How is a high blood pressure built up in a glomerulus? Why is this high blood pressure needed?

Here’s my assessment.

The how

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.

The what

The glomerular blood hydrostatic pressure (##”GBHP”##) of 55 mmHg favours filtration.

However, the solute concentration in the glomerular capillaries is greater than that on the other side of the glomerular membrane.

Thus there is a blood colloid osmotic pressure (##”BCOP”##) of 30 mmHg that opposes filtration.

The Bowman’s capsular hydrostatic pressure (##”CHP”##) of 15 mmHg in the Bowman’s capsule also opposes filtration.

The net filtration pressure (##”NFP”##) is

##”NFP” = “GBHP- BCOP – CHP” = “55 mmHg – 30 mmHg – 15 mmHg” = “10 mmHg”##

The why

The glomerular filtration rate (##”GFR”##) is directly proportional to the ##”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.