27. A healthy individual has the cardiac output of 5.5 L and heart rate of 72 beats per minute. The stroke volume of the individual is ______mL.
Stroke Volume Calculation
Introduction
The cardiovascular system continuously pumps blood throughout the body to supply oxygen and nutrients while removing carbon dioxide and metabolic waste products. The efficiency of the heart is evaluated using several physiological parameters, among which cardiac output, heart rate, and stroke volume are the most important. These parameters are closely related and are frequently tested in competitive examinations because they help assess normal cardiac function as well as various cardiovascular disorders.
Cardiac output is the total volume of blood pumped by one ventricle of the heart in one minute. It depends on two factors: the number of heartbeats per minute (heart rate) and the amount of blood ejected during each heartbeat (stroke volume). Therefore, if any two of these values are known, the third can be easily calculated using a simple physiological equation.
Correct Answer
Correct Answer: 76.4 mL (approximately 76 mL)
Detailed Explanation
The relationship between cardiac output, heart rate, and stroke volume is expressed by the following equation:
Cardiac Output = Heart Rate × Stroke Volume
To calculate stroke volume, rearrange the equation:
Stroke Volume = Cardiac Output ÷ Heart Rate
First, convert cardiac output into milliliters.
Cardiac Output = 5.5 L/min = 5500 mL/min
Now substitute the values into the formula.
Stroke Volume = 5500 ÷ 72
Stroke Volume = 76.39 mL
After rounding to one decimal place:
Stroke Volume ≈ 76.4 mL per beat
Therefore, a healthy individual with a cardiac output of 5.5 L/min and a heart rate of 72 beats per minute ejects approximately 76.4 mL of blood with each heartbeat.
Step-by-Step Calculation
| Parameter | Value |
|---|---|
| Cardiac Output | 5.5 L/min = 5500 mL/min |
| Heart Rate | 72 beats/min |
| Formula | Stroke Volume = Cardiac Output ÷ Heart Rate |
| Calculation | 5500 ÷ 72 = 76.39 mL |
| Final Answer | 76.4 mL |
Understanding the Formula
Each heartbeat ejects a fixed amount of blood from the left ventricle into the systemic circulation. This amount is known as the stroke volume. When stroke volume is multiplied by the number of heartbeats occurring every minute, the result is the total volume of blood pumped per minute, known as cardiac output.
Cardiac Output = Stroke Volume × Heart Rate
This relationship is one of the most fundamental equations in cardiovascular physiology and is widely used in both clinical medicine and physiological research.
Important Cardiovascular Parameters
| Parameter | Definition | Normal Adult Value |
|---|---|---|
| Heart Rate | Number of heartbeats per minute | 60–100 beats/min |
| Stroke Volume | Blood pumped by one ventricle in one beat | 70–80 mL/beat |
| Cardiac Output | Blood pumped by one ventricle per minute | 4–8 L/min |
| Ejection Fraction | Percentage of ventricular blood ejected during systole | 55–70% |
Relationship Between Cardiac Output, Stroke Volume, and Heart Rate
| If This Increases | Effect on Cardiac Output |
|---|---|
| Heart Rate | Cardiac output increases (within physiological limits) |
| Stroke Volume | Cardiac output increases |
| Both Heart Rate and Stroke Volume | Cardiac output increases significantly |
| Heart Rate decreases markedly | Cardiac output decreases unless stroke volume compensates |
Clinical Significance of Stroke Volume
Stroke volume is an important indicator of cardiac performance. It reflects the pumping efficiency of the ventricles and is influenced by preload, afterload, and myocardial contractility. During exercise, stroke volume increases to meet the higher oxygen demand of skeletal muscles. In contrast, conditions such as heart failure, myocardial infarction, severe blood loss, or cardiomyopathy reduce stroke volume, resulting in decreased cardiac output and impaired tissue perfusion.
Final Answer
Stroke Volume = 76.4 mL (approximately 76 mL per beat)
A healthy individual with a cardiac output of 5.5 L/min and a heart rate of 72 beats/min has a calculated stroke volume of approximately 76.4 mL per heartbeat, obtained using the formula:
Stroke Volume = Cardiac Output ÷ Heart Rate = 5500 ÷ 72 = 76.4 mL
