Cancer, the blood and circulation

This page tells you about the blood and how it circulates around the body. It can help you to understand how cancer can sometimes spread to other parts of the body in the blood circulation. It can also help you to understand how some types of cancer can raise or lower the number of blood cells in the bloodstream. You can also read about how some cancer treatments can lower the production of blood cells. 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. Blood also carries away waste products so that they can be removed from the body. Without a blood supply, cells and body tissues die.

How blood circulates

The blood moves around the body in 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.

Diagram showing the circulatory system

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.

The circulation can help to explain how some cancers spread to particular parts of the body. For example, 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.

You can read more about how cancer can spread.

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.

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.

Neutrophils

The most common type of white blood cells are the neutrophils (sometimes called leucocytes). 

Diagram of a white blood cell

There are between 2,000 and 7,500 of these cells per cubic millimetre of blood. They are important for fighting infection. Some chemotherapy and biological therapy drugs can temporarily lower the levels of neutrophils and reduce resistance to infection.

Lymphocytes

The next most common type of white blood cell is lymphocytes. 

Diagram of a lymphocyte

A normal lymphocyte count is between 1,500 and 4,500 cells per cubic millimetre of blood. Lymphocytes are involved in making antibodies to fight infection. There are B lymphocytes and T lymphocytes.

Other types of white blood cells

Other white blood cells are present in smaller numbers in the circulating blood. In a cubic millimeter of blood there are between

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

Lymphocytes, eosinophils and basophils are collectively called granulocytes.

The range quoted as normal for blood cell counts varies. The figures on this page 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. 

Diagram of a red blood cell

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 in 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.

Platelets

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

A normal platelet count is between 150,000 and 440,000 per cubic millimetre of blood.

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 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 possible to collect stem cells from the bone marrow or the blood and freeze them. The stem cells can then be used as part of high dose chemotherapy treatment called stem cell transplant or bone marrow transplant.

You can find information about stem cell and bone marrow transplants.

Effects of cancer drugs on blood cells

Some types of cancer drugs can lower the number of white blood cells, red blood cells and platelets in your blood for a time. Drugs that can cause this include some chemotherapy drugs and some biological therapy drugs.

Developing blood cells multiply all the time as they mature in the bone marrow and are then released into the blood. Some cancer drugs can slow the production of blood cells by the bone marrow so they are not released as quickly into the blood. Then the number of circulating blood cells goes down.

The level of white cell counts goes down first, because many white cells naturally die off within a few days. Normally these are replaced by newly developed white cells. But cancer drugs may kill some of the developing white cells. It usually takes a week or two before more cells can be made in the bone marrow and get back into the blood.

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 cancer drug treatment course. If your red blood cell level gets very low you may need to have a blood transfusion.

The platelet level may also drop. If it does, you may get nose bleeds, or notice a red rash on your skin like tiny bruises. You may then need 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.

For information about how cancer treatment can affect blood cells, look at our page about your blood, bone marrow and cancer drugs.

You can find out if particular drugs cause low levels of blood cells in our section about cancer drugs.

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