How does blood flow through the heart start?

Blood flow through the heart begins in the right atrium, where deoxygenated blood from the body's systemic circulation enters after traveling through the superior and inferior venae cavae. This blood then fills the right atrium, which serves as a receiving chamber. As the atrium contracts, blood is pushed through the tricuspid valve into the right ventricle.

From there, the right ventricle pumps the blood to the lungs through the pulmonary artery for oxygenation. This sequence is crucial for maintaining proper circulation and ensuring that oxygen-poor blood is sent to the lungs before it is circulated to the rest of the body.

The other options describe different pathways of blood flow but do not represent the starting point. For example, blood moving from the left atrium to the left ventricle occurs after the oxygenated blood has returned from the lungs, while blood flow from the right ventricle to the body represents the exit of the blood after it has already traveled through the heart. The flow from the lungs to the left atrium reflects the subsequent step after the lungs have oxygenated the blood.