Kidney

Kidneys are a pair of bean-shaped organs that lie on either side of the spine in abdominal cavity on dorsal wall. The mammalian adult kidney filters the entire volume of human body’s blood, about 200 liters, every day.

 Kidneys are the main excretory organs, which help in the separation and elimination of nitrogenous waste and toxin and maintain the balance of salts and pH necessary to sustain life. Nephron is the structural and functional unit of kidneys. They are microscopic, thin, long structures comprising of glomerulus, tubules, collecting duct and peritubular capillaries. Each kidney has more than 1 million nephrons. But if these nephrons are destroyed, kidney lacks the ability of making new nephrons.

 During development, a complex dialogue mediated by molecular signals orchestrates the assembly of hundreds of thousands of nephrons from the resident progenitor cell populations. This process requires several steps but only a partial picture of the interplay of gene regulation that prompts the creation of kidney is known.

Blood enters into glomeruli through microscopic capillaries. Here, it gets filtered of waste products and fluid while essential nutrients, salts, red blood cells, proteins, and large molecules are retained in the capillaries. In addition to wastes, some useful substances are also filtered out. The filtrate is then collected in a sac called Bowman's capsule. The tubules are the next step in the filtration process. The tubules are lined with highly functional cells which process the filtrate, reabsorbing water and chemicals useful to the body while secreting some additional waste products into the tubule. Kidneys also release hormones that have important functions in the body. Erythropoietin which stimulates the bone marrow to produce red blood cells. Renin, which regulates blood volume and blood pressure. Active form of vitamin D, which regulates absorption of calcium and phosphorus from foods, promoting formation of strong bone.

Reference:

Biology Scholar's textbook, page no. 306,307.

http://en.wikipedia.org/wiki/Kidney