Question

In a normal kidney, which of the following conditions would cause an increase in the glomerular filtration rate (GFR)?
(a) Constriction of the afferent arteriole
(b) A decrease in the pressure of the glomerulus
(c) An increase in the capsular hydrostatic pressure
(d) A decrease in the concentration of plasma proteins in the blood
(e) A decrease in the net glomerular filtration process

Accepted Answer

Step 1: Understand that the glomerular filtration rate (GFR) depends on the net filtration pressure, which is influenced by hydrostatic and osmotic pressures in the glomerulus and Bowman's capsule. The main equation for net filtration pressure (NFP) is:

$$ 
	ext{NFP} = P_{GC} - P_{BC} - \pi_{GC} 
$$  
where $$P_{GC} is $$the glomerular capillary hydrostatic pressure, $$P_{BC} is $$the Bowman's capsule hydrostatic pressure, and $$\pi_{GC} is $$the glomerular capillary oncotic pressure (due to plasma proteins). Step 2: Analyze how constriction of the afferent arteriole affects $$P_{GC}. $$Constricting the afferent arteriole reduces blood flow into the glomerulus, which lowers $$P_{GC}$$ and thus decreases GFR. So, option (a) would decrease GFR, not increase it. Step 3: Consider the effect of a decrease in glomerular pressure ($$P_{GC}). $$Since $$P_{GC}$$ drives filtration, a decrease in this pressure reduces the net filtration pressure and therefore decreases GFR. So, option (b) would decrease GFR. Step 4: Evaluate the impact of an increase in capsular hydrostatic pressure ($$P_{BC}). An $$increase in $$P_{BC}$$ opposes filtration by pushing fluid back into the glomerulus, reducing net filtration pressure and GFR. So, option (c) would decrease GFR. Step 5: Examine the effect of a decrease in plasma protein concentration, which lowers the oncotic pressure ($$\pi_{GC}). $$Since oncotic pressure opposes filtration, a decrease in $$\pi_{GC}$$ reduces the force pulling fluid back into capillaries, thereby increasing net filtration pressure and GFR. Therefore, option (d) would cause an increase in GFR.

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Key Concepts

Glomerular Filtration Rate (GFR)

Afferent and Efferent Arteriole Dynamics

Starling Forces in Glomerular Filtration