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What the blood does
The blood flows throughout the body. It:
- carries food (nutrients) and oxygen to all the cells of the body
- carries away waste products that the body needs to get rid of
Without a blood supply, cells and body tissues die.
How blood circulates
The blood flows around the body in blood vessels (tubes) called arteries, veins and capillaries. This is the circulatory system. The heart pumps the blood through the circulatory system.
Arteries
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.
Capillaries
Capillaries 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 from the cells. The capillaries connect with the smallest veins in the body.
Veins
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 gets pumped back around the body.
Blood circulation and cancer spread
The blood always circulates through the body in the same direction. It carries oxygen, carbon dioxide and many other substances. As the blood circulates through the gut (digestive system 
), it picks up digested food products and carries them to the liver where it gets stored or used.
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. So if 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
- 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 3 main types of cells in the blood. These are:
- white blood cells
- red blood cells
- platelets
A full blood count (FBC) measures the number of red cells, white cells and platelets in your blood. There are several different types of white blood cells including neutrophils and lymphocytes.
There isn’t an exact value of normal for blood counts. The range of figures quoted as normal varies slightly between laboratories and also differs between men and women.
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 how the body deals with an infection or anything else the body recognises as ‘foreign’. The body can make white blood cells very quickly. They have a short life. Some only live for a few hours, others for a few days.
Your white blood cell count may go up if you have surgery or an infection.
Neutrophils
The most common type of white blood cells are the neutrophils (sometimes called leucocytes).
They are important for fighting infection. Some chemotherapy and targeted cancer drugs can lower the levels of neutrophils for a short time. This reduces your resistance to infection.
Lymphocytes
The next most common type of white blood cell is lymphocytes.
Lymphocytes help to make 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.
These are called eosinophils, basophils and monocytes. They are sometimes collectively called granulocytes.
Red blood cells
Red blood cells give the blood its red colour. They contain a pigment called haemoglobin.
Red blood cells attach to oxygen and carry it in the blood to the tissues. When they get to an area that needs oxygen, 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.
How and where are blood cells are made?
Your body makes blood cells in the bone marrow. The bone marrow is the soft inner part of your bones. You make blood cells in a controlled way, as your body needs them.
All blood cells start as the same type of cell, called a stem cell. In adults, there are blood stem cells in the bone marrow, inside the skull, ribs, sternum (breast bone), spine and pelvis.
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 different types of cells can develop from a single blood stem cell.
It is possible to collect stem cells from the bone marrow or the blood and freeze them. Doctors can then use stem cells as part of high dose chemotherapy treatment called stem cell transplant or bone marrow transplant.
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 some time. Drugs that can cause this include some chemotherapy drugs and some targeted cancer 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.
Low level of white blood cells
The level of white cell counts goes down first because many white cells naturally die off within a few days. Usually, these get replaced by new white blood cells , but cancer drugs may kill some of the developing cells.
It usually takes a week or 2 for the bone marrow to make more cells and release them back into the blood.
A drop in white blood cells can lead to an increased risk of infection.
Low level of red blood cells
Mature red blood cells live for about 3 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 treatment. If your red blood cell level gets very low, you may need to have a blood transfusion.
A drop in red blood cells can make you feel tired and you may become breathless.
Low level of platelets
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.
