Which of the following will cause a decrease in the glomerular filtration rate?
a. An increase in blood pressure.
b. An increase in hydrostatic pressure in the glomerular capillaries.
c. An increase in total blood volume.
d. Constriction of the renal afferent arteriole.
e. Constriction of the renal efferent arteriole.
The correct answer and explanation is:
The correct answer is: d. Constriction of the renal afferent arteriole.
Explanation:
Glomerular Filtration Rate (GFR) is the rate at which blood is filtered through the glomeruli in the kidneys. It depends primarily on the pressure gradient across the glomerular capillaries, which is influenced by blood flow and vascular resistance in the afferent and efferent arterioles.
- Afferent arteriole: Brings blood into the glomerulus.
- Efferent arteriole: Carries blood away from the glomerulus.
How GFR is regulated:
- Increased blood pressure (a) or increased hydrostatic pressure in glomerular capillaries (b) usually increases GFR because higher pressure pushes more plasma through the filtration membrane.
- Increased total blood volume (c) tends to increase renal blood flow and pressure, which can raise GFR, not decrease it.
- Constriction of the afferent arteriole (d) reduces blood flow into the glomerulus, lowering hydrostatic pressure inside the glomerular capillaries and thus decreasing GFR.
- Constriction of the efferent arteriole (e) causes blood to back up in the glomerulus, increasing glomerular capillary hydrostatic pressure and typically increasing GFR, at least initially.
Why constriction of the afferent arteriole decreases GFR:
- When the afferent arteriole narrows, less blood enters the glomerulus.
- This reduces the hydrostatic pressure inside the glomerular capillaries.
- Lower pressure means less force pushing plasma through the filtration barrier.
- Consequently, less fluid is filtered into the Bowman’s capsule, reducing GFR.
Summary:
- Constriction of the afferent arteriole → Decreased blood flow into glomerulus → Decreased glomerular hydrostatic pressure → Decreased GFR.
This mechanism is critical in physiological regulation of kidney filtration and can be influenced by hormones and neural signals to maintain homeostasis.