Which structure enters the left atrium posteriorly?

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The entering structure is the four pulmonary veins. Oxygenated blood returns from the lungs and drains into the left atrium via these veins, which open into the posterior wall of the atrium. This arrangement places the left atrium to receive the lung venous return directly from the back of the chamber, allowing blood to flow smoothly into the left ventricle for systemic circulation. In contrast, the superior and inferior vena cavae bring deoxygenated blood into the right atrium, and the pulmonary artery carries blood away from the right ventricle to the lungs, not into the left atrium. So, the only structure that enters the left atrium posteriorly is the pulmonary veins.