American Medical Association Family Medical Guide - American Medical Association [530]
The kidneys
Each day, the kidneys filter more than 200 quarts of blood. Blood from the renal artery first passes through the glomeruli. These tiny, globelike tangles of blood vessels in the outer part of the kidney (the cortex) filter a fluid containing nutrients and wastes from the blood. The fluid then flows into the center of the kidney (the medulla) through long, thin tubes called tubules. The tubules are surrounded by blood vessels that reabsorb the nutrients from the fluid. The filtered blood passes through a cavity called the renal pelvis, leaves the kidneys through the renal vein, and returns to the circulation. The filtered fluid, which contains waste products from the blood, continues down through the tubules, which form collecting ducts that lead into the ureters.
The male urinary tract
The bladder
The bladder is a hollow, muscular organ that can store about 1 pint of urine. Urine flows down the ureters from the kidneys to the bladder. As the bladder fills with urine, its elastic, flexible walls expand. Nerves in the bladder signal the brain when the bladder is full. When you urinate, the bladder walls contract to expel urine through the urethra. The muscle layers within the bladder walls and the sloping angle at which the ureters enter the bladder combine to act as a valve to prevent urine from flowing backward through the ureters into the kidneys.
The female urinary tract
Male and female urinary tracts
The structure of the male urinary tract differs slightly from the structure of the female urinary tract. The male urethra is about 10 inches long and provides an outlet for semen as well as urine. The female urethra is about 1½ inches long and lies, with the bladder, just in front of the reproductive organs.
Infections, Inflammation, and Injury
In a healthy person, there are usually no microorganisms in the urinary tract and urine is sterile. However, infectious agents, especially bacteria, can enter the urinary tract from outside the body by moving up the urethra into the bladder. Microorganisms can also travel through the bloodstream to the urinary tract from other parts of the body. Once the infectious agents enter the urinary tract, they can multiply and spread, causing inflammation and disrupting normal functioning. Infections of the urinary tract can be acute (short term, usually severe) or chronic (long-lasting).
Infection of the kidney is called pyelonephritis. Infection of the bladder is called cystitis. Kidney inflammation (nephritis) can occur without an infection, such as when it is caused by medication. Glomerulonephritis, inflammation of the glomeruli (the network of tiny blood vessels inside the nephrons, the basic filtering units of the kidneys), can also occur without infection.
Direct injury to the kidneys from a blow is rare because the kidneys are located at the back of the abdominal cavity and are protected by the rib cage and the fat and muscles of the back. The bladder is also well protected inside the pelvis. Direct injury to the urinary tract usually occurs along with severe injuries that require hospitalization, such as from a motor vehicle collision or a bullet wound.
Acute Pyelonephritis
Acute pyelonephritis is an infection of the kidneys that develops suddenly. The infection and the inflammation it causes affect mainly the tissue in the kidneys that contains tiny filtering units (nephrons). In most cases, the bacteria causing the infection come from the tissues surrounding the opening of the urethra (the tube that carries urine from the bladder out of the body). If the bacteria multiply, they can enter the urethra and move up through the bladder and ureter to the kidneys. Acute pyelonephritis occurs more frequently in women than in men because the rectum is closer to the urethra in women and the urethra is shorter in women, increasing the likelihood that bacteria will enter the urinary tract.
Acute