The Urinary System

(G & H. Chap 19)

The urinary system consists of the two kidneys, two ureters (leading from kidney to bladder), the urinary bladder and the urethra. The kidneys produce urine which is conveyed by the ureters to the urinary bladder, where it is stored until discharged via the urethra. The urine contains metabolic waste products, such as urea, uric acid and creatinine, certain foreign substances or their breakdown products, electrolytes, and a variable amount of water. These urinary components reflect the functions of the kidneys in excretion, electrolyte balance, acid base balance, and water balance.

General Structure

The kidney, which seems very complicated at first, is really simple and repetitious. Each human kidney consists mainly of over one million uriniferous tubules.

Each kidney is a bean-shaped organ, located in the abdominal cavity. A connective tissue capsule covers the outer surface of the kidney. The medial border of the kidney is concave and contains a hilus through which the renal vessels and nerves pass, and through which the origin of the ureter, called the pelvis, leaves. The kidney is divided into an outer cortex, and an inner medulla.

The medulla contains from 8 to 12 conical structures called pyramids. The bases of the pyramids face the cortex, and the apices face the hilus. A pyramid consists of numerous tubules and blood vessels which converge from the base toward the apex. The tissue between the pyramids is called the renal column. Although the renal columns are situated in the medulla, they are continuous with the cortex.

The cortex consists of numerous straight and convoluted tubules, blood vessels, and the renal corpuscles (or Malpighian corpuscles). Together, a pyramid, its' overlying cortical tissue, and the neighboring tissue in the renal columns, are referred to as a lobe of a kidney. The lobes of the kidney are further subdivided into lobules; each lobule consists of a central medullary ray and the surrounding cortical material between two interlobular arteries. Medullary rays project from the base of each pyramid, and consist of one or more collecting tubules, together with straight portions of several nephrons, the functional unit of the kidney.

The renal pelvis (which is the expanded origin of the ureter) divides into smaller funnel-shaped branches called the minor calyces; these collect urine from the apex of the pyramid. The portion of the pyramidal apex which projects into the minor calyx is the papilla.

Nephron

The functional unit of the kidney is the nephron. It consists of a tuft of capillaries called the glomerulus, and a urinferous tubule. The glomerulus is contained within a double layered cup, called Bowmans capsule. Together, the glomerulus and Bowman's capsule constitute the renal corpuscle (or Malpighian corpuscle). Each uriniferous tubule exits from the renal corpuscle as a continuation of Bowman's capsule. The sequential segments of the uriniferous tubule are: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, which finally opens into a collecting tubule. The average length of a uriniferous tubule in man is about 20 inches. These tubules are described more fully below.

The renal (Malpighian) corpuscle.

The renal corpuscles are the filtration apparati of the kidney, producing as much as 180 liters of plasma filtrate during a 24 hour period. Blood enters the glomerular capillaries through the afferent arteriole and exits the glomerulus through the efferent arteriole. Thus the glomerulus is an arterial portal system. The glomerulus consists of a tuft of 4-6 primary capillaries, which is surrounded by a double walled epithelial capsule. Thus the glomerulus sits in a pouch, which is Bowman's capsule. The inner layer of Bowman's capsule closely envelops the capillaries of the glomerulus, and is called the visceral layer, while the outer layer forms the wall of the capsule, and is called the parietal layer; it consists of a simple squamous epithelium. Between the two layers of Bowman's capsule is the urinary space, which receives the plasma filtrate produced by the filtration apparatus of the renal corpuscle. The production of this plasma filtrate is the first step in the process of urine filtration. Usually a complex filtration barrier prevents cells and large molecules, such as proteins from entering the urinary space.

The visceral (inner) layer of Bowman's capsule consists of modified epithelial cells, called podocytes, which envelop the capillaries with hundreds of foot processes. The foot processes of one podocyte interdigitate with the foot processes of neighboring podocytes. The narrow filtration slits between foot processes are bridged by a very thin membrane, the filtration slit membrane, which plays an important role in the filtration of plasma. Blood enters the glomerular capillaries through the afferent arteriole and exits through the efferent arteriole. The pressure in the efferent arteriole is greater than the pressure in the afferent arteriole, so that fluid leaks out of the glomerular capillaries by ultrafiltration. Proteins and other large molecules do not pass through the filtration barrier--however water and small solutes pass freely. Protein which does manage to pass through the filtration barrier is reabsorbed by endocytosis in the proximal convoluted tubule.

