Normal Blood Cells

Background

There are 3 main types of blood cells:

Red Cells - These transport oxygen from the lungs to the body organs and return with carbon dioxide which the lungs then blows off.

The normal blood contains 5 million, million red cells per litre of blood!!

White Cells - These fight infections. There are two main types - neutrophils and lymphocytes.

The normal blood contains about 5 thousand million white cell per litre of blood!!

Platelets - These cells help the blood clot.

There are about 200 thousand million platelets per litre of blood!!

The cells circulate suspended in plasma which consists mostly of water mixed with proteins, sugars and fat - 40% of the blood is cells with the rest plasma.

Blood Cell Production

All the blood cells are produced in the bone marrow which is a spongy material at the centre of bones. All blood cells are derived from a single cell type - the stem cell - which resides in the marrow and makes up only 1 in 10,000 of all marrow cells.

The production of marrow / blood cells is highly regulated so that enough but not too many cells are produced. The cells are released from the marrow into the blood. Once the cells have aged and become less effective they are removed from the blood by the spleen and destroyed.

Red Cells

These are stimulated to grow by several marrow growth factors the most important of which is erythropoetin a hormone secreted from the kidneys. Production is extremely fast with about 3 million red cells made each second and so a good supply of nutrients especially folic acid and vitamin B 12 is essential.

Haemoglobin is produced by the red cells with 640 million molecules present in each cell. It is haemoglobin which gives blood it's red colour but more importantly it carries the oxygen and carbon dioxide around the body. At the centre of the haemoglobin molecule is iron and therefore deficiency of iron rapidly leads to anaemia.

A normal red cell survives about 120 days.

White Cells

These are stimulated to grow by many marrow growth factors one of which G-CSF has been artificially synthesised and is available for clinical use. White cells go through various stages of maturation within the bone marrow ultimately emerging as neutrophils which are then released into the blood. The average life of a neutrophil is only 24 - 36 hours. Their main role is to kill bacteria and fungi although they also have activity against herpes viruses. Antibodies which are produced from B-lymphocytes help them.

There are two types of lymphocytes:

• T lymphocytes which kill viruses and cancer cells.
• B lymphocytes transform in to plasma cells, which produce antibodies that can either directly kill microbes and viruses or help neutrophil in this process.

Lymphocytes reside in all body organs but particularly in the lymph glands, bone marrow, spleen and liver - the so-called lymphoid tissues. Lymphocytes live many, many years.

Platelets

These are also stimulated to grow by many different marrow growth factors the most important of which is thrombopoetin. Platelets live 10-14 days with the majority (70-80%) circulating in the blood with the rest residing in the spleen.