In coronary microvascular dysfunction, which vessels regulate distribution of blood flow to match local tissue metabolism?

• A. Small arterioles (resistance vessels) and coronary capillaries
• B. Epicardial arteries
• C. Large veins
• D. Venous sinusoids

Answer: (A)

The main idea here is that how much blood reaches a specific area of the heart muscle is controlled by the microcirculation, particularly the small resistance vessels and the capillary network. These tiny arterioles can constrict or dilate to adjust resistance, and they respond to the metabolic needs of the tissue. When a region of myocardium has higher activity, signals (like adenosine, CO2, low O2) cause the arterioles to dilate, lowering resistance and increasing blood flow to that area. Capillaries can also be recruited to increase the exchange surface as needed, further supporting matching blood flow to metabolism.

Epicardial arteries are large conduit vessels that deliver blood to downstream microvasculature; their tone affects overall perfusion pressure more than the fine-tuned, local distribution driven by metabolic demand. Veins and venous sinusoids are involved in returning blood to the heart and do not regulate flow distribution to meet tissue metabolic needs.