Pre-Veterinary Small Animal Practice Test

Red blood cell lifespan is the amount of time a mature cell circulates before being removed by the spleen as it ages. Mature RBCs lack a nucleus, can't repair damage, and gradually become less deformable after weeks of circulation. The body keeps numbers steady by producing new cells in the bone marrow to replace those cleared by macrophages. In most domestic mammals, this turnover averages about 100 days (humans are often cited around 120 days, but 100 days is the standard teaching value used in many pre-vet materials). So the best choice is about 100 days; shorter times like 30 or 60 days are too brief for a typical RBC, and 180 days is longer than usual.