The Urinary System and Kidney Structure

The urinary system removes waste products and excess water from the body and helps keep the composition of the blood stable. It consists of two kidneys, two ureters, the bladder and the urethra. The kidneys do the filtering work; the ureters carry the resulting urine to the bladder, which stores it; and the urethra is the tube through which urine finally leaves the body. Together these organs regulate fluid balance, salts and the removal of dissolved wastes.

Each kidney is a bean-shaped organ that filters the blood passing through it. Blood enters under pressure and is processed by about a million tiny filtering units called nephrons, where water, salts and waste are first separated from the blood. Much of the water and many useful substances are then reabsorbed back into the bloodstream, while the remaining waste and surplus water form urine. In this way the kidneys not only excrete waste such as urea but also control blood pressure, the body's acid balance and the concentration of important minerals.

The urine produced by the kidneys drains into the bladder, a muscular, expandable sac that stores it until it is convenient to release. When the bladder empties, the urine passes out through the urethra. Because the kidneys continuously clean the blood, their failure is serious, and conditions affecting them are treated with measures ranging from medication to dialysis, a process that performs the filtering function artificially when the kidneys cannot.

Frequently asked questions

What are the main organs of the urinary system?

The two kidneys, the two ureters, the bladder and the urethra. The kidneys filter the blood, and the other organs transport, store and release the urine.

What does the kidney actually do?

It filters the blood through tiny units called nephrons, removing waste and excess water as urine while returning useful substances to the bloodstream and helping regulate fluid and salt balance.

What is dialysis?

A treatment that artificially filters the blood when the kidneys can no longer do so, removing waste and excess fluid that the failing kidneys cannot.