Blood and circulation

Blood circulation system

Your blood circulation is a network of arteries, veins and capillaries carrying blood to every part of the body. You have around five litres of blood circulating in your body (1).

The blood carries the oxygen you breathe in through the lungs to the body’s cells. It picks up carbon dioxide carrying it back to the lungs, where you breathe it out (1).

Blood also carries nutrients to the cells and carries waste products from the cells to the kidneys and liver to be processed and removed from the body (1).

Find out more: Blood cancers and blood disorders.

Blood cells

Blood is made of clear fluid and blood cells. Around half (55%) of it is the straw-coloured fluid, plasma. The remainder (45%) is blood cells (1).

There are three main types of blood cells (1):

• white cells help to protect the body from infection
• red cells carry oxygen and carbon dioxide
• platelets help the blood to clot.

There are billions of cells in your blood. They are constantly being used up and need to be replaced. Every day, on average, you make around 200 billion red cells, 10 billion white cells and 400 billion platelets (2).

How are blood cells made?

Most blood cells are made in the bone marrow – all your red cells and platelets and about two-thirds of your white cells. The remaining white cells start off in the bone marrow but finish developing in the organs of the lymphatic system, the spleen, lymph nodes and thymus (2).

Neutrophils are the most numerous type of white blood cell. They are extremely important in fighting infection (4). If you’re having chemotherapy, you may be told that you are ‘neutropenic’. That means that your neutrophil count is very low and you are at risk of picking up infections (5).

Find out more: What are blood stem cells?