Final answer:
Blood enters the left atrium through the pulmonary veins, which carry oxygenated blood from the lungs back to the heart. This occurs after gas exchange in the lungs and is part of the pulmonary circulation process, which finishes with the oxygen-rich blood being pumped out to the body via the left ventricle.
Step-by-step explanation:
The correct answer to the question of blood entering the left atrium is via the pulmonary veins. This is the route by which oxygenated blood returns from the lungs to the heart. After oxygen exchange occurs in the pulmonary capillaries, the blood, now rich in oxygen, travels through the pulmonary veins to the left atrium.
It is important to note that the bicuspid valve, also known as the mitral valve, is not involved at this stage as it separates the left atrium and the left ventricle, rather than being the point of entry for blood into the left atrium.
To address the other options mentioned, the bicuspid valve is incorrect in this context as it regulates blood flow within the heart, not blood entry into it. The superior and inferior vena cava carry deoxygenated blood from the systemic circuit into the right atrium, so they are not related to the left atrium's blood input.
The pulmonary artery is responsible for transporting deoxygenated blood from the right ventricle to the lungs for oxygenation and thus is the vessel that precedes the pulmonary veins in the circuit.
To summarize, the systemic circulation ends when deoxygenated blood enters the right atrium, and the pulmonary circulation concludes as oxygenated blood enters the left atrium via the pulmonary veins, before moving into the left ventricle for systemic distribution.