Ultrafiltration

Ultrafiltration is the filtration of blood under high pressure in the Bowman's capsule. Small molecules are forced out of the glomerulus into the capsule, while blood cells and large proteins stay in the blood.High hydrostatic pressure in the glomerulus is created because the afferent arteriole bringing blood in is wider than the efferent arteriole taking blood out. This pressure pushes fluid out of the capillaries into the capsule, even though the plasma proteins give the blood a lower water potential.The fluid passes through three layers, which together form the filter:1. the capillary endothelium, which has pores (fenestrations) between and through its cells
2. the basement membrane, a mesh of collagen and glycoproteins, which is the actual filter: it stops all blood cells and molecules with a relative molecular mass above about 69 000, including most plasma proteins
3. the podocytes, cells lining the capsule with foot-like projections and gaps between them, through which the filtrate passes into the capsuleThe glomerular filtrate contains water, glucose, amino acids, urea, mineral ions and vitamins, at about the same concentrations as in plasma. It contains no blood cells and almost no plasma proteins.The rate at which filtrate forms is the glomerular filtration rate (GFR), about 125 cm³ min⁻¹ in a healthy adult.Worked example: in one day, 125 × 60 × 24 = 180 000 cm³ (180 dm³) of filtrate is produced. Urine output is only about 1.5 dm³ per day, so more than 99% of the filtrate must be reabsorbed.Ultrafiltration is not selective: useful substances are filtered along with waste, so they must be reabsorbed later.