<P> The nephron (from Greek νεφρός--nephros, meaning "kidney") is the 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 an encompassing 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 0.8 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 foot processes of the podocytes 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 . </P> <P> Bowman's capsule has a space that collects the filtrate, called Bowman's space; the filtering capillaries of the glomerulus, and the 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 podocytes (foot processes). The tubule has five anatomically and functionally different parts: the proximal tubule, which has a convoluted section 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 collecting ducts . Nephrons have two lengths with different urine concentrating capacities: long juxtamedullary nephrons and short cortical nephrons . </P> <P> The four mechanisms used to create and process the filtrate (the result of which is to convert blood to urine) are filtration, reabsorption, secretion and excretion . Filtration occurs in the glomerulus and is largely passive: it is dependent on the intracapillary blood pressure . About one - fifth of the plasma is filtered as the blood passes through the glomerular capillaries; four - fifths continues into the peritubular capillaries . Normally the only components of the blood that are not filtered into Bowman's capsule are blood proteins, red blood cells, white blood cells and platelets . Over 150 liters of fluid enter the glomeruli of an adult every day: 99% of the water in that filtrate is reabsorbed . Reabsorption occurs in the renal tubules and is either passive, due to diffusion, or active, due to pumping against a concentration gradient . Secretion also occurs in the tubules and is active . Substances reabsorbed include: water, sodium, chloride, glucose, amino acids, lactate, magnesium, calcium, phosphate, uric acid, and bicarbonate . Substances secreted include urea, creatinine, potassium, hydrogen, and uric acid . Some of the hormones which signal the tubules to alter the reabsorption or secretion rate, and thereby maintain homeostasis, include (along with the substance affected) antidiuretic hormone (water), aldosterone (sodium, potassium), parathyroid hormone (calcium, phosphate), atrial natriuretic peptide (sodium) and brain natriuretic peptide (sodium). A countercurrent system in the renal medulla provides the mechanism for generating a hypertonic interstitium, which allows the recovery of solute - free water from within the nephron and returning it to the venous vasculature when appropriate . </P> <P> Some diseases of the nephron predominantly affect either the glomeruli or the tubules . Glomerular diseases include diabetic nephropathy, glomerulonephritis and IgA nephropathy; renal tubular diseases include acute tubular necrosis and polycystic kidney disease . </P>

Where is the blood first filtered in a nephron