Q.Describe the structure of the circulatory system and the heart, and explain the idea of double circulation (pulmonary and systemic circulation).

Cardiac Output: The Engine's Flow Rate

Think of the heart as a pump — not a pipe, but a muscular pump that pushes blood through a closed circuit. Every time it beats, it ejects a certain volume of blood. The question is: how much blood does the heart deliver to the body per minute? That's exactly what cardiac output measures.

Intuition first. If you squeeze a water bottle once and 100 mL comes out, that's one squeeze's worth. But if you squeeze it 70 times in a minute, the total water delivered in that minute is $70 \times 100 = 7000$ mL. Your heart works the same way: each beat ejects a volume (stroke volume), and the number of beats per minute (heart rate) determines the total flow per minute.

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The Precise Definition

Cardiac output ($CO$) is the volume of blood pumped by one ventricle (usually the left ventricle) per minute. It is given by:

$CO = HR \times SV$

where: …