The kidney tubules.

As the glomerular filtrate passes through the various tubules of the kidneys, it undergoes a number of changes: 1) Many of the substances within the filtrate are reabsorbed, some partially (e.g. sodium) and some completely (e.g. glucose). 2) Certain substances (e.g. creatinine) are added to the filtrate by secretory activity of the tubule cells. 3) The volume of the filtrate is reduced substantially; and 4) the urine is made hypertonic. The loops of Henle and collecting tubules, parallel to similarly arranged blood vessels, serve as the basis for the mechanism that is instrumental in concentrating the urine.

Proximal thick segment.

This consists of the proximal convoluted tubule and the descending thick tubule, which is the first part of the descending limb of Henle. The proximal thick segment, or proximal tubule, is made up of large, simple, cuboidal cells. The apical border is covered with tall microvilli, which constitute a brush border. The apical surface of the cells contains numerous pinocytotic vesicles, which function in the endocytosis of protein from the lumen of the tubule. A Na+/K+ ATPase is localized in the basolateral membranes and is responsible for transporting Na+ ions out of these cells. Numerous mitochondria are located at the base of these cells, and provide ATP to fuel the pump. As Na⁺ is actively pumped out of these cells into the interstitial space, chloride and water follow passively, and the urinary volume is reduced by 80%. Nearly all of the filtered glucose, amino acids, ascorbic acids, HCO₃- and K+ are also actively absorbed.

Loop of Henle.

The loop of Henle includes portions of the thick proximal and thick distal tubules, but its most characteristic feature is the thin limb. The epithelium of the thin limb is simple squamous. At both the proximal and distal ends of the thin loop of Henle, there is a sudden transition to the thicker epithelium of the proximal and distal tubules. The thin limb plays an important role in the concentration of urine, mainly because of its hairpin-loop configuration, and its permeability to salts and water.

Distal tubule

The distal tubule comprises the ascending straight portion (which is also the ascending thick limb of the loop of Henle), and the distal convoluted tubule. The cells of the distal tubule are cuboidal to columnar, and are less eosinophilic than cells of the proximal tubule. The ascending straight portion of the distal tubule is specialized for the active transport of Na⁺, but the tube is impermeable to water. The removal of NaCl, and the retention of water, make the interstitial tissue hypertonic, and the contents of the tubule hypotonic. Thus the ascending distal tubule is the second most important site for the absorption of NaCl (after the proximal tubule). Several distal tubules join to a collecting tubule.

Collecting tubules

The collecting tubules are lined by cuboidal to columnar cells with pale cytoplasm and clearly demarcated borders, darkly staining nuclei, and round apices which bulge into the tubule lumen. Numerous collecting ducts open into a minor calyx, and the epithelium suddenly becomes transitional. Normally the collecting tubules are impermeable to water. However, the epithelium of the collecting ducts is responsive to antidiuretic hormone (ADH) secreted by the posterior pituitary. If water intake is limited, ADH is secreted and the epithelium of the collecting ducts becomes permeable to water, which is reabsorbed.

Juxtaglomerular apparatus.

Just outside of the glomerulus, the wall of the afferent arteriole possesses smooth muscle cells, which contain conspicuous granules: these are called the juxtaglomerular (JG) cells. Closely apposed to these JG cells is a specialized region of the distal convoluted tubule, called the macula densa (dense spot). To form the macula densa, the distal convoluted tubule curves around and makes close contact with the JG cells. The cells of the macula densa are taller, narrower and closer together than other cells in the distal tubule, and thus their nuclei appear clustered. The granules of the JG cells contain an enzyme, renin, which is released in response to a decrease in extracellular fluid volume (e.g. after significant blood loss). Renin acts on a blood protein, angiotensinogen, converting it to angiotensin I. This is converted in the lung endothelial cells to angiotensin II, which is a potent vasoconstrictor and which also stimulates the release of aldosterone from the adrenal cortex. Aldosterone acts on the distal tubule causing more sodium to be absorbed from the tubule, thereby increasing the osmolarity of the extracellular fluid. This in turn stimulates the release of ADH (antidiuretic hormone) from the posterior pituitary, which increases the reabsorption of water from the collecting ducts, thus restoring extracellular fluid volume. The macula densa probably plays a role in transmitting information about the flow of urine and its osmolarity to the JG cells, thus controlling the reabsorption of H₂0 from the collecting ducts.

