Urology => Urinary System
Urinary System, system of organs that produces and excretes urine from the body. Urine is a transparent yellow fluid containing unwanted wastes, mostly excess water, salts, and nitrogen compounds. The major organs of the urinary system are the kidneys, a pair of bean-shaped organs that continuously filter substances from the blood and produce urine. Urine flows from the kidneys through two long, thin tubes called ureters. With the aid of gravity and wavelike contractions, the ureters transport the urine to the bladder, a muscular vessel. The normal adult bladder can store up to about 0.5 liter (1 pt) of urine, which it excretes through the tubelike urethra.
An average adult produces about 1.5 liters (3 pt) of urine each day, and the body needs, at a minimum, to excrete about 0.5 liter (1 pint) of urine daily to get rid of its waste products. Excessive or inadequate production of urine may indicate illness and doctors often use urinalysis (examination of a patient's urine) as part of diagnosing disease. For instance, the presence of glucose, or blood sugar, in the urine is a sign of diabetes mellitus; bacteria in the urine signal an infection of the urinary system; and red blood cells in the urine may indicate cancer of the urinary tract.
STRUCTURE AND FUNCTION
The kidneys lie embedded in fat tissue on either side of the backbone at about waist level. Each fist-sized kidney is reddish-brown, weighs 140 to 160 g (5 to 6 oz), and is similar in shape to the kidney beans sold at the supermarket.
On the inner border of each kidney is a depression called the hilum, where the renal artery, the renal vein, and the ureter connect with the kidney (the adjective renal is from the Latin term renalis, meaning of or near the kidneys). The renal artery delivers over 1700 liters (450 gal) of blood to the kidneys each day, which these organs filter and return to the heart via the renal vein. Each kidney contains about 1 million microscopic coiled channels, called nephrons, which perform this critical blood-filtering function and produce urine in the process.
The bulblike upper portion of the kidney's nephrons filters water; urea, the nitrogen-containing breakdown product of protein; salts; glucose; amino acids, the building blocks of proteins; yellow bile compounds from the liver; and other trace substances from the blood. As this material moves through a long, looped tubule, many of these filtered materials are reabsorbed into the blood to be reused by the body to maintain normal body functions. Less than 1 percent of the water and other materials remain behind to be excreted as waste products in the urine.
These waste materials then pass from the nephrons into a funnel-shaped area called the renal pelvis. From the renal pelvis, waste trickles out of the kidney into the ureter, which is about 25 to 30 cm (10 to 12 in) long and about 0.5 cm (0.2 in) in diameter. The ureter empties into a hollow, muscular sac called the urinary bladder. A valvelike flap of tissue at the point of entry into the bladder prevents urine from flowing backward into the ureter. The urinary bladder is able to expand and contract according to how much urine it contains. As it fills with urine, the walls of the bladder stretch and become thinner, with the bladder itself lengthening to 12.5 cm (5 in) or more and holding up to about 0.5 liter (1 pt) of urine. A ringlike sphincter muscle surrounds the bladder's outlet and prevents spontaneous emptying.
As the bladder becomes full, stretch-sensitive receptors in its walls are stimulated, and the person becomes aware of the fullness. When the person is ready to urinate, or expel urine, the sphincter relaxes and urine flows from the bladder to the outside through the urethra. In females, the urethra is about 3.8 cm (1.5 in) long and is strictly a urinary passage. In males, the urethra is about 20 cm (8 in) long; it passes through the penis and also serves to convey semen during sexual intercourse.
In addition to their vital role in ridding the body of wastes through the production of urine, kidneys play important regulatory roles. They maintain water balance, ensuring that the amount of water in body tissues remains at a constant level. So, for example, if a person drinks a lot of water one day, but little water the next, the kidneys are able to adapt by regulating the water balance in the tissues. The kidneys also control calcium levels in the blood to maintain healthy bones. They aid in regulating the acid-base balance of the blood and body fluids so that all body processes can proceed smoothly. By controlling salt levels, the kidneys help regulate blood pressure. Finally, they stimulate the body to make red blood cells, the primary component of healthy blood. Properly functioning kidneys are so vital to health that if they cease to function, death follows within days.
All vertebrates dispose of excess water and other wastes by means of kidneys. The kidneys of fish and amphibians are comparatively simple, while those of mammals are the most complex. Fish and amphibians absorb a great deal of water and, as a result, must excrete large quantities of urine. In contrast, the urinary systems of birds and reptiles are designed to conserve water; these animals produce urine that is solid or semisolid.
DISORDERS OF THE URINARY SYSTEM
Urologists are physicians who specialize in treating urinary system disorders. One of the most serious disorders urologists treat is renal failure, which slows or stops the filtration of blood, causing toxic waste products to build up in the blood. Acute renal failure, which occurs suddenly, may be caused by bacterial infection, injury, shock, congestive heart failure, drug poisoning, or severe bleeding following surgery. Treatment may include drugs to address the underlying cause or to stimulate proper kidney function, blood transfusions, surgery, or, in some cases, kidney dialysis, in which the blood is mechanically filtered.
Chronic renal failure is a progressive deterioration of kidney function over a long period of time. It can be caused by diseases such as hypertension, diabetes, lupus erythematosus, acquired immune deficiency syndrome (AIDS), and a form of cancer called myeloma. If diagnosed early the degenerative process can be slowed, but not reversed, by interventions that can range from simply restricting fluid intake and protein consumption (proteins are the main source of waste products) to taking drugs to address the underlying disease that is damaging the kidneys. Some patients may go on to develop end-stage renal failure, a life-threatening condition that requires long-term dialysis or a kidney transplant.
Urinary calculi, commonly known as kidney stones, result from the gradual buildup of crystallized salts and minerals in the urine. Kidney stones can cause intense pain if they obstruct a passageway that carries urine. Usually, the stones pass through and out of the urinary tract on their own. If they fail to pass out of the body, they can be removed surgically or broken up nonsurgically by an ultrasound technique called lithotripsy.
Bacterial infections, most frequently caused by the common intestinal bacterium Escherichia coli, can occur in any part of the urinary system. Antibiotics usually conquer the infection, although recurrent and chronic infections are not uncommon. The incidence of kidney and bladder cancer has risen markedly in the last 30 years. These cancers have been linked to various causative agents, primarily cigarette smoking, abuse of analgesics, obesity, and certain industrial chemicals. Treatment typically includes removal of cancerous tissue, followed by radiation therapy.
Among the many inherited and congenital disorders of the urinary system are polycystic renal diseases, in which numerous cysts form in the kidney, reducing the amount of functioning renal tissue. Kidney dialysis or transplantation usually is necessary to avoid kidney failure and death. Hypospadias is a birth defect in which the male urinary opening is misplaced on the penis; it may be under the head of the penis or as far away as the scrotum. Surgery before the child reaches 24 months can correct the defect, permitting normal urination and, later, sexual intercourse.