The bone marrow and blood formation

In finding out more about a blood cancer or blood disorder, you may find it helpful to understand more about your bone marrow, stem cells, and blood.

Bone Marrow and Blood Formation

Bone marrow is the spongy tissue that fills the cavities inside your bones. Most of your blood cells are made in your bone marrow. The process by which blood cells are made is called haemopoiesis.

As an infant, haemopoiesis takes place at the centre of all bones. In later life, it is limited to the hips, ribs and breastbone (sternum). Some of you may have had a bone marrow biopsy taken from the bone at the back of your hip (the iliac crest).

You might like to think of the bone marrow as the blood cell factory. The main workers at the factory are the blood stem cells. They are relatively small in number but are able, when stimulated, to reproduce vital numbers of red cells, white cells and platelets. All blood cells need to be replaced because they have limited life spans.

There are two main families of stem cells, which develop into various types of blood cells.

Myeloid (‘my-loid’) stem cells develop into red cells, white cells (neutrophils, eosinophils, basophils and monocytes) and platelets.
Lymphoid (‘lim-foid’) stem cells develop into two other types of white cells called T-cells and B-cells.

Growth factors and cytokines

All normal blood cells have a limited survival in circulation and need to be replaced on a continual basis. This means that the bone marrow remains a very active tissue throughout your life. Natural chemicals in your blood called growth factors or cytokines control the process of blood cell formation.

Different growth factors stimulate the blood stem cells in the bone marrow to produce different types of blood cells.

These days some growth factors can be made in the laboratory (synthesised) and are available for use in people with blood disorders. For example, granulocyte-colony stimulating factor (G-CSF) stimulates the production of white cells called neutrophils while erythropoeitin (EPO) stimulates the production of red cells.

Blood

Blood consists of blood cells and plasma. Plasma is the straw coloured fluid part of the blood that blood cells use to travel around your body. The blood cells consist of red cells, white cells and platelets.

Red Cells and Haemaglobin

Red cells contain haemoglobin (Hb), which gives the blood its red colour and transports oxygen from the lungs to all parts of the body. The body uses this oxygen to create energy.

The normal haemoglobin range for a man is approximately 130 – 170 g/L, while the normal haemoglobin range for a woman is approximately 120 -160 g/L.

Red cells are by far the most numerous blood cell and the proportion of the blood that is occupied by blood cells is called the haematocrit. A low haematocrit suggests that the number of red cells in the blood is lower than normal.

The normal haematocrit for a man is between 40% and 52%, while the normal haematocrit for a woman is between 36% and 46%.

Anaemia

Anaemia is a reduction in the number of red cells or low haemoglobin. Measuring either the haematocrit or the haemoglobin will provide information regarding the degree of anaemia.

If you are anaemic you will feel run down and weak. You may be pale and short of breath or you may tire easily because your body is not getting enough oxygen. In this situation a red cell transfusion may be given to restore the red cell numbers and the haemoglobin to more normal levels.

White Blood Cells

White blood cells, also known as leukocytes, fight infection. There are different types of white cells that fight infection together and in different ways.

  • Neutrophils: kill bacteria and fungi.
  • Eosinophils: kill parasites.
  • Basophils: work with neutrophils to fight infection.
  • Monocytes: work with neutrophils and lymphocytes to fight infection; they also help with antibody production and act as scavengers to remove dead tissue. These cells are known as monocytes when they are found in the blood and macrophages when they migrate into body tissues to help fight infection.
  • T-cells: kill viruses, parasites and cancer cells; produce cytokines.
  • B-cells: make antibodies which target microorganisms.

When your white cell count drops below normal you are at risk of infection. The normal adult white cell count varies between 3.7 and 11 x 109/L.

Neutropenia

Neutropenia is the term used to describe a lower than normal neutrophil count. If you have a neutrophil count of less than 1 (1 x 109/L) you are considered to be neutropenic and at risk of developing frequent and sometimes severe infections. The normal adult neutrophil count varies between 2.0 and 7.5 x 109/L.

Platelets

Platelets are disc-shaped cellular fragments that circulate in the blood and play an important role in clot formation. They help to prevent bleeding. If a blood vessel is damaged (e.g. by a cut), the platelets gather at the site of injury, stick together and form a plug to help stop the bleeding.

The normal adult platelet count varies between 150 and 400 x 109/L.

Thrombocytopenia

Thrombocytopenia is the term used to describe a reduction in the platelet count to below normal.  If your platelet count drops below 20 (20 x 109/L), you are at risk of bleeding and you tend to bruise easily. Platelet transfusions are sometimes given to bring the platelet count back to a safe level.

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