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.

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:

where:

• $HR$ = heart rate (beats per minute, bpm)
• $SV$ = stroke volume (mL per beat)

The units are typically L/min or mL/min. For a healthy adult at rest:

• $HR \approx 72$ bpm
• $SV \approx 70$ mL/beat
• $CO \approx 72 \times 70 = 5040$ mL/min $\approx 5$ L/min

That's roughly the entire blood volume circulating once every minute.

Why "One Ventricle"?

The right ventricle pumps the same volume to the lungs, and the left ventricle pumps the same volume to the body. They work in series, so the cardiac output is the same for both — you only calculate it once. The total blood pumped by both ventricles combined would be $2 \times CO$, but that's not what the term means.

What Changes Cardiac Output?

Two levers: heart rate and stroke volume.

• Heart rate increases during exercise (up to 150–200 bpm) or decreases during sleep (as low as 40–60 bpm in athletes).
• Stroke volume depends on:
  • Preload — how much blood fills the ventricle before contraction (more stretch → stronger squeeze, up to a point)
  • Contractility — how forcefully the heart muscle contracts
  • Afterload — the resistance the heart must push against (e.g., high blood pressure increases afterload, reducing stroke volume) …