8.3.2—Wall thickness differences

Cardiac muscle thickness is an adaptation to the pressure and distance that each chamber must supply. Thicker muscle can generate a greater pressure when it contracts; wall thickness is not defined by the oxygen content of the blood.

• Atria — thin walls: thin muscular walls → only a short push is needed → atrial contraction generates enough pressure to move blood into the ventricles, but does not need to drive it around a circuit.
• Ventricles — thicker walls: thicker, more muscular walls → a stronger squeeze is possible → ventricular contraction raises pressure and ejects blood through the semilunar valves.
• Right ventricle — thinner than the left: blood travels from the right ventricle to the nearby lungs → a lower pressure is sufficient → its wall can be thinner while still powering the pulmonary circuit.
• Left ventricle — thickest wall: blood must travel from the left ventricle through the systemic circuit to the body → high pressure is required → the much thicker muscle generates the force for systemic delivery.
• Septum: the muscular septum separates the right and left sides → the two circuits remain distinct and blood does not mix between them.

Boundary: this card explains structural pressure demand. The timed systole/diastole and valve sequence belong to the cardiac-cycle card.

The left ventricle is thicker because it pumps farther and needs higher pressure, not because its blood is “more oxygenated”. Do not infer a cardiac-cycle time point from wall thickness alone. No image generated or bound.