Red blood cells, known as erythrocytes, are the most abundant cells in the bloodstream, and they each contain about 250 million molecules of hemoglobin, a very large quaternary protein that is essential for carrying oxygen throughout the body. red blood cells survive for about 120 days and, unlike all other blood cells, they lack a nucleus and most organelles. a specific type of blood stem cell called a hemocytoblast is crucial in the developmental pathway of all blood cells, including red blood cells, which ultimately leads to the variety of mature blood cells in a human circulatory system. which of the following is the most reasonable explanation for the drastic structural