Urine Formation

Urine formation happens in three successive steps: ultrafiltration (glomerular filtration), selective reabsorption, and tubular secretion (augmentation).

Ultrafiltration. Because the afferent arteriole is wider than the efferent arteriole, and both are wider than the glomerular capillaries themselves, blood is forced through the glomerulus under unusually high pressure — the glomerular hydrostatic pressure (GHP), normally about 55 mm Hg. This outward-pushing pressure is opposed by two inward-pulling forces: the osmotic pressure of the blood (normally about 30 mm Hg, generated mainly by plasma proteins that cannot cross the capillary wall) and the hydrostatic pressure already present in the capsular space (normally about 15 mm Hg). The net result is the effective filtration pressure (EFP), normally about 10 mm Hg (EFP = GHP − (blood osmotic pressure + capsular hydrostatic pressure), i.e. 10 = 55 − (30 + 15)). Under this pressure, the extremely thin, fenestrated capillary walls of the glomerulus become permeable to almost everything in the blood except blood cells and large molecules such as plasma proteins — so plasma minus its proteins is forced out through the filtration slits into the capsular space. Roughly 600 ml of blood passes through each kidney every minute, and the resulting glomerular filtrate (deproteinised plasma, also called primary urine) is produced at a rate of about 125 ml/minute, or around 180 litres per day. This filtrate is alkaline and still contains urea, amino acids, glucose, pigments and inorganic ions — essentially all the useful substances of plasma, minus the proteins — and it now flows on into the proximal convoluted tubule.

Selective reabsorption. The PCT, being highly coiled, slows the flow of filtrate right down, and its cuboidal cells' microvilli give it an enormously enlarged absorptive surface — together these features make the PCT extremely efficient at reclaiming useful substances. Reabsorption here happens both actively (ATP-driven, against a concentration gradient) and passively (by simple diffusion). High-priority substances such as glucose, amino acids, vitamin C, and the ions Ca2+, K+, Na+ and Cl− are actively reabsorbed, while lower-priority substances such as water, sulphates and nitrates are passively reabsorbed. Altogether, about 99% of the original glomerular filtrate is reclaimed by reabsorption occurring in the PCT and DCT combined. …