Erythrocytes do not have a nucleus, mitochondria or endoplasmic reticulum etc. but are very efficient. Thus, the factor more efficient are as :● Their small size helps the red blood cell travel through the finiest of capillaries.● Absence of nucleus provides more surface area for absorbing oxygen.● Absence of mitochondria prevents oxygen utilisation by itself.● Absence of endoplasmic reticulum provides it flexibility so that it can easily pass through capillaries.