Glomerular Filtration: Net Filtration Pressure

Net filtration pressure (NFP) is the total pressure that drives filtration across the glomerular membrane. It is calculated using the formula NFP = GBHP - CHP - BCOP, where GBHP is glomerular blood hydrostatic pressure, CHP is capsular hydrostatic pressure, and BCOP is blood colloid osmotic pressure. Under normal conditions, NFP averages about 10 mmHg.

Glomerular blood hydrostatic pressure (GBHP), averaging 55 mmHg, is the blood pressure inside glomerular capillaries. It promotes filtration by pushing water and solutes through the filtration membrane into the capsular space. This is the primary driving force for the filtration process.

Capsular hydrostatic pressure (CHP), averaging 15 mmHg, is a back pressure exerted against the filtration membrane by fluid already present in the capsular space and renal tubule. It opposes filtration and reduces the net pressure available to drive fluid across the membrane.

Blood colloid osmotic pressure (BCOP), averaging 30 mmHg, arises from plasma proteins like albumin, globulins, and fibrinogen in glomerular capillaries. These proteins create an osmotic gradient that pulls water back into the capillaries, opposing the filtration of fluid across the membrane.

In certain kidney diseases, glomerular capillaries become damaged, allowing plasma proteins to enter the filtrate. This reduces blood colloid osmotic pressure in the capillaries, increasing net filtration pressure and causing more fluid to be filtered than normal, potentially leading to disorders of the urinary system.

Three main pressures determine net filtration pressure: glomerular blood hydrostatic pressure (55 mmHg) promotes filtration, while capsular hydrostatic pressure (15 mmHg) and blood colloid osmotic pressure (30 mmHg) oppose it. Together, these pressures regulate how much fluid filters from blood into the capsular space.

Normal net filtration pressure is approximately 10 mmHg. This value results from subtracting the opposing pressures (CHP and BCOP totaling 45 mmHg) from the driving pressure (GBHP of 55 mmHg), allowing a standard amount of blood plasma to filter into the capsular space.