Erythrocytes, also referred to as Red Blood Cells (RBCs) is a significant cellular component of blood. These cells circulate in the blood carrying oxygen from the lungs to all the tissues of the body. It is responsible for imparting blood with its characteristic colour. Mature erythrocytes in humans are rounded, small and biconcave, as though dumbbell-shaped. As the cell is flexible, it can reform to take up a bell shape when it passes through the super tiny blood vessels.
Structure of Erythrocytes
RBCs or erythrocytes exhibit a diameter of 7-8 µm possessing an atypical structure in comparison to most other body cells of humans. These cells resemble a donut, they are biconcave wherein their periphery is thicker than their central portion. Courtesy this feature, the total surface of the cell membrane is maximized enabling exchange of gases and their transport.
These cells are anuclear and do not have any other intracellular organelles as they are lost in erythropoiesis. There are two main structures – cytoplasm engirdled by a cell membrane.
Cytoplasm – It is filled with haemoglobin which in turn contains acidophilia causing erythrocytes to stain intense red with eosin on the samples of tissues stained with hematoxylin and eosin.
Cell membrane – This membrane is a lipid layer containing two types of membrane proteins – peripheral and integral.
The peripheral membrane proteins extend into the cytoplasm only, as they are found on the inner surface of the plasma membrane. The proteins are interconnected by several intracellular filaments which form a complex mesh-like cytoskeletal network through the inner cell membrane. This network is responsible for imparting strength and elasticity to RBCs enabling its even passage into the thinnest and smallest capillaries without any breakage/leakage.
Integral membrane proteins are innumerable, stretching throughout the thickness of the cell membrane. It binds haemoglobin serving as anchor points for the cytoskeletal network of RBCs. Additionally, they express antigens of ABO blood groups. Erythrocyte surface antigens are necessary for blood transfusions.
Red blood Cells – Function
Erythrocytes are covered with a membrane comprising proteins and lipids. While the nucleus is absent, it contains a red iron-rich protein – haemoglobin, which binds to oxygen. Additionally, red blood cells extract carbon dioxide from your body and carry it all the way to the lungs for it to be exhaled.
Red blood cells are synthesized in the bone marrow where they are usually. Their life span is approximately 120 days after which they die. The primary role of these red cells along with its haemoglobin is to pass oxygen from gills/lungs to all tissues of the body, and then carrying carbon dioxide (a by-product of metabolism) to the lungs for its exhalation.
The oxygen-carrying pigment in invertebrates is passed free in the plasma. In vertebrates, the concentration of this pigment in the red cells is more efficient which indicates the significant development of evolution. The biconcave shape of the cells enables exchange of oxygen at a steady rate over the largest area possible. Erythrocytes help in determining the type of blood group too.
Exchange of gases:
Haemoglobin in lung capillaries associates with the inhaled oxygen to form oxyhaemoglobin imparting cells with its red color. The oxygen-rich erythrocytes then pass through the arteries until they arrive at the tissue capillaries. Here the oxygen is liberated from the haemoglobin and diffuses into the body tissues. On the other hand, CO2 binds to haemoglobin forming oxyhaemoglobin imparting their color. Erythrocytes rich in carbon dioxide travel to venous blood towards the heart finally moving to the lungs. The carbon dioxide in these lung capillaries is released from the capillaries in the exchange for fresh oxygen.
Life cycle of Erythrocytes
Erythrocytes’ life cycle involves three stages – production, maturity and destruction. Through the erythropoiesis, which is the production of erythrocytes, a sub-process of haematopoiesis occurs in the red bone marrow. The initial stages of haematopoiesis lead to the creation of an erythroid stem cell known as Colony Forming Unit – Erythroid (CFU-E). It marks the beginning of this process driven by erythropoietin – hormone. These cells are found in erythroid islands in the bone marrow, where they multiply and differentiate towards mature RBCs. The process of differentiation gives rise to cells – erythrocytes, erythroblasts, proerythroblasts and reticulocytes.
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