Which sequence correctly describes the path of blood through the heart starting from the right atrium?

• A. Right atrium -> Right ventricle -> Lungs -> Left atrium -> Left ventricle
• B. Right atrium -> Left atrium -> Right ventricle -> Left ventricle -> Lungs
• C. Right ventricle -> Right atrium -> Lungs -> Left atrium -> Left ventricle
• D. Left atrium -> Left ventricle -> Lungs -> Right atrium -> Right ventricle

Answer: (A)

The path starts with blood entering the right atrium and follows the flow through the heart and lungs in a single, unbroken loop: from the right atrium into the right ventricle, then to the lungs for oxygenation, back from the lungs into the left atrium, and finally into the left ventricle to be pumped out to the body. This sequence reflects the separation of the pulmonary and systemic circuits and the direction of blood flow: deoxygenated blood from the body collects in the right atrium, moves to the right ventricle, goes to the lungs via the pulmonary artery, returns oxygenated to the left atrium via the pulmonary veins, and then is pumped to the body by the left ventricle.

Other sequences don’t fit because they either skip the ventricle and lungs after the right atrium, or they start with a chamber that would not be the starting point when tracing blood from the right atrium. For example, moving directly from the right atrium to the left atrium bypasses the right ventricle and lungs, and starting with the right ventricle omits the essential step of flowing through the right atrium first. Starting with the left atrium would imply a flow pattern that isn’t initiated from the right atrium.