Question

The stroke volume of a normal human heart is approximately:

• A. \(30\) mL
• B. \(50\) mL
• C. \(70\) mL
• D. \(100\) mL

Hint:

Cardiac Output (\(CO\)) is Stroke Volume (\(70\) mL) multiplied by Heart Rate (\(72\) bpm), which equals roughly \(5040\) mL/min or \(5\) liters per minute.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Stroke volume is the volume of blood ejected by each ventricle during a single heart beat. It is a critical measure of the heart's pumping efficiency.
Key Formula or Approach:
Stroke Volume (\(SV\)) is calculated as the difference between the End-Diastolic Volume (\(EDV\)) and the End-Systolic Volume (\(ESV\)):
\[ SV = EDV - ESV \]

Step 2: Detailed Explanation:
In a healthy resting adult:
1. The End-Diastolic Volume (volume of blood in the ventricle just before contraction) is about \(120\) mL.
2. The End-Systolic Volume (volume of blood remaining after contraction) is about \(50\) mL.
3. Therefore: \( SV = 120\text{ mL} - 50\text{ mL} = 70\text{ mL} \).
This means that in every beat, each ventricle pumps out about \(70\) mL of blood.

Step 3: Final Answer:
The approximate stroke volume is \(70\) mL.

Answer 2

Rather than only computing EDV minus ESV, each numeric option can be checked against what is physiologically realistic for a resting adult heart by weighing it against typical heart rate and cardiac output figures.

1. 30 mL: A resting adult's cardiac output is about 5 litres per minute at a heart rate near 75 beats per minute. If stroke volume were only 30 mL, cardiac output would come out to roughly \( 30 \times 75 = 2250 \) mL per minute, barely 2.25 litres, far too low to meet the body's resting oxygen demand, so this value is unrealistically small.
2. 50 mL: At 75 beats per minute, a 50 mL stroke volume gives \( 50 \times 75 = 3750 \) mL per minute, still under 4 litres, below the accepted resting cardiac output range, so this undershoots normal physiology as well.
3. 70 mL: At 75 beats per minute, a 70 mL stroke volume gives \( 70 \times 75 = 5250 \) mL per minute, right in the well established resting cardiac output range of about 5 to 5.5 litres per minute, matching accepted physiological values for a healthy adult.
4. 100 mL: At 75 beats per minute, a 100 mL stroke volume gives \( 100 \times 75 = 7500 \) mL per minute, well above normal resting cardiac output and closer to the kind of output seen during moderate exercise, so this is too high for a resting value.

Checking each option this way, only 70 mL produces a cardiac output that matches normal resting physiology.

So the correct answer is 70 mL.