Typically, all vertebrates have a pair of kidneys, which function on the filtration - reabsorption - secretion principle. Only in a few teleost fishes, the kidney is aglomerular (without a glomerulus) and functions on the absorption - reabsorption - secretion principle as in the malpighian tubules of insects. The functional unit of the vertebrate kidney is the nephron or the uriniferous tubule. A small fish may have only a few dozen nephrons in its kidneys; a large mammal may have several million. A mammalis nephron begins with the renal corpuscle or the Malpighian body which consists of a double-walled cup, the Bowman's capsule, enclosing a knot of blood capillaries called glomeplus.
Figure: Schematic diagram of mammalian kidney and nephoron
Blood is brought to the kidney by the renal artery which branches and sub-branches into interloper arteries and finally into the afferent arteriole which gives rise to the capillary network of the glomerulus. An efferent arteriole formed by the confluence of the capillaries takes blood away from the Bowman's capsule. The Bowman's capsule latter continues into a long convoluted tubule which is distinguished into the proximal and distal convoluted tubules respectively. The distal convoluted tubules from different nephorons join to form the collecting tubule which carry the urine into the renal pelvis from where the ureter starts. The proximal and distal tubules are present in all vertebrates, but in birds and mammals a new U-shaped hair-pin-like segment called Henle's loop is present between proximal and the distal tubules. In the following section we shall study structural variations in the vertebrate kidney.