed by the proximal convoluted tubule.  However, the loop of Henle, distal convoluted tubule, and collecting duct also reabsorb solute and water (see figure 26.15).  Most of the materials reabsorbed have a nutritional value.  Some of the substances that are absorbed are: glucose, Na+, K+, PO4-3, amino acids, creatine, sulfate, uric acid, ascorbic acid, acetoacetic acid, B-hydroxybutyric acid, and water.  If the reabsorption of any solute is prevented or inhibited, as is the case with some medications and diseases, water will be retained in the filtrate to prevent hypertonicity and the person will excrete a greater volume of urine.  Diuretics act by preventing the reabsorption of Na+ and, hence, of water.  Under usual circumstances, glucose and amino acids are completely reabsorbed from the filtrate.  Therefore, the urine does not normally contain glucose and amino acids.

                2.) Secretion
                    Solutes are secreted into the filtrate by the distal convoluted tubule and collecting duct (see figure 26.15).  Some of the substances secreted are K+, H+, Cl--, HCO3--, NH4+, creatinine, ethereal sulfates, and steroid glucuronides.

            c. Now, let’s examine what happens in each section of the nephron and collecting duct (see figure 26.15).

                1.) Filtration occurs across glomerular walls.  Filtrate enters the renal capsule.

                2.) The filtrate passes to the proximal convoluted tubule.  75% of the water and solute is passively and actively reabsorbed by the proximal convoluted tubule.

                3.) The filtrate passes to the descending arm of the loop of Henle.  Water passively diffuses out of the descending arm of the loop of Henle into the hypertonic blood surrounding the descending arm.

                4.) The filtrate passes to the ascending arm of the loop pf Henle.  Na+, K+, and Cl-- are actively reabsorbed by the ascending arm of the loop of Henle.

                5.) The filtrate passes to the distal convoluted tubule.  Na+ and Cl-- are reabsorbed from and H+ secreted into the filtrate by the distal convoluted tubule.

                6.) The filtrate passes to a collecting duct.  Many solutes are reabsorbed and secreted across the wall of the collecting duct (see figure 26.15).

            d. Effect of hormones on reabsorption and secretion
                There are several hormones that influence reabsorption and secretion by the nephron and collecting duct.
                    1.) ADH (antidiuretic hormone), released by the pituitary gland, increases the reabsorption of water primarily by the collecting duct.
                    2.) Aldosterone, released by the adrenal cortex, 1) increases the reabsorption of Na+ and 2) increases the secretion of H+ by the distal convoluted tubule and collecting duct.  Increased water reabsorption occurs as a consequence of the increased Na+ reabsorption.  The H+ secreted replaces the Na+ reabsorbed.  The H+ is produced by the following reaction:

            H2O  +  CO2    ®   H2CO3   ®    H+   + HCO3--

                        H+ lowers the pH of the urine (urine becomes more acid).  The HCO3-- above is absorbed into the blood where it acts as a buffer.

                    3.) PTH (parathyroid hormone) released by the parathyroid glands 1) increases the reabsorption of Ca+2 and  2) increases the secretion of PO4-3 by the distal convoluted tubule and collecting duct.

        2. Micturition (urination)
            The emptying of the urinary bladder is called micturition.  Urine constantly flows along the ureters from the kidneys into the urinary bladder.  As the bladder fills, it stretches to relieve the buildup of pressure.  However, when the volume of the bladder reaches 300-350 mL, sensory receptors in the wall of the bladder detect stretch.  A nervous impulse is sent from the receptors to the spinal cord (see figure 26.19).  The spinal cord then sends out a message along a parasympathetic nerve to the detrusor muscle of the bladder (see figure 26.19).  The message tells 1) the detrusor m. to contract and 2) the internal urethral sphincter to relax.  If the person chooses to micturate, he/she voluntarily relaxes the external urethral sphincter (see figure 26.19).  If he/she chooses not to micturate, he/she voluntarily keeps the external urethral sphincter closed.  As the bladder continues to fill with urine, pressure will continue to build up.  Eventually, the person will lose the ability to keep the external sphincter closed and the bladder will empty.