External compression of the left renal artery with preserved perfusion pressure is explained by which arteriolar adjustment?

External compression of the left renal artery with preserved perfusion pressure is explained by which arteriolar adjustment?

• A. Decreased afferent arteriolar resistance
• B. Increased efferent arteriolar resistance
• C. Increased renal blood flow
• D. Systemic hypertension

Answer: (A)

This question hinges on how the kidney autoregulates blood flow and filtration pressure through arteriolar tone. When the left renal artery is externally compressed, the blood inflow to that kidney tends to drop. To keep perfusion pressure and glomerular filtration going, the kidney lowers resistance in the afferent arteriole by dilating it. This increases blood flow into the glomerulus and helps preserve the glomerular capillary pressure despite upstream compression. If instead the afferent arteriole constricted (increased resistance) or the efferent arteriole constricted, renal blood flow or glomerular pressure would be affected differently and wouldn’t restore perfusion pressure in the same way. The mechanism that best explains preserved perfusion pressure under upstream compression is decreased afferent arteriolar resistance.

This question hinges on how the kidney autoregulates blood flow and filtration pressure through arteriolar tone. When the left renal artery is externally compressed, the blood inflow to that kidney tends to drop. To keep perfusion pressure and glomerular filtration going, the kidney lowers resistance in the afferent arteriole by dilating it. This increases blood flow into the glomerulus and helps preserve the glomerular capillary pressure despite upstream compression.

If instead the afferent arteriole constricted (increased resistance) or the efferent arteriole constricted, renal blood flow or glomerular pressure would be affected differently and wouldn’t restore perfusion pressure in the same way. The mechanism that best explains preserved perfusion pressure under upstream compression is decreased afferent arteriolar resistance.