An experimental substance is infused intravenously at a constant rate into a healthy volunteer. The substance is known to selectively constrict the efferent arteriole in renal glomeruli. The rate of infusion is closely controlled during the experiment to allow for only mild constriction of the efferent arteriole. Which of the following changes in glomerular filtration rate (GFR) and filtration fraction (FF) is most likely to occur during the infusion of this substance?

Question

An experimental substance is infused intravenously at a constant rate into a healthy volunteer.  The substance is known to selectively constrict the efferent arteriole in renal glomeruli.  The rate of infusion is closely controlled during the experiment to allow for only mild constriction of the efferent arteriole.  Which of the following changes in glomerular filtration rate (GFR) and filtration fraction (FF) is most likely to occur during the infusion of this substance?

Quizplus · Accepted Answer

Blood circulates in the kidney along the following path: interlobar arteries→arcuate arteries→radial arteries→afferent arterioles→glomerular capillaries→efferent arterioles→peritubular capillaries→interlobar veins. Glomerular filtration rate (GFR) normally equals approximately 125 ml/min, although this number can vary between 90 to 130 by gender and between different subjects. The GFR depends on the difference in hydrostatic and oncotic pressures between the glomerular capillaries and Bowman's capsule. 11ec160b_3cf2_f77b_9249_35d9e3c17fe9_MD0001_00 Constriction of the efferent arteriole produces a significant increase in glomerular capillary hydrostatic pressure because of the reduction in glomerular blood outflow. This produces a corresponding increase in GFR. Efferent arteriolar constriction also reduces renal plasma flow (RPF). The increase in GFR along with the decrease in RPF leads to an increased filtration fraction (FF) because FF=GFR/RPF (Choice D). As RPF decreases, the slower capillary flow allows more time for the filtration of plasma across the glomerular membrane. This leads to an increased concentration of non-filterable plasma proteins within the glomerular capillaries, thus elevating capillary oncotic pressure. As efferent arteriolar constriction increases past a certain point, this increase in capillary oncotic pressure begins to oppose, and eventually overwhelm, the increase in capillary hydrostatic pressure also produced by efferent arteriolar constriction. Thus, when RPF is low secondary to severe efferent arteriolar constriction, the substantially increased capillary oncotic pressure results in an overall decrease in GFR (Choice A). (Choice B) Selective constriction of the afferent arteriole decreases plasma flow into the glomerular capillaries, reducing capillary hydrostatic pressure and thus decreasing the GFR. Constriction of the afferent arteriole results in relatively equal decreases in RPF and GFR (FF=GFR/RPF), so the filtration fraction remains unchanged. Educational objective: Selective vasoconstriction of the efferent arteriole (up to certain extent) increases hydrostatic pressure in the glomerular capillaries, and therefore increases the glomerular filtration rate. As efferent arteriolar constriction continues to increase, the glomerular filtration rate begins to decrease due to a flow-mediated rise in oncotic pressure in the glomerular capillaries. The filtration fraction always increases with increasing efferent arteriole constriction.

An Experimental Substance Is Infused Intravenously at a Constant Rate