The Blood

Although blood looks like a red liquid, if some is left in a test tube it separates out into a pale liquid called plasma and a solid layer of blood cells.
The blood is about 55% plasma and 45% cells. Plasma is mostly water with some proteins and other chemicals dissolved in it. There are three main types of cells in the blood

• White blood cells
• Red blood cells
• Platelets

White blood cells
There are several different types of white cells in the blood in differing amounts. They all play a part in the immune response. This is the response of the body to infection, or anything else the body recognises as 'foreign'. These blood cells can be made very quickly and generally have a short life. Some only live for a few hours, others for days.

There isn't an exact 'normal' figure for blood counts. 'Normal' for a large man wouldn't be the same as for a small woman. But generally the normal white cell count is between about 4,000 and 11,000 per cubic millimeter of blood. If you have surgery or an infection, your white blood cell counts will go up within a day or two.

The most numerous of the white blood cells are the neutrophils. There are between 2,000 and 7,500 of these per cubic millimeter of blood. They are important for fighting infection. If you have chemotherapy, particularly high dose, your doctors will probably talk about your neutrophil count.

The next most numerous are the lymphocytes. A normal lymphocyte count is between 1,300 and 4,000 per cubic millimeter of blood. Lymphocytes are involved in making antibodies as part of the immune response.

There are other white blood cells that are present in smaller numbers in the circulating blood. There are between

• 40 and 400 eosinophils
• 0 and 100 basophils
• 200 and 800 monocytes

per cubic millimeter of blood. As we've said, the range quoted as normal for blood cell counts does vary. These figures have come from the Oxford Handbook of Clinical Medicine.

Red blood cells
Red blood cells give the blood its red colour. There are more than 4 or 5 million of them in every cubic millimeter of blood. A red blood cell can live for up to 120 days.

Red blood cells are able to attach to oxygen to carry it within the circulation to the tissues. When they get to an area where the oxygen is needed, they give it up and pick up carbon dioxide which they carry back to the lungs. A shortage of red blood cells is called anaemia. The role of the red blood cell in carrying oxygen explains why very anaemic people usually feel breathless.

Platelets
Platelets are really bits of much bigger cells called megakaryocytes. A normal platelet count is between 150,000 and 440,000 per cubic millimeter of blood.

Platelets are very important in blood clotting. They clump together to form a plug if bleeding occurs. Then they release other chemicals that help the blood to clot and the blood vessel to be repaired.

How and where blood cells are made
All the different types of blood cells develop from one type of cell called a 'blood stem cell'. In adults, blood stem cells are normally found in the red bone marrow inside the bones. Blood cells are made in the bone marrow in the skull, ribs, sternum (breast bone), spine and pelvis.

The stem cells divide and multiply to make the blood cells. These cells differentiate (develop and mature) as they grow into white cells, red cells or platelets. It is now possible to collect stem cells and freeze them. Then give them back to you after high dose chemotherapy treatment. There is information about stem cell and bone marrow transplants elsewhere in this section of CancerHelp UK.