Erythrocytes or Red Blood Cells: 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.,,
A healthy individual has 12-16 gms of haemoglobin in every 100 ml of blood. These molecules play a significant role in transport of
respiratory gases. RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs).
Basic and important functions of RBCs:
- Delivers oxygen from the lungs to the tissues all through the body
- Facilitates carbon dioxide transport
- Acts as a buffer and regulates hydrogen ion concentration
- Contributes to blood viscosity
- Carries blood group antigens and Rh factor
BCs or erythrocytes exhibit a diameter of 7-8 µm possessing an atypical structure in comparison to most other body cells of humans. The RBC structure resembles a donut, they are biconcave wherein their periphery is thicker than their central portion. Courtesy to 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 RBC membrane is a two-dimensional structure comprising a cytoskeleton and a lipid bilayer bound together. The lipid bilayer has different types of cholesterol, phospholipids, sphingolipids, and integral membrane proteins like the glycophorin.
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.
Life cycle of Erythrocytes or Red Blood Cells
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.
Stages of Erythrocytes or Red Blood Cells
|Stage of Erythropoiesis||Corresponding events|
|Proerythroblast||Initiation of the process of haemoglobin synthesis|
|Early normoblast||Disappearance of nucleoli|
|Intermediate normoblast||Stage where haemoglobin starts to appear|
|Late normoblast||Disappearance of nucleus|
|Reticulocyte||Formation of reticulum
From the site of production, the cell enters capillary
|RBC matures||Distinct donut shaped biconcave cell achieved
Disappearance of reticulum,
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By Team Learning Mantras