**Core Concept**
When a person transitions from standing to lying down, gravitational forces affecting blood distribution in the body are reduced. This shift leads to improved venous return to the heart due to decreased venous pooling in the lower extremities and abdominal veins.

**Why the Correct Answer is Right**
In the standing position, gravity causes venous blood to pool in the legs and abdomen, reducing venous return to the heart. Upon lying down, this pooling is minimized, and venous return to the heart increases immediately. This is due to the removal of gravitational pull on the venous system and enhanced venous compliance in the supine position. The increase in venous return directly boosts cardiac output via the Frank-Starling mechanism.

**Why Each Wrong Option is Incorrect**
Option A: Heart rate does not increase immediately upon supine position change. It may rise slightly in response to increased venous return, but the primary change is in venous return, not heart rate.
Option C: Cerebral blood flow is maintained or slightly increased due to vasodilation, but it does not increase significantly or immediately compared to venous return.
Option D: Blood flow at lung apices actually increases in supine position due to improved perfusion, not decreases. Gravity-driven perfusion differences are reduced, not enhanced.

**Clinical Pearl / High-Yield Fact**
In supine position, venous return increases significantly—up to 20–30%—leading to a transient rise in cardiac output. This is why orthostatic hypotension is more common in standing than lying positions.

✓ Correct Answer: B. Venous return to hea increases immediately