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The blood and circulation

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This page tells you about the blood and circulation. There is information about

 

What the blood does

The blood circulates throughout the body. It carries nutrients (food) and oxygen to all the cells of the body. And carries away waste products so that they can be removed from the body. Without a blood supply, cells and body tissues die.

 

The circulation

Diagram showing the circulatory system of the body

The blood moves around the body inside the circulatory system. This is made up of blood vessels (tubes) called arteries, veins and capillaries. The blood keeps moving through these blood vessels because it is pumped by the heart.

Arteries carry blood that is full of oxygen from the heart to all parts of the body. As the arteries get further and further away from the heart, they get smaller and smaller. Eventually they turn into capillaries. These are the smallest blood vessels. They go right into the tissues. Here the blood in the capillaries gives oxygen to the cells and picks up the waste gas, carbon dioxide, from the cells.

The capillaries are connected to the smallest veins in the body. The veins get bigger and bigger as they carry the blood back towards the heart. The blood passes through the right side of the heart and goes to the lungs where it gets rid of carbon dioxide and picks up more oxygen. It then passes through the left side of the heart and is pumped back around the body.

The blood always circulates through the body in the same direction. As well as oxygen and carbon dioxide, many other substances are carried in the blood. The blood circulating through the digestive system picks up digested food products and carries them to the liver to be used or stored.

Diagram showing a cancer cell stuck in a small blood vessel (capillary)

The circulation can help explain why some cancers nearly always spread to the same place. Cancers of the colon (large bowel) often spread to the liver. This is because blood circulates from the bowel through the liver on its way back to the heart. If there is a cancer in the large bowel, and some cancer cells escape into the circulation, they may stick in the liver as the blood passes through. They can then begin to grow into secondary cancers.

 

What is in blood

Although blood looks like a red liquid, if left in a test tube it separates out into a pale liquid called plasma and a solid layer of blood cells.

Diagram of what is in blood

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

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 a few days.

Diagram of a white blood cell

There isn't an exact 'normal' figure for the number of cells in your blood (called your blood count). '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 cells per cubic millimetre of blood. If you have surgery or an infection, your white blood cell counts will go up within a day or two.

The most common type of white blood cells are the neutrophils (sometimes called leucocytes). There are between 2,000 and 7,500 of these per cubic millimetre of blood. They are important for fighting infection. If you have chemotherapy, and especially if you have high doses, your neutrophil count usually drops quite quickly.

The next most common are the lymphocytes. A normal lymphocyte count is between 1,500 and 4,500 cells per cubic millimetre of blood. Lymphocytes are involved in making antibodies as part of the immune response. There are both B lymphocytes and T lymphocytes.

Other white blood cells 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. Lymphocytes, eosinophils and basophils are collectively called granulocytes.

As we've said, the range quoted as normal for blood cell counts varies. These figures are taken from the Oxford Handbook of Clinical Medicine.

 

Red blood cells

Red blood cells give the blood its red colour. They contain a pigment called haemoglobin. There are more than 4 or 5 million red blood cells in every cubic millimetre of blood. A red blood cell can live for up to 120 days (about 4 months). Doctors can measure the amount of haemoglobin (Hb) in our blood. They measure the Hb level in grams per decilitre of blood (g/dL) or grams per litre of blood (g/L). Healthy men usually have a level between 13 to 18 g/dL (130 to 180 g/L). Healthy women usually have a level between 11.5 to 16.5 g/dL (115 to 165g/L). 

Red blood cells attach to oxygen and carry it within the blood 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.

Diagram of a red blood cell

 

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 to help stop bleeding. They then release other chemicals to clot the blood and repair the blood vessel.

Diagram of a platelet

 

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.

Diagram showing where blood cells are made in the body

The stem cells divide and multiply to make the blood cells. These cells develop and mature (differentiate) as they grow into white cells, red cells or platelets. The diagram below shows how the various different types of cells can develop from a single blood stem cell.

Diagram showing how blood cells are made

It is now possible to collect stem cells and freeze them. Then give them back to you after high dose chemotherapy treatment. 

You can read about stem cell and bone marrow transplants.

 

Cancer drugs and blood cells

If you are having chemotherapy, the number of white blood cells, red blood cells and platelets in your blood is likely to go down for a time. 

Developing blood cells multiply all the time as they mature. Chemotherapy kills cells that are actively multiplying. So blood cells are killed by the chemotherapy drugs. The white cell counts go down first, because many white cells naturally die off within a few days. Normally these are replaced by newly developed white cells. But chemotherapy will have killed some of the developing white cells. It usually takes a week or two before more cells can be made.

Mature red blood cells live for about three months, so there are fewer multiplying at any one time. So you often don't get low in red cells (anaemic) until further into your chemotherapy course. If your red blood cell level gets very low you may have a blood transfusion.

Chemotherapy can also make your platelet level drop. If it does, you may get nose bleeds, or notice a red rash on your skin like tiny bruises. Your doctor may then want you to have a platelet transfusion. After high dose chemotherapy it can take longer for the platelet count to get back to normal than any other blood cell count.

Some biological therapies can also lower your levels of white blood cells or red blood cells. 

Find out more about the effect of treatment on blood cells.

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Updated: 29 August 2013