Kidney and Urinary Tract Disorders

Normally, a person has two kidneys. The rest of the urinary tract consists of the following:

Kidney and urinary tract disorders can involve one or both kidneys, one or both ureters, the bladder, or the urethra, and in men, the prostate, one or both testes, or the epididymis. Problems with the male reproductive system often manifest as persistent erection, scrotal pain, scrotal swelling, or blood in the semen.

Urinalysis, the testing of urine, may be necessary in the evaluation of kidney and urinary tract disorders and can also help evaluate bodywide disorders such as diabetes or liver problems. A urine sample is usually collected using the clean-catch method or another sterile method. For example, a method to obtain an uncontaminated urine sample involves passing a catheter through the urethra into the bladder.

The blood flow to the kidneys needs to be intact for the kidneys to function properly. Any interruption of or reduction in the blood flow can cause kidney damage or dysfunction and, if long-standing, increased blood pressure (hypertension). When blood flow in the arteries supplying the kidneys is completely blocked, the entire kidney or a portion of the kidney supplied by that artery dies (kidney infarction). Kidney infarction can lead to the inability of the kidneys to process and excrete the body's waste products (kidney failure).

About 80,000 new cases of bladder cancer are diagnosed every year in the United States. According to 2019 estimates, more than 17,500 people die of bladder cancer every year. About 3 times as many men as women develop bladder cancer.

One or more fluid-filled sacs (cysts) sometimes develop in the kidneys without causing serious kidney problems.

Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly.

The kidneys filter and cleanse the blood. They also maintain the body’s balance of water, electrolytes (such as sodium, potassium, bicarbonate, and chloride), and nutrients in the blood.

The kidneys constantly produce urine, which flows through two tubes (the ureters) to the bladder, where urine is stored (see figure Viewing the Urinary Tract). The lowest part of the bladder (the neck) is encircled by a muscle (the urinary sphincter) that remains contracted to close off the channel that carries urine out of the body (the urethra), so that urine is retained in the bladder until it is full.

This chapter includes a new section on COVID-19 and acute kidney injury (AKI).

Each kidney contains about 1 million filtering units (glomeruli). The glomeruli are made up of many microscopic clusters of tiny blood vessels (capillaries) with small pores. These blood vessels are designed to leak fluid from the bloodstream into a system of miniature tubules. The tubules secrete and reabsorb chemicals and substances from the fluid to cause it to become urine. The urine then drains from the tubules into larger and larger tubes until it leaves the kidney. Normally this filtering system permits fluid and small molecules (but almost no protein or blood cells—Asymptomatic Proteinuria and Hematuria Syndrome) to leak into the tubules. Diseases that affect the kidneys can be divided into three categories based on the way they affect different parts of the kidneys:

Urinary tract obstruction is a blockage that inhibits the flow of urine through its normal path (the urinary tract), including the kidneys, ureters, bladder, and urethra.

Stones (calculi) are hard masses that form in the urinary tract and may cause pain, bleeding, or an infection or block of the flow of urine.

In healthy people, urine in the bladder is sterile—no bacteria or other infectious organisms are present. The tube that carries urine from the bladder out of the body (urethra) contains no bacteria or too few to cause an infection. However, any part of the urinary tract can become infected. An infection anywhere along the urinary tract is called a urinary tract infection (UTI).