Red Blood Cells and Its Functions

Red blood cells are the more numerous type of cells present in the blood. They are also called as RBCs, red blood corpuscles, erythrocytes, etc. The red blood cells are red in color due to the presence of hemoglobin in their cytoplasm. An adult human male has about 5-6 million per cubic millimeter and has 20 to 30 trillion red blood cells at any given time in circulation. An adult human female has slightly less number such as 4-5 million erythrocytes per cubic millimeter. ​The erythrocytes or RBCs moves by the push of the blood flow in arteries and blood flow pull in the veins and squeezing through the capillaries. The flexibility of RBCs helps then to squeeze through smaller blood vessels and capillaries.

Photo credit: fi.edu 

Red Blood Cell Structure

The mature red blood cells are are flexible and biconcave shaped cells with no nucleus or organelles. The biconcave shape of the red blood cells provides them with good surface area compared to the volume that helps diffusion of gases. RBCs have a diameter of about 7.5 microns and the thickness at the periphery is 2 microns and at the center it is 1 micron. RBCs do not contain cellular structures such as nucleus, mitochondria, ribosomes, endoplasmic reticulum, or centrioles.

The cytoplasm of RBC is fully composed of hemoglobin and it is interwoven in the structure of the RBCs. About 95% of the dry weight of the RBC is due to the presence of hemoglobin. The hemoglobin is an iron containing compound that helps in the transport of oxygen and carbon dioxide through the blood. The presence of the hemoglobin gives the red blood cell its red color. The cell membrane of the erythrocytes contains lipids and proteins. The membrane of the red blood cells play important role in regulating their surface deformability, flexibility, adhesion, and immune recognition. These functions depend on the properties of the cell membrane. The red cell membrane is composed of 3 layers, the exterior glycocalyx that is rich in carbohydrates, a lipid bilayer, which contains transmembrane proteins, and the membrane skeleton which is a structural network of proteins located on the inner surface of the lipid bilayer. The lipid bilayer is composed of cholesterol and phospholipids.​

Photo credit: virtualmedicalcentre.com

​Red Blood Cell Formation

RBCs have a lifespan of 120 days in the blood stream. There is a huge loss of RBCs daily, and so to keep up with the number, there must be sufficient number of RBCs produced daily. Red blood cells are developed from 2 types of tissues:

1) Myloid tissue which is the red bone marrow, that produces RBCs. 

2) Lymphoid tissues include lymph nodes, the thymus, and the spleen.​

The formation of blood cells starts from the 3rd week of intrauterine life, which is called erythropoiesis. The most primitive form of RBC derives from pluripotent stem cells. The pluripotent stem cells divide and differentiate to produce progenitor cells. From the progenitor cells, pronormoblast develops, which is converted into early normoblast, intermediate normoblast, and the late normoblast to form reticulocyte. The reticulocyte later is turned into a matured erythrocyte or the RBC. The factors that influence the formation of red blood cells are hematopoietic growth factors such as erythropoietin, interoleukins, stem cell factor, colony stimulating factor, thrombopoietin, some vitamins, and minerals iron and copper.

A normal increase in RBC production occurs in severe hypoxemia and conditions of less oxygen in the air.

Functions of RBC

RBCs collect oxygen from the lungs and release it to the cells and tissues whereas they take up carbon dioxide from the cells and tissues and release it to the lungs. Hemoglobin present in the red blood cells helps in the transport of oxygen and carbon dioxide to and fro between the lungs and the cells.

​When there is a shear stress in the constricted blood capillaries, the RBCs release ATP which makes the vessel wall to relax and dilate to allow normal flow of the blood.

Due to the deoxygenation of the hemoglobin molecules the red blood cells release S-nitrosothiols that acts to dilate the blood vessels directing more blood to the areas of oxygen depletion.

Exposure of RBCs to physiological levels of shear stress activates them to synthesize nitric oxide enzymatically thus contributing to the regulation of the vascular tone.

Erythrocytes produce hydrogen sulfide that helps to relax the blood vessel walls.

RBCs helps in playing a role in the immune mechanism of the body. When lysed by the pathogenic bacteria, the hemoglobin in the RBCs releases free radicals that breaks down the cell wall of the pathogenic bacteria and thereby killing it.

Diseases Involving the Red Blood Cells

Anemia is a disease characterized by a low oxygen carrying capacity of the blood due to decrease in number of red blood cells or abnormality of the red blood cells. There are different types of anemia such as:

Iron deficiency anemia occurs due to the decreased intake or decreased absorption of iron from the diet.

Sickle cell anemia is a genetic disease due to the presence of abnormal hemoglobin molecules.

Hemolytic anemia is characterized by excessive breakdown of the red blood cells.

Thalassemia is a genetic disease that causes an abnormal ratio of hemoglobin.

Pernicious anemia is a disease where there is a decreased absorption of vitamin B12, which is necessary for the production of hemoglobin.

Spherocytosis is a disease that causes a defect in the red blood cell's cytoplasm where the RBCs become spherical in shape.

Aplastic anemia is disease caused by the inability of the bone marrow to produce blood cells.

Polycythemia is a disease characterized by an abnormal increase in number of red blood cells.

Hemolytic transfusion reaction is the destruction of the donated red blood cells after a blood transfusion. It is mediated by the antibodies present in the recipient's blood.

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