The efferent arteriole has a smaller diameter than the afferent arteriole. This causes a blood back-up in the glomerular capillaries. Therefore, fluid and solute are forced out from the blood and into the glomerular capsule. ... Thus there is a blood colloid osmotic pressure ( BCOP ) of 30 mmHg that opposes filtration
The afferent artery is wider than efferent artery so when the blood moves from afferent artery to efferent artery a high pressure is developed.
The glomerulus is supplied blood by the afferent arteriole, then the efferent arteriole takes away blood from the glomerulus. The afferent arteriole has a larger diameter than the efferent arteriole. The efferent arteriole, due to its smaller diameter creates some resistance to blood flow causing a blood back-up which creates a high pressure in the glomerular cavity. Therefore fluid and solute are forced out from the blood and into the glomerular capsule. Most of this filtrate is returned to the blood via the peritubular capillary bed surrounding the nephron.
The glomerular filtration rate (GFR) is directly proportionate to the net filtration pressure (NFP). The GBHP must be precisely regulated because a drop of less than 20 % will stop filtration completely. If that happens, many substances discharged in the urine will be resorbed. The maintenance of sodium balance and control of body fluid volume will be severely compromised.
The glomerular filtration rate (GFR) is directly proportionate to the net filtration pressure (NFP).