The kidneys are bean-shaped, fist-sized organs that filter the blood in the body to remove wastes and to maintain proper levels of various substances in the blood. As one might imagine, they provide a valuable function in the body, ensuring that wastes do not accumulate, potentially causing health problems.
About 20% of the blood pumped out by the heart goes directly to the kidneys via the renal artery. Once the blood enters the kidneys, it is pumped up into the nephrons, tiny structures which filter the blood. Cleaned blood is sent out from the renal vein, returning to the heart for circulation to the rest of the body, while waste products drain to the bladder through the ureters. These organs are equipped to handle a very high volume of blood every 24 hours, and each kidney can even adapt to do the job independently, in the event that one fails or is removed.
When blood enters a nephron, the nephron absorbs material that is useful to the body, allowing the rest to circulate through so that it reaches the ureter. These structures regulate the components of the blood, compensating for a large intake of salt, for example, or a reduced water intake. The goal of the kidneys is to keep the volume of water in the body constant while maintaining the composition of the blood by removing waste and keeping concentrations of various substances constant. In the process, they also keep the acid/base balance in the blood stable, regulate the body's blood pressure, maintain calcium, and stimulate the production of red blood cells.
The kidneys are quite efficient, extracting the maximum amount of value from the blood and producing surprisingly little waste. They work with the intestinal tract and the sweat glands to help remove wastes and toxins from the body, keeping the body healthy and in a state of homeostasis. However, they can break down, causing serious health problems as toxins accumulate in the blood stream, rather than being filtered out. Kidney failure can happen to the elderly and to people with renal diseases, and it requires prompt medical treatment, often including dialysis, a mechanical blood filtering treatment.
One sign that the kidneys are failing is blood in the urine, because normally they do not filter red blood cells. Doctors can also test urine to look for specific proteins and other materials which can indicate problems.