Kidneys are bean-shaped organs, each about the size of a fist. They are located near the middle of the back, just below the rib cage. The kidneys are sophisticated reprocessing machines. Every day, the kidneys process about 200 quarts of blood to sift out about 2 quarts of waste products and extra water. The waste and extra water become urine, which flows to the bladder through tubes called ureters.
Structure and Function
The primary functions of the kidneys are osmoregulation and filtration. That is, the kidneys filter small molecules and ions from the blood and support organismal homeostasis by balancing the concentration of water and salts in the bloodstream. The kidneys also secrete hormones which regulate things like thirst and blood pressure.
Each kidney is composed of ~1,000,000 functional filter structures called nephrons. Each nephron contains a filter head (glomerulus) with re-absorptive tubules draining into a collecting duct.
Blood passing through the capillaries in the glomerulus passes over a sieve formed by specialized cells called podocytes. The filtrate which passes through the sieve then travels through the tubules where water and salts still needed by the body are reabsorbed into the bloodstream, concentrating the urine. The urine then passes into the collecting duct system where it is further concentrated as more water is reabsorbed. Finally, the urine passes through a papillary duct into the renal calyces, then into the pyelum and out through the ureter and into the bladder.
The re-absorptive machinery in the kidneys is so efficient that only 1 liter of urine is produced for every ~100 liters of primary filtrate produced in the glomerulus. This efficiency is important physiologically because it allows the body to conserve water and salts. Diabetes insipidus is a disease state in which the ability of the kidneys to reabsorb water is somehow compromised, usually due to a deficiency in the hormone vasopressin (AVP).