The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney. Blood is filtered as it passes through three layers: the endothelial cells of the capillary wall, its basement membrane, and between the podocyte foot processes of the lining of the capsule. The tubule has adjacent peritubular capillaries that run between the descending and ascending portions of the tubule. As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged (some are added, others are removed); first with the interstitial fluid outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—urine—exits: it is composed of water, metabolic waste, and toxins.
The interior of Bowman's capsule, called Bowman's space, collects the filtrate from the filtering capillaries of the glomerular tuft, which also contains mesangial cells supporting these capillaries. These components function as the filtration unit and make up the renal corpuscle. The filtering structure (glomerular filtration barrier) has three layers composed of endothelial cells, a basement membrane, and podocyte foot processes. The tubule has five anatomically and functionally different parts: the proximal tubule, which has a convoluted section called the proximal convoluted tubule followed by a straight section (proximal straight tubule); the loop of Henle, which has two parts, the descending loop of Henle ("descending loop") and the ascending loop of Henle ("ascending loop"); the distal convoluted tubule ("distal loop"); the connecting tubule, and the last part of nephron the collecting ducts. Nephrons have two lengths with different urine-concentrating capacities: long juxtamedullary nephrons and short cortical nephrons.
