In human blood, what is the average life span of a red blood cell (RBC) in circulation?

Introduction / Context:
Red blood cells, also called erythrocytes, are essential for transporting oxygen from the lungs to tissues and carrying carbon dioxide back to the lungs. They circulate in the bloodstream for a characteristic period before being removed and recycled. This question checks your knowledge of the approximate average life span of human red blood cells.

Given Data / Assumptions:

• The question concerns normal, healthy human RBCs in the bloodstream.
• Several possible life spans are given in days.
• No disease states or abnormal conditions are implied.

Concept / Approach:
In standard physiology, the average life span of an RBC in human circulation is about 120 days. During this time, RBCs pass repeatedly through narrow capillaries and experience mechanical stress and oxidative damage. Eventually, older cells lose flexibility and are removed from circulation by macrophages, mainly in the spleen. To answer the question, we choose the option that matches this established value.

Step-by-Step Solution:
Step 1: Recall the textbook value for RBC life span, which is approximately four months, or around 120 days.Step 2: Compare this with the options provided. Option A is 120 days, which matches the known value.Step 3: Option B, 99 days, is too low and not a standard physiological figure.Step 4: Option C, 101 days, and option D, 111 days, are also not standard accepted values.Step 5: Conclude that option A is correct because it matches the widely accepted average life span of human RBCs.

Verification / Alternative check:
A simple way to remember this fact is that RBC life span is often quoted as about four months. Considering that 30 days times 4 equals 120 days, this matches option A exactly. Medical and physiology textbooks consistently use this approximate number when discussing erythrocyte turnover.

Why Other Options Are Wrong:
Option B is wrong because 99 days is not commonly cited as an RBC life span and underestimates the time RBCs normally survive. Options C and D are also incorrect as they do not correspond to the familiar four month period used in human physiology. Using non standard values can lead to confusion in clinical and exam situations.

Common Pitfalls:
Students may confuse RBC life span with other time related values such as platelet life span or plasma turnover. Others may think of round numbers such as 100 days and guess incorrectly. Learning a few key numerical facts in physiology, including the 120 day life span of RBCs, helps in answering many multiple choice questions quickly and accurately.

Final Answer:
120 days.