HY USMLEQ#12 Renal Physiology  Step 1

Level: Step 1 Renal Physiology ( inspired by ChatGPT) 

A 29-year-old man takes part in a physiology experiment. His mean arterial pressure (MAP) is slowly increased from 90 mmHg to 140 mmHg. However, despite this increase in blood pressure, his renal blood flow and glomerular filtration rate remain constant. Which of the following mechanisms is primarily responsible for maintaining constant renal blood flow in this situation? 

 

A. Activation of the sympathetic nervous system

B. Angiotensin II-mediated efferent arteriole constriction

C. Tubuloglomerular feedback via the macula densa

D. Myogenic response of afferent arterioles

E. Increased secretion of atrial natriuretic peptide (ANP)

 

✅ Correct D. Myogenic response of the afferent arterioles 

 

Education Objective: 

The kidneys maintain relatively constant renal blood flow and glomerular filtration rate across a range of blood pressures (90 to 140 mmHg). This is due to renal autoregulation, which involves 2 mechanisms: 

1. First Mechanism: Stretch of the afferent arterioles from increased pressure leads to reflex vasoconstriction. This protects the glomerular capillaries from high pressures and keeps the flow stable. 

2. Second Mechanism: Tubuloglomerular feedback. The macula densa senses the increased NaCl in the distal tubules (from higher GFR). Send signals to constrict the afferent arterioles and reduce GFR and plasma blood flow.  Important but slower and more of a backup to the myogenic response. 

 

❌ A. Sympathetic activation: Would decrease renal blood flow, not stabilize it. 

❌ B. Angiotensin 2: Constricts the efferent arteriole, maintains GFR during hypotension. 

❌ C. Macular densa: Works in conjunction, but the primary mechanism is myogenic in this pressure range. 

❌ E. ANP: Affects sodium excretion and vascular tone, not primary in autoregulation.