45 Net Filtration Pressure
Three forces affect glomerular filtration rate (volume filtered per unit time); hydrostatic pressure of the blood in the glomerulus, hydrostatic pressure of the fluid in the capsular space, and colloid osmotic force of the blood in the glomerulus. Hydrostatic pressure within the glomerular capillaries promotes or enhances filtration. The other two forces oppose filtration and are: back pressure from hydrostatic pressure of the fluid within the capsule and colloid osmotic pressure caused by the plasma proteins within the glomerular capillaries. Subtracting the opposing pressures from the promoting pressure yields the net filtration pressure.
Hydrostatic Pressure – Resistance from fluid in the tubule – Osmotic Colloidal Pressure = Net Filtration Pressure
The glomerular hydrostatic pressure (i.e., the blood pressure in glomerular capillaries) is the primary force responsible for pushing water and solutes from blood across the filtration membrane. This unusually high pressure (55 mm Hg) is opposed by two forces that resist the influx of fluids: the capsular hydrostatic pressure exerted by fluid already in the glomerular capsule and the blood colloid osmotic pressure caused by proteins present in blood plasma (e.g., albumen, fibrinogen). Net filtration pressure averages 10 mm Hg (i.e., 55 mmHg −15 mmHg − 30 mmHg = 10 mmHg). This is in contrast to the 0.3 mmHg net pressure found in most capillaries of the body.
The glomerular filtration rate (GFR) is the total amount of filtrate formed by the two million renal corpuscles in the kidneys divided by time. The average GFR is 125 milliliters (4 ounces) per minute, which adds up to nearly 140 liters (50 gallons) a day. The kidneys must maintain a relatively consistent GFR to prevent homeostatic imbalances in body fluids. If the GFR is too high, needed substances will rush through the renal tubules too quickly to be completely absorbed, and they will be lost in urine. A GFR that is too low will allow waste products to accumulate in the plasma, ultimately leading to illness and death if not corrected.