Final answer:
Effective filtration pressure equals zero when capillary hydrostatic pressure matches blood colloidal osmotic pressure, resulting in no net fluid movement across the capillary wall. This state usually occurs at the midpoint of a capillary bed and is essential for maintaining fluid balance within tissues.
Step-by-step explanation:
When the effective filtration pressure equals zero, it indicates a state where there is no net movement of fluid across the capillary wall. This happens when the capillary hydrostatic pressure (CHP) is balanced by the blood colloidal osmotic pressure (BCOP). At this equilibrium point, the movement of fluid out of the capillary is exactly the same as the movement of fluid into the capillary.
In the context of the glomerulus, filtration occurs when the glomerular hydrostatic pressure is higher than the luminal hydrostatic and osmotic pressures in Bowman's capsule. However, if these pressures equalize, it would result in no net glomerular filtration.
Within a capillary bed, these dynamics vary across different points. Near the arterial end, the CHP is typically higher than the BCOP, leading to net filtration. At the midpoint, the pressures equalize (CHP = BCOP), leading to no net movement, and near the venous end, the BCOP exceeds the dwindling CHP, resulting in net reabsorption of fluid into the capillary.