Concentration and dilution of Urine.

The mammalian kidney can rid the body of water by producing a copious volume of dilute urine (e.g. after drinking a pitcher of beer) or conserve water by producing a small amount of concentrated urine (e.g. during heavy exercise or sweating). This is achieved in the loop of Henle which forms a countercurrent multiplier system that generates an osmotic gradient in the medullary interstitium. This will be discussed at length in your Physiology and Biochemistry courses.

Blood supply.

Each kidney receives a large artery, the renal artery, which enters the kidney at the hilus. The renal artery divides and each branch sends interlobar branches into the kidney substance, which travel between the pyramids as far as the junction between the medulla and cortex. At this point they turn to run along the base of the pyramids and are called arcuate arteries. Small interlobular arteries branch off at right angles and travel through the cortex towards the capsule. Interlobular arteries form the boundaries of renal lobules, which consist of a medullary ray and the adjacent cortical labyrinth. From the interlobular arteries arise the afferent arterioles, which supply blood to the capillaries in the glomeruli. The glomerular capillaries reunite to form the efferent arteriole. Some efferent arterioles nourish the uriniferous tubules in the cortex. These then drain into the interlobular veins which join the arcuate veins. Other efferent arterioles descend into the medulla alongside the loops of Henle. These are called the vasa recta (straight vessels), which nourish the tubules of the medulla, and play an important role in the countercurrents concentration of urine. These vessels then loop back towards the cortex and drain into the arcuate veins. Blood from the arcuate veins empties into the interlobular and then into interlobar veins which converge to form the renal vein, through which blood leaves the kidney.

Bladder and Urinary Passages.

Urine from all of the collecting ducts from each pyramid drains through a papilla into a minor calyx, several of which combine to form a major calyx, and then to the renal pelvis, the ureter, the bladder (where it is temporarily stored) and finally leaves the body through the urethra. All of these structures have the same general organization, namely a mucosa (epithelium plus lamina propria), a muscularis and an adventitia or serosa. The mucosa of these organs consists of transitional epithelium (except the urethra).

The ureters conduct urine from the renal pelvis to the bladder. They enter the bladder obliquely so that a valve is formed that prevents the backflow of urine. The muscle layers of the ureter, and the other urinary passages are smooth. It is arranged in bundles which are arranged into inner and outer longitudinal layers, and a middle circular layer. Waves of muscular contraction move urine down the ureters.

The muscular layers of the bladder are very thick and strong, and are not as regularly arranged as in the ureters.

Transitional epithelium of the calyces, ureters and bladder is highly impermeable to water and solutes, and can stretch enormously to contain a large volume of urine, or contract to expel it. The exit of urine from the bladder to the urethra is controlled by a combination of smooth and striated muscle systems. Smooth muscle forms a circular ring, the internal sphincter, as the urethra leaves the bladder. There is also an external sphincter of striated muscle, which is generally the method of last resort for retaining urine.

The urethra is the tube that carries urine from the bladder to the exterior. In the male, sperm and seminal fluids empty into the urethra during ejaculation.

The female urethra is short, only 3-5 cm long. It is lined with stratified or psuedostratified squamous epithelium. A lamina propria as well as inner circular and outer longitudinal muscle layers are present. The extreme shortness of the female urethra permits relatively easy access of microorganisms to the bladder, resulting in the characteristic condition called acute cystitis.

The male urethra is 18-20 cm long. The portion closest to the bladder is lined with transitional epithelium which changes to pseudostratified or stratified epithelium. Striated muscle fibers form a sphincter around the initial part of the urethra. The spongy portion of the urethra receives numerous glands of Littre that provide a mucous secretion that lubricates the epithelial lining of the urethra. _.Enlargement of the prostate in older men (benign prostatic hypertrophy) compresses the urethra, and causes difficulty with the elimination of urine.