Wall thickness indicates pressure differences in blood flow

Steps:

• Blood exits the heart via ventricles: right to lungs (low pressure), left to body (high pressure).
• Thicker walls generate higher pressure to match circulatory demands.
• Atria receive blood and have thinner walls for lower internal pressure.
• Comparing atrial and ventricular walls highlights overall pressure variation for pulmonary vs. systemic circulation.

Why D is correct:

• Ventricular walls are thicker than atrial walls to pump blood at higher pressure to the body, while atrial contraction supports lower-pressure flow toward the lungs, per the principle that muscle thickness correlates with force/pressure output.

Why the others are wrong:

• A: Thinner walls in R and S (likely atria) confirm general structure but don't specify exit pressure differences to lungs vs. body.
• B: Valve strength affects flow direction, not pressure generation for different paths.
• C: Greater muscularity in T vs. U suggests side-specific differences but not overall atrial-ventricular pressure contrast.

Final answer: D