Question

The diameter of a human red blood cell is
(a) 15 μm
(b) 8.2 μm
(c) 2 μm
(d) 50 μm

Answer 1

Hint: The greatest diameter of circulating platelets is 2 μm while the average diameter of an erythrocyte is about four times the diameter of a platelet. RBCs are mainly disc-shaped biconcave cells.

Complete answer:
Human red blood corpuscle has a diameter of 8.2 μm. The thickness of an RBC varies from 2.2 μm at the periphery to 1 μm at the centre.
- Blood mainly consists of formed elements (blood cells) and plasma (liquid portion of blood).
- Plasma consists of serum and fibrinogen.
- The formed elements of blood cells are RBC (erythrocytes), WBC (leukocytes) and platelets (thrombocytes).
- They do not contain a nucleus, ribosomes, endoplasmic reticulum (ER), mitochondria, centriole etc.
The normal size of an RBC:
- Diameter is in the range of 6.2 to 8.2 μm.
- The difference in its thickness as stated in the hint is due to its biconcave shape.
Advantages of biconcavity:
- As the central thickness is not more than 1 μm, oxygen does not have to travel great distances for diffusion.
- While passing through minute capillaries, these cells can squeeze through the capillaries easily.
- Large surface area is provided for absorption of oxygen.
- A condition where biconcavity is lost is known as spherocytosis.
- Rouleaux formation seen in red blood cells is responsible for the property of ESR.
So, the correct answer to this question is 8.2 μm.

Note: Rouleaux formation: When taken out of the blood vessel, the red blood cells pile up one above another, like a pile of coins. This property of RBC is known as the rouleaux formation.
- ESR stands for Erythrocyte Sedimentation rate.