Table of Contents
Blood formation begins in the womb; the placenta serves as a source of connection between the mother and her baby, and though fetal and maternal blood does not mix, blood is continually purified and circulated during pregnancy. Blood cells are continually produced and destroyed in the body until a person dies.
The process by which red blood cells are formed and mature is called erythropoiesis. It occurs in different organs in the womb, and after birth.
In fetal life, blood production is divided into three stages; the first stage is maintained for the first two months, and here blood is produced in the mesenchyme of the yolk sac, this is called the mesoblastic stage.
In the second stage (hepatic stage) which begins from the third month, the liver, spleen, and lymphoid tissue begins to produce red blood cells.
The last stage known as the myeloid stage occurs in the third trimester of pregnancy, and at this point, red cells are produced in the liver, and bone marrow.
Blood production continues in the bone marrow for up to twenty years of age. The bone marrow of long bones such as the humerus, the radius and ulna, femur, tibia and fibula, etc, and in flat bones like the scapula, hip bones, etc. are responsible for the production of red blood cells.
After 20 years of age, fat begins to be deposited in the bone marrow of long bones making them yellow in colour. When this happens, erythropoiesis can no longer occur in these bones hence it will continue in the liver.
Blood cell formation begins from stem cells which are cells that have the ability to divide and differentiate into other types of cells. The stages involved are highlighted below
Following the formation of colony-forming unit erythrocytes, red cells will undergo six more stages before they become mature. These stages include:
As these stages are undergone, the cells become smaller in size (proerythroblasts are aver 20µ in size while mature red cells are 7.2µ in diameter), the nucleus shrinks gradually until it disappears (mature red cells do not have a nucleus), staining properties begin to change, and haemoglobin is formed.
Red cells have a lifespan of 120 days after which they are destroyed in the spleen; hence the spleen is usually regarded as the graveyard of red blood cells. As the older red cells squeeze through the capillaries of the spleen, they are lysed (broken down), and haemoglobin is released.
The released haemoglobin is broken down into
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