Red blood cells are also called as erythrocytes. Cellular component of blood, millions of which in the circulation of vertebrates give the blood its characteristic colour and carry oxygen from the lungs to the tissues. The mature human red blood cell is small, round, and biconcave; it appears dumbbell-shaped in profile. The cell is flexible and assumes a bell shape as it passes through extremely small blood vessels. It is covered with a membrane composed of lipids and proteins, lacks a nucleus, and contains hemoglobin-a red ironrich protein that binds oxygen.
Answer the following questions in context to RBC.
(i) Biconcavity and small size are unique feature of human erythrocytes. These features help in __________ and __________ .