What is the average lifespan of a red blood cell in circulation?

The average lifespan of a red blood cell in circulation is approximately 120 days. This places the correct answer in the range of days. Red blood cells are produced in the bone marrow and, once released into circulation, they transport oxygen to tissues and carbon dioxide back to the lungs for exhalation. Over time, they undergo changes that signal they need to be removed from circulation, primarily by the spleen. The relatively short lifespan is a crucial aspect of their function, as the body continually produces new red blood cells to replace those that are lost or have expired. The other choices — months, years, and hours — do not accurately reflect the biological processes involved in red blood cell production and degradation.