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The urinary system or renal system maintains the balance of various substances and water in the body. It consists of two kidneys, two ureters (one from each kidney) which are, tubes that drain urine from the kidneys into the bladder (a storage sac), and the urethra (the tube that transports the urine out of the body). Muscles help control the release of urine from the bladder. See DaVita: How Kidneys Work for an explanatory video and NIDDKD: The Kidneys and How They Work for more details on how the kidney functions.
The kidneys, a pair of bean-shaped organs, are located at the bottom of the ribcage in the right and left sides of the back. The kidneys receive blood from the aorta, filter it, and send it back to the heart with the right balance of chemicals and fluid for use throughout the body. The urine created by the kidneys is moved out of the body via the urinary tract.
The kidneys control the quantity and quality of the fluids within the body. They also produce: Erythropoietin (EPO), which stimulates the bone marrow to make red blood cells; Renin, which helps control blood pressure, and Calcitriol, the active form of vitamin D, which is needed to maintain calcium for teeth and bones and for normal chemical balance in the body. When the kidneys are not working properly, watse products and fluids can build up to dangerous levels creating a life-threatening situation. Among the important substances the kidneys help to control are sodium, potassium, chloride, bicarbonate, pH, calcium, phosphate and magnesium.
Although the body is equipped with two kidneys, you can function with one reasonably healthy kidney if the other is damaged or removed. However, when kidney - or "renal"- function drops below 25% (peopel with two healthy kidneys have 100% of their kidney function), serious health problems can occur, and when function drops below 10-15%, critical intervention in the form of dialysis or kidney transplantation becomes necessary to maintain life. This is called end stage renal dsease (ESRD or kidney failure).
A sudden loss of kidney function, over a few hours or days, is called acute kidney injury (AKI, formerly called acute renal failure or ARF). When kidney damage and decreased function occurs over time and/or lasts longer than 3 months, it is called chronic kidney disease (CKD). The next part of this article provides information on some of the causes of these conditions.
Last Review Date: June 3, 2020