Glomerular filtration is the term used for the filtration of blood in progress at any time in the glomerulus thus creating the filtrate in the nephrons. The three adaptations relating to the capillaries in the kidney that make filtration of large liquid volumes in the glomerulus possible, are:
- The hydrostatic pressure in the renal capillaries is high (higher than in other capillaries).
- The capillaries in the glomerulus have a large surface (branches and folds inside the glomerulus).
- The membrane of the capillaries are more perforated than other capillaries making them more permeable.
If required, vasoconstriction at the efferent arterioles can further increase the hydrostatic pressure gradient in the glomerulus (between the capillaries and the hydrostatic pressure in the tubule).
Because of the large surface area and permeability of the capillaries the relatively high blood pressure in the glomerulus means they are capable of filtering large volume of blood. During a day over 180 liters of filtrate is produced, while all other capillary beds in the body produce approximately 4.2 litres of fluid (which is drained by the lymphatic system). In addition to water and salts that are relatively small substances, larger molecules such as sugars, amino acids, nitrogen waste (urea) are also filtered into the nephrons but larger molecules such as cells or proteins are not filtered.