AIIMS November 2011 – MCQ 190

A is false-Fluid movement across the glomerulus is governed by Starling’s forces, being proportional to the permeability of the membrane and to the balance between the hydraulic and oncotic pressure gradients.
GFR = LpS x (Δhydraulic pressure – Δoncotic pressure)
= LpS x [(Pgc – Pbs) – s (πp – πbs)]

where Lp is the unit permeability (or porosity) of the capillary wall, S is the surface area available for filtration, Pgc and Pbs are the hydraulic pressures in the glomerular capillary and Bowman’s space, πp and πbs are the oncotic pressures in the plasma entering the glomerulus and in Bowman’s space, and s represents the reflection coefficient of proteins across the capillary wall (with values ranging from 0 if completely permeable to 1 if completely impermeable). Since the filtrate is essentially protein-free, πbs is zero and s is one. Thus,

(Eq. 2) GFR = LpS x ( Pgc – Pbs – πp)

The GFR in normal adults is approximately 95 ± 20 mL/min in women and 120 ± 25 mL/min in men.

B is true-
The glomerular capillaries are uniquely interposed between two arterioles. As a result, the Pgc (glomerular capillaries pressure) is determined by three factors: the aortic pressure; the resistance at the afferent arteriole; and the resistance at the efferent arteriole. The ability to regulate arteriolar resistances permits rapid regulation of the GFR through changes in the Pgc. Constriction of the afferent arteriole reduces both Pgc and GFR, since less of the systemic pressure is transmitted to the glomerulus; dilation of the afferent arteriole, on the other hand, enhances both of these parameters. In comparison, constriction of the efferent arteriole retards fluid movement from the glomerulus into the efferent arteriole, increasing Pgc and GFR; dilation of the efferent arteriole facilitates fluid entry into the efferent arteriole, diminishing both of these parameters.

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