What structure is responsible for the contraction of the ventricles in the heart?

The structure that is primarily responsible for the contraction of the ventricles in the heart is the atrioventricular node (AV node). The AV node acts as a key component of the heart's electrical conduction system, which orchestrates the heartbeat. Specifically, the AV node receives electrical impulses from the sinoatrial node (SA node), which initiates the heartbeat and is located in the right atrium.

Once the AV node receives the signal, it delays it slightly to allow the atria to fully contract and push blood into the ventricles. After this delay, the AV node transmits the electrical impulse down to the ventricles through the bundle of His and Purkinje fibers, resulting in the contraction of the ventricles. This contraction is crucial for pumping blood to the lungs and the rest of the body.

The other structures mentioned have different roles: the vena cava is responsible for carrying deoxygenated blood to the heart, coronary arteries supply blood to the heart muscle itself, and the sinoatrial node acts as the primary pacemaker that initiates the heartbeat but does not directly cause ventricular contraction.