What is the GFR?
It is the ultra-filtration of plasma in the glomerulus. The glomerular filtration rate is the amount of blood filtered by the kidney’s glomerulus into the bowman’s capsule per unit of time. It is abbreviated as GFR. Its normal value is 125ml/min or 180L/24 hours. The rate of filtration is influenced by the interplay of several important forces.
THE BATTLE OF FORCES
There are two kinds of forces involved: HYDRA (hydrostatic force) and ONCA (oncotic force). These opposing forces are at war in the kidney’s glomeruli and influence the glomerular filtration rate. These are also known as starling forces.
Forces favouring the filtration:
- Glomerular hydrostatic pressure: It is an outward force favouring the filtration. Normal value is 60 mmHg.
- Colloid osmotic pressure in the bowman’s capsule: since the protein content in the filtrate is zero, its value is zero.
Here, HYDRA of glomerular capillaries and ONCA of bowman’s capsule teamed up to favour filtration.
Thus, the total outward force favouring the filtration is 60+0= 60 mmHg.
Forces inhibiting the filtration:
- Colloid osmotic pressure in the glomerular capillaries: normal value is 32 mmHg.
- Hydrostatic pressure in the bowman’s capsule: normal value is 18 mmHg.
ONCA of glomerular capillaries and HYDRA of bowman’s capsule together oppose filtration.
Thus, the total inward force inhibiting the filtration is 32+18 = 50mmHg.
Therefore, the net filtration pressure is 60 – 50 = 10 mmHg. This net filtration pressure is responsible for the filtration of 125 ml of fluid in glomeruli per minute.
Besides the starling’s forces, glomerular filtration also depends upon the filtration coefficient(k) which depends upon the permeability of glomerular capillaries and their surface area.
Hope you understood. Thank you!
-Contributed by Ariba Ali