Glomerular filtration is a passive process in which hydrostatic pressure forces fluids and solutes through a membrane. The glomeruli can be viewed as simple mechanical filters because filtrate formation does not consume metabolic energy.
The glomerulus is a much more efficient filter than other capillary beds. One reason is that its filtration membrane has a large surface area and is thousands of times more permeable to water and solutes. Furthermore, glomerular blood pressure is much higher than that in other capillary beds (approximately 55 mmHg as opposed to 18 mmHg or less), resulting in a much higher net filtration pressure. As a result of these differences, the kidneys produce about 180 L of filtrate daily, in contrast to the 2 to 4 L formed daily by all other capillary beds of the body combined.
Molecules smaller than 3 nm in diameter such as water, glucose. amino acids, and nitrogenous wastes pass freely from the blood into thr glomerular capsule. As a result, these substancesusually show similar concentration in the blood and the glomerular filtrate. Larger molecules pass with greater difficulty, and those larger than 5 nm are generally barred from entering the tubule. Keeping the plasma proteins in the capillaries maintains the colloid osmotic (oncotic) pressure of the glomerular blood, preventing the loss of all its water to the renal tubules. The presence of proteins or blood cells in the urine usually indicates a problem with the filtration membrane.
Net filtration pressure
The net filtration pressure, responsible for filtrate formation, involves forces acting at the glomerular bed. Glomerular hydrostatic pressure, which is essentially glomerular blood pressure, is the chief force pushing water and solutes out of the blood and across the filtration membrane. Although theoretically the colloid osmotic pressure in the capsular space of the glomerular capsule “pulls” the filtrate into the tubule, this pressure is essentially zero because virtually no proteins enter the capsule.
Glomerular filtration rate
The glomerular filtration rate is the volume of filtrate formed each minute by the combined activity of all 2 million glomeruli of the kidneys. Factors governing filtration rate at the capillary beds are :
- Total surface area available for filtration
- Filtration membrane permeability
- Net filtration pressure
In adults the normal glomerular filtration rate in both kidneys is 120-125 ml/min. Because glomerular capillaries are exceptionally permeable and have a huge surface area (collectively equal to the surface area of the skin), huge amounts of filtrate can be produced even with the usual modest net filtration pressure. The opposite side of this “coin” is that a drop in glomerular pressure of only 18% stops filtration altogether.
The glomerular filtration rate is directly proportional to the net filtration pressure, so any change in any of the pressures acting at the filtration membrane changes both the net filtration pressure and the glomerular filtration rate. In the absence of regulation, an increase in arterial (and glomerular) blood pressure in the kidneys increases the glomerular filtration rate.