Question

Regarding the pattern of coronary blood flow, how does flow in the right coronary artery (RCA) differ from that in the left coronary artery (LCA) during the cardiac cycle?

• A. RCA receives appreciable flow in both systole and diastole (biphasic), whereas LCA flow occurs predominantly in diastole
• B. RCA flow occurs almost entirely in diastole, while LCA flow is biphasic
• C. Both RCA and LCA are perfused only during systole
• D. Both RCA and LCA are perfused only during diastole

Hint:

Low RV pressure lets the RCA flow in systole too — biphasic; LCA is mainly diastolic.

Answer 1

The Correct Option is A

Step 1: Understand why coronary flow is phasic. During systole the contracting myocardium compresses the intramural coronary vessels, throttling flow; in diastole the muscle relaxes and flow resumes. The degree of systolic throttling depends on the intramural (chamber) pressure each artery's bed is exposed to.

Step 2: Left coronary artery. The LCA supplies the thick, high-pressure left ventricle. During systole the LV develops very high wall tension (~120 mmHg), which strongly compresses the vessels and nearly abolishes left coronary inflow. Hence LCA flow is predominantly diastolic.

Step 3: Right coronary artery. The RCA mainly supplies the thin-walled right ventricle, whose systolic pressure is low (~25 mmHg). This generates little compressive force, so the RCA continues to receive substantial flow during systole as well as diastole — giving a biphasic pattern (flow in both systole and diastole).

Step 4: Match the recall key. The annotation “d + s” (diastole + systole) refers to the RCA receiving flow in both phases due to the low RV pressure — exactly the biphasic pattern.

Step 5: Why the others are wrong. It is the LCA (not RCA) that is mainly diastolic, so option 2 is reversed. Neither artery is perfused exclusively in systole or exclusively in diastole, eliminating options 3 and 4.

Key fact: RCA flow is biphasic (systole + diastole) because RV pressure is low, whereas LCA flow is mainly diastolic because high LV systolic pressure compresses its vessels.