Blood is a specialized body fluid. It has four main components namely plasma, white blood cells, platelets, and red blood cells. The primary function of the blood includes

  1. Transportation of oxygen and nutrients to the lungs and tissues

  2. Formation of blood clots via clotting factors

  3. Mounting an immune response via white blood cells

  4. Transportation of waste products to liver and kidneys for excretion

  5. Regulation of body temperature



  1. Blood makes up 7% of the weight of the human body.

  2. There are 150 Billion red blood cells in one ounce of blood.

  3. There are 2.4 Trillion red blood cells in one pint of blood.

  4. The human body manufactures 17 million red blood cells per second. If stress precipitates the body can produce 7 times that amount.

  5. A red blood cell is 7 microns in size.

  6. Red blood cells have a life span of 120 days.



The red blood cells(RBC) are highly specialized cells, approx 7.8 µm in diameter and in the form of biconcave disc, a shape which provides a large surface to volume ratio for transportation of gases(CO2 and O2). When blood is centrifuged the volume of packed red cell range between 42 and 54 percent in men and between 37 and 47 percent in women. The normal red cell count ranges between 4 million and 6 million per cubic millimeter.

The RBC membrane is composed of lipids, proteins, and carbohydrates in an organized manner. The red blood cell undergoes distortion when passing through small passages but once the stress is removed it springs back into shape. The RBC is impermeable to the hemoglobin inside but is permeable to water, oxygen, carbon dioxide, glucose, and urea among other substances.

The membrane of the red blood cell has a group of molecules on its surface which confers blood group specificity to it. These molecules are antigens that are capable of producing antibodies if injected in persons lacking them.


About 95% of RBC consists of Hb which is necessary for oxygen transport. There are 4 iron atoms in each molecule of Hb which can bind to 4 atoms of oxygen. The affinity of Hb for oxygen is such that at oxygen concentration present in the lungs nearly 95% of Hb is saturated with oxygen. As it moves through the tissues differential oxygen gradient leads to the transfer of oxygen from the Hb to the tissues.



Food rich in iron helps in maintaining healthy red blood cells. Vitamin B2, B3, and B12 found in eggs, whole grains, and bananas are also required as so is Folate. However, it is to be remembered that red blood cells in mammals are anucleate, i.e. devoid of the nucleus and thus have very little reparative ability and no division ability. One of the reasons for absence of the nucleus is that the RBC needs to pass through very narrow capillaries and the presence of the nucleus would have led to size constraint for the same.

If the RBC is constrained for space it can release multiple factors including ATP, Nitric oxide, and hydrogen sulfide all of which are directed to the vessel wall for dilation and relaxation, which can provide help only to a little extent.


RBC is produced via a process named erythropoiesis from stem cells to mature red blood cells in 7 days. Through this process, red blood cells are continuously produced in the bone marrow of large bones. The production is stimulated by the hormone erythropoietin synthesized in the kidney. The functional lifetime of RBC is 120 days. Aged RBC undergoes eryptosis or planned RBC death by making the membrane of RBC susceptible and recognizable to macrophages.


Problems of the red blood cells include different type of anemias and are-

  1. Iron deficiency anemia- The most common type of anemia due to low iron in the diet, sudden loss of blood, chronic loss of blood.

  2. Sickle cell anemia- An inherited disease in which RBC is half-moon shaped. The change in shape makes the cells sticky and may cause blockage in flow sometimes. The life span of sickle cells is also 1/10th of normal RBC’s.

  3. Hemolytic anemia- It occurs when RBCs are destroyed within the body by an abnormal process, for eg: due to G6PD deficiency.

  4. Fanconi anemia- Rare inherited disorder where bone marrow isn’t able to make sufficient blood products.

  5. Pernicious anemia- It is an autoimmune disorder wherein the body lacks intrinsic factor, required to absorb vitamin b12 from food.

Pernicious anemia


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