Identifying heart chambers by position and wall thickness

Steps:

• Examine the diagram's cross-section to locate upper and lower chambers.
• Note wall thickness: thicker walls indicate ventricles, thinner indicate atria.
• Identify right side (thinner ventricle wall) as W and left side (thicker wall) as X.
• Confirm W and X are lower chambers, distinguishing them from atria.

Why D is correct:

• In mammalian hearts, the right ventricle (W) pumps deoxygenated blood to lungs via thinner wall, while left ventricle (X) pumps oxygenated blood to body via thicker wall for higher pressure (definition of ventricular structure).

Why the others are wrong:

• A: Reverses left-right order; left ventricle wall is thicker than right.
• B: Atria are upper chambers with thin walls for receiving blood, not matching W/X positions.
• C: Reverses atrium order and confuses with lower ventricular positions.

Final answer: D