Production and destruction of blood cells

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.

Production of red blood cells

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

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.

After fetal life

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

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.

General processes involved in blood formation

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

• Uncommitted pluripotent hematopoietic stem cells.
  These are stem cells present in the bone marrow; they are called uncommitted because at this stage they are not designed to give a particular type of blood cell. They are also referred to as pluripotent cells because of their ability to form different types of blood cells. They are seen more in the fetuses and in the blood present in the umbilical cord; adults have only a few of these cells.
• Committed pluripotent hematopoietic stem cells
  At this point, the cells have become committed to forming different types of blood cells, hence are of two types.
  Colony-forming blastocytes and Lymphoid stem cells. Lymphoid stem cells will give rise to lymphocytes and natural killer cells, while colony-forming blastocytes are further differentiated into the following:
• Colony-forming unit erythrocytes (CFU – E)
  These cells will give rise to red blood cells (erythrocytes)
• Colony-forming unit megakaryocyte (CFU – M)
  These cells will develop to form blood platelets
• Colony-forming unit granulocytes/monocytes (CFU – GM)
  These cells will differentiate to form both monocytes and granulocytes which include neutrophils, basophils, and eosinophils.

Maturation and destruction of red blood cells

Following the formation of colony-forming unit erythrocytes, red cells will undergo six more stages before they become mature. These stages include:

• Proerythroblast
• Early normoblast
• Inermediate normoblast
• Late normoblast
• Reticulocyte
• Mature red blood cells

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.

Destruction of red blood cells

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

• Globin is a protein that is taken back to circulation, stored, and reused.
• Iron combines with a substance called apoferritin to form ferritin. Ferritin is stored and reused.
• Porphyrin is degraded to biliverdin, and then to bilirubin. Bilirubin is excreted in the bile, part of it is broken down to stercobilin which gives faeces its colour, and urobilin which gives urine its colour.