venae cavae

Meaning: The venae cavae (singular vena cava) are the two largest veins in the body. They return blood from the systemic circuit to the right atrium. The superior vena cava drains the head, neck, and upper limbs. The inferior vena cava drains the trunk and lower body below the diaphragm. Systemic veins are low-pressure, high-compliance vessels, so return of blood depends on skeletal muscle contraction, one-way valves in limb veins, and pressure changes during breathing.

On the MCAT: You may be asked to trace the path of a red blood cell: left ventricle → aorta → arteries → capillaries → venules → veins → venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary arteries → lungs. Other questions ask about oxygen content or pressure at a given point in the circuit. Some ask what happens to venous return and cardiac output if flow through a vena cava is obstructed. Expect a drop in right-heart filling and therefore in output.

Common confusion: Students sort vessels by oxygen content instead of direction of flow. Arteries carry blood away from the heart and veins carry blood toward it. The venae cavae carry deoxygenated blood, but the pulmonary veins carry oxygenated blood to the left atrium, and the pulmonary arteries carry deoxygenated blood to the lungs.