In human physiology, what is the average total volume of blood in the body of a healthy adult?

Introduction / Context:
Knowing the approximate volume of blood in the human body is important in physiology, medicine, and first aid. It helps in understanding shock, blood loss, and the effects of blood donation. This question asks you to recall the average total blood volume in a healthy adult.

Given Data / Assumptions:
- The person considered is a typical healthy adult, not a small child or an unusually large individual.
- Blood volume is usually expressed in litres (L).
- The options provide different ranges that overlap slightly.

Concept / Approach:
A common estimate is that blood volume is roughly 7 to 8 percent of body weight. For a 70 kilogram adult, this works out to about 5 to 6 litres. Smaller adults may have slightly less, and larger adults may have slightly more, but the standard teaching value is about 5 L, sometimes extending up to 6 L for larger individuals. Volumes much above 7 L are not typical for an average adult, and volumes below 3 L would be too low for normal physiology.

Step-by-Step Solution:
Step 1: Recall the rule of thumb that adult blood volume is approximately 7 to 8 percent of body weight. Step 2: For a 70 kg adult, calculate 0.07 * 70 = 4.9 L, close to 5 L. Step 3: Recognize that ranges like 3 to 5 L might be appropriate for much smaller or underweight individuals but are low as a general adult average. Step 4: Note that 7 to 8 L or 8 to 9 L would be unusually high and not commonly used as textbook averages for a typical adult. Step 5: Conclude that 5 to 6 litres is the best approximate average for a healthy adult.

Verification / Alternative check:
Physiology textbooks often quote blood volume figures in the range of about 4.5 to 6 L for adults, depending on size and sex, with 5 L frequently used as an average. Clinical guidelines for blood donation and transfusion also assume around 5 L of total blood for a 70 kg adult, confirming that the range 5 to 6 L is the most accurate among the options given.

Why Other Options Are Wrong:
Option B: A volume of 7 to 8 litres would indicate a much larger than average blood volume and is not considered the typical range for a standard adult.
Option C: 8 to 9 litres is even more extreme and would be far above normal physiological values for most adults.
Option D: 3 to 5 litres may be closer to the lower end and might fit some smaller adults, but as an average range, 5 to 6 litres is more accurate and widely accepted.

Common Pitfalls:
A common error is to remember only 5 litres and overlook that the question asks for a range, leading to overestimation or underestimation. Another pitfall is confusing blood volume with total body water or plasma volume; blood volume includes red cells and plasma and is smaller than total body water. Keeping the 7 to 8 percent rule in mind helps anchor your understanding.

Final Answer:
The correct answer is about 5 to 6 litres of blood in an average adult because this range closely matches the typical blood volume calculated from body weight and used in medical practice.