Mammalian red blood cells contain hemoglobin as its primary unit for oxygen and carbon dioxide transportation. In order to increase the efficiency of transport (increase the amount of oxygen and carbon dioxide transported per red blood cell), the red blood cells' structure are designed to be biconcave. This efficient structure of the red blood cells increases it function compared to a spherical red blood cell (low surface area, i.e. hereditary spherocytosis). This is an example of the relationship between structure and function of the red blood cells. Also, the biconcave shape of the red blood cell makes it more resilient to early destruction in the small capillaries and the spleen (since the spleen is mostly responsible for red blood cell turnover, which is every 120 days).
