Within that single drop, there bustle great armies of cells: 250,000,000 red blood cells,

The red cells scurry through the intricate network of the vascular system, carrying oxygen from the lungs to every cell in the body and removing the carbon dioxide. So tiny are these cells that a stack of 500 of them would only be 0.04 inches [0.1 cm] high. Yet, a stack of all the red cells in your body would soar up to 31,000 miles [50,000 km]! After about 120 days of making the trip through the body 1,440 times a day, the red cell is retired. Its iron-rich core is efficiently recycled, the rest disposed of. Every second, three million red cells are removed, while the same number of new ones are made in the marrow. How does the body know that a red cell has reached the right age for retirement? Scientists are mystified. But without this system of replacing old red cells, according to one chemist, “our blood would be thick as concrete in a couple of weeks.”