Humans have a right and a left kidney, which are small, oval organs lying to either side of the spine. They are protected in the back by the muscles of the loins. The primary function of the kidneys is to filter metabolic waste products, excess sodium, and water from the blood and help eliminate them from the body. This is done through a process known as glomerular filtration.
Each kidney is supplied with blood by a large renal (kidney) artery, a direct branch off the aorta, the major artery supplying blood to the body in the upper abdomen. In healthy adults, the two kidneys together receive approximately 20% of the blood pumped by the heart, which translates into one liter of blood per minute to the kidneys. Every liter of blood contains approximately 50% red and white blood cells. The other 50% is plasma, 20% of which is filtered off as the glomerular filtrate. The term kidney failure generally means a loss of glomerular filtration or a reduction in glomerular filtration rate.
Glomerular filtration and tubular re-absorption
Each kidney contains approximately one million filtering units called glomeruli, in which arterial blood passes through very specialized tiny blood vessels, called capillaries. The blood pressure within the capillaries forces much of the watery component, or glomerular filtrate of the blood to exit into the space surrounding these capillaries. This area is called Bowman's space, and it forms the beginning of the kidney tubules. The cells and protein of the blood are unable to cross into Bowman's space and remain confined within the capillaries.
The glomeruli reside in the outer region of the kidney known as the cortex. After leaving the glomerulus, each of the one million or so renal tubules follows a course beginning in the cortex, descending in a loop (the loop of Henle) into the inner zone of the kidney (the medulla) and then ascending back into the cortex before joining the tubules from other glomeruli to eventually drain into the renal pelvis, a lined space within the kidney. The pelvis of the kidney in turn drains via the ureter into the bladder. Finally, the bladder drains via the urethra to the outside world, through the process of urination or micturition.
The complex course of the tubules is designed to process the glomerular filtrate so that those constituents that are needed by the body, for example most of the salt and water, are retained, while the waste substances such as urea, which is derived from protein breakdown and creatinine, derived from muscle breakdown, leave the body. During the course of this filtration process other substances, such as drugs, are secreted into the glomerular filtrate along the tubules, so that larger quantities of toxins and waste products can be passed in the final urine. Hemodialysis or peritoneal dialysis are used when the kidneys are no longer able to filter and clean the blood.
Red blood cell production
The kidneys have other discrete functions, including the production of erythropoietin, which controls the rate of production of red blood cells. If the kidneys produce insufficient erythropoietin, anemia is the consequence.
Vitamin D production
Another function of the kidneys is to activate vitamin D. Vitamin D is derived from the diet or the effects of sunlight on the skin, but has to be converted to a final active product in the kidneys. If the kidneys can no longer perform this function, symptoms such as bone disease with bone pain or fracturing can occur.