Without an efficient disposal system, poisonous wastes would collect in body tissues. In fact, several body systems provide exits for unwanted by-products of the processes of living. Lungs remove carbon dioxide and some water and heat. The skin gives off heat and water with some salts and urea, a product of protein breakdown. The digestive tract expels feces containing indigestible food, some salts, bile pigments, and water. But most excretion removal of the products of metabolism—involves the kidneys, ureters, bladder, and urethra, which combine to form the urinary system.
The kidneys continuously filter the blood. A network of tubules then process the filtrate. Some water is reabsorbed, along with electrolytes such as potassium. Acids, waste products, excess salt, and excess water are disposed of in the urine. The amount of water and electrolytes that are reabsorbed varies as the kidneys work to maintain a healthy balance in the body.
Kidneys work astonishingly hard. Each minute they process about 2.75 pints (1.3 liters) of blood one-quarter of the amount pumped out by the heart in this time. All blood travels through the kidneys nearly 20 times every hour. Fifteen times a day they purify the body’s entire fluid contents handling a total of about 50 gallons (190 liters). Most of the fluid is reabsorbed, but approximately one-thousandth of the total volume passes out of the body as urine.
The kidneys are also important because they regulate several hormone systems. For example, the kidneys produce a hormone called erythropoietin. This hormone travels via the bloodstream from the kidney to the bone marrow, where it stimulates the production of red blood cells. Diseased kidneys often do not produce enough erythropoietin and therefore people with kidney disease frequently have a low red blood cell count.
The kidneys also initiate the renin-angiotension system. In this system, renal tubular cells produce an enzyme called renin in response to perceived low blood pressure. Renin causes the body to retain more salt, which in turn causes the retention of fluids. This raises blood pressure. Renin also stimulates the production of a chemical substance called angiotensin, which causes blood vessels to constrict, and this also raises blood pressure. The kidneys usually stop producing renin after blood pressure reaches a certain level. However, something interferes with this system in some people. In such cases blood pressure remains elevated and hypertension results.
Remarkably enough, if one kidney is diseased or damaged, the other usually copes adequately.
From the outside, kidneys resemble a pair of purplish-brown beans the size of a man’s fist. Each weighs about 5 ounces (140 grams). They lie on either side of the backbone, their concave sides facing inward. Cut open, a kidney reveals two major areas: a pale outer layer called the cortex, and a dark inner mass, the medulla. The cortex is made mostly of blood filtration units venal corpuscles and tubules. The medulla contains tubules (loops of Henle) that collect the dilute filtrate and reabsorb most of the water, and the collecting ducts for the final concentrated filtrate that becomes urine. Blood for filtration flows into a kidney from the renal artery, which divides and subdivides into tiny branches. Treated blood leaves the kidney through a network of small veins that feed into the large renal vein.
How the blood is filtered
About a million blood filtration units, or nephrons, are packed into each kidney. Each nephron is a coiled tubule with a loop, and measures 1—2.5 inches (2.5—6 centimeters). The head of the nephron is in the cortex; it leads ultimately to a collecting duct that passes down through the medulla before opening into a large collecting area in the renal pelvis at the kidney’s core. Each nephron’s outer end (in the cortex) forms a double-walled cup known as a Bowman’s capsule. This envelops a bulging knot of capillaries called a glomerulus. Blood driven through the glomerulus forces small molecules out through the capillary walls into the Bowman’s capsule. Blood cells and protein molecules too large to pass through the filter remain in the capillaries. Filtration can occur at great speed because each kidney’s glomeruli have an overall area of approximately a quarter of the total surface area of the body. The resulting filtrate includes water and dissolved sugar, salts, and urea.
In this way, the nephron removes impurities from blood, but leaves the latter too concentrated. So water and other valuable substances are returned to the blood as the filtrate trickles through the nephron’s convoluted tubule, around a hairpin turn (the loop of Henle), and up again toward the cortex. Throughout this journey, salts, sugars, and even some urea filter out of the tubule and back into nearby capillaries.
By the time the fluid has passed through all the nephron tubule and on to a collecting tube and then into the hilum, the kidney’s main collecting area in the renal pelvis, it has been converted into urine, a concentrated urea solution with some salts and other wastes, which must be removed from the body.
Ureters, bladder, and urethra
From the kidneys, drops of urine continuously run down through two narrow, muscular tubes called ureters, each about 10 inches (25 centimeters) long. The urine collects in a muscular storage bag, the bladder. The rate at which urine leaves the kidneys depends on the amount of water in the body. If the body is dehydrated, the kidneys release no more than a cupful of concentrated urine daily. If the body is abundantly supplied with water, daily output can rise to more than 5.25 gallons (20 liters) of very dilute urine. Urine normally leaves each kidney at a rate of about one drop a minute. Contracting smooth muscles in the ureter’s wall squeeze the urine down into the bladder, which is walled by thick layers of smooth muscle, with a ring of skeletal muscle the external urethral sphincter around the narrow outlet at its base.
Emptying the bladder involves a spinal reflex action that makes the bladder contract, and a consciously directed order from the brain relaxing the external sphincter. Children gain bladder control only when they are old enough to learn to master both muscular actions. However, even adults cannot retain urine when the bladder is overly full.
During urination, also known as micturition, urine from the bladder leaves the body through a tube or channel called the urethra. The urethra is longer in men than in women because it passes through the prostate gland and the penis.