Vertebrates have two kidneys that are in the back of the abdominal cavity on either side of the aorta. Each kidney has a coat of connective tissue called renal capsule. Inner portion of the kidney is called medulla, the region between the capsule and the medulla is cortex. There are three kinds of vertebrate kidneys:
(1) Pronephros:
Pronephros appears only briefly in many vertebrate embryos and not at all in mammalian embryos. In some vertebrates pronephros is first osmoregulatory and excretory organ of the embryo (tadpoles and other amphibians larvae); in other hag fishes it remains as the functioning kidney.
(2) Mesonephros:
During embryonic development of amniotes or during metamorphosis in amphibians, mesonephros replaces pronephros. Mesonephros is the functioning embryonic kidney of many vertebrates and also adult fishes and amphibians.
(3) Metanephros:
Metanephros gives way during embryonic development to metanephros in adult reptiles, birds and mammals.
Physiological differences between these kidney types are primarily related to the number of blood filtering units they contain. Pronephric kidney contains fewer blood filtering unit than either the mesonephric or metanephric kidneys. Larger number of filtering units in the meta has allowed vertebrates to face the rigorous osmoregulatory and excretory demands of freshwater and terrestrial environments.
Metnephric kidney work:
The filtration device of metanephric kidney consists of over one million individual filtration, secretion and absorption structures called nephrons. At the beginning of nephron is filtration apparatus called glomerular capsule (Bowman’s capsule). The capsules are in the cortical (outermost) region enters and branches into fine network of capillaries called glomerulus. The walls of these glomerular capillaries contain small perforations called filtration slits that act as filters. Blood pressure forces fluid through these filters. Fluid is now known as glomerular filtrate and contains small molecules such as glucose, ions (Ca2+, PO43+), and urea or uric acid.
Because the filtration slits are so small, large proteins and blood cells remain in the blood and leave the glomerulus via efferent (outgoing) arteriole. Efferent arteriole then divides into a set of capillaries called peritubular capillaries that wind profusely around the tubular portions of the nephron. Eventually they merge to form veins that carry blood out of the kidney.
Beyond the glomerular capsule are proximal convolute tubule, the loop of the nephron (loop of Henle) and the distal convoluted tubule. At various places along these structures, glomerular filtrate is selectively reabsorbed, returning certain ions (Na+, K+, Cl-) to the blood stream. Both actively (ATP – requiring) and passive transport are involved potentially harmful compounds such as Hydrogen (H+) and ammonium (NH4+) ions, drugs and various other foreign material are secreted into the nephron lumen. In the last portion of the nephron called collecting duct, final water re-absorption takes place so that the urine contains an ion concentration well above that of the blood. Thus the filtration secretion and re-absorption activities of nephron do not simply remove wastes. They also maintain water and ion balance, homeostasis.
Mammalian and to a lesser extent avian and reptilian, kidneys can remove for more water from the glomerular filtrate than can the kidneys of amphibians. For example human urine is four times as concentrated as blood plasma, a camel’s urine is eight times as concentrated, a gerbil’s is 14 times as concentrated and some desert rates and mice have urine more than 20 times as concentrated as their plasma. This concentrated waste enables then to live in dry or desert environments where little water is available for them to drink. Most of their water is metabolically produced from the oxidation of carbohydrates fats and proteins in the seeds that they eat.
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