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.