Glomerular Filtration

Glomerular filtration is the first and entirely passive step of urine formation. Blood arriving at the glomerulus through the wide afferent arteriole is under an unusually high pressure - about 55 mmHg, much higher than in any other capillary bed in the body - because the afferent arteriole is wider than the efferent arteriole it drains into. This glomerular hydrostatic pressure pushes water and small dissolved substances out of the capillaries and into Bowman's capsule; it is opposed by two forces pulling back the other way, the colloidal osmotic pressure of plasma proteins (about 30 mmHg) and the capsular hydrostatic pressure of fluid already in Bowman's capsule (about 15 mmHg). The result is a net filtration pressure of about 10 mmHg (55 - (30 + 15)), which is what actually drives filtration. Because the filtration membrane bars any molecule larger than about 5 nm from crossing, the fluid that results - the glomerular filtrate - is essentially blood plasma with the large plasma proteins removed, as the table alongside this section shows: water, amino acids, glucose, urea, uric acid, creatinine and inorganic ions all pass through at almost the same concentration as in plasma, while protein concentration collapses from 80.0 to just 0.05 g/dm3. Altogether the kidneys produce around 180 litres of this filtrate every 24 hours, at a ra …