Erythropoiesis
The process of red cell synthesis is known as erythropoiesis.
In adults, it takes place in sites like bone marrow of long bones, pelvis, and sternum. Decreased oxygen delivery to tissues, stimulates secretion of erythropoietin from the kidney, which is the hormone that regulates late stages of erythropoiesis. The early stages are regulated by interleukin-3 and stem cell factor.
Erythropoiesis starts from the stem cells in bone marrow. They pass several stages of cell division and differentiation, to generate normoblasts. With each of these steps, the size and cellular content decrease. Normoblasts lose their nucleus when they differentiate into reticulocytes. We call them reticulocytes , because they contain an mRNA reticulum, which codes for hemoglobin. It disappears when they mature into erythrocytes, as the last step of erythropoiesis.
Structure and Function
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On average, the mature erythrocyte measures 7.5 micrometers in diameter, and 2.5 micrometers in height. They are biconcave shaped, which gives them this pale colour appearance in the middle third, Â when you look at them under a microscope. This shape increases the effective area available for oxygen diffusion.
The shape is maintained with the support of a cytoskeleton, made of various proteins. But at the same time, they let the cell to be flexible, which allows RBCs to squeeze through tiny capillaries.
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A red cell usually contains about 270 million hemoglobin molecules. Hemoglobin is a quaternary structure protein which is made up of four peptide chains and four iron atoms. They can carry up to four oxygen molecules. When it’s bound to oxygen, it gives these cells the characteristic red color.
Adaptations
RBC’s transport oxygen and carbon dioxide. They have several unique characteristics, which helps them to carry out this function effectively. Unlike other cells in the human body, they do not contain mitochondria, which means their only means of energy production are glycolysis and HMP shunt. Although these processes are less efficient compared to aerobic respiration, they do not use oxygen, which is a huge advantage in effective oxygen transportation. If the carriers themselves used up the oxygen, tissues would get hypoxia, right? In addition, they lack nuclei and other cell organelles such as the endoplasmic reticulum, which saves a lot of energy and space, which will again be utilized for carrying hemoglobin.
Destruction
After entering the circulation, the red cells have a lifespan of 120 days. When they get old, diseased, or dysfunctional, macrophages in the reticuloendothelial system will engulf them and break them into pieces. Hemoglobin inside them is degraded into haem and globin parts. Protease enzymes, then degrade globin chains into amino acids, which gets recycled to help in protein synthesis. The haem component gets degraded into bilirubin and iron. Iron ends up as hemosiderin, and bilirubin gets metabolized in the liver, and later excreted in bile.
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