What are the main components of blood?

Blood consists of blood cells and plasma (the fluid used to transport cells around the body). The three main types of blood cells are red blood cells, white blood cells, and platelets. Each of these has important functions to perform.

Bone marrow produces blood cells

Bone marrow is a spongy tissue that fills up the cavities of bones. Bone marrow acts as a blood cell ‘factory’ that releases blood cells into the bloodstream when they are mature, and when the body has a special need for them (for example, to help mend a cut or bruise).

In a healthy body, the bone marrow produces three types of blood cells: red blood cells, white blood cells, and platelets.

How many cells are made?

Blood cell growth and development are carefully controlled to produce the correct number of each type of cell to keep the body healthy. About 3 million red cells and 120,000 white cells are produced every second.

How long do blood cells live?

Cells normally stay inside the bone marrow until they are mature enough to circulate in the blood and perform their various functions properly. All blood cells naturally grow old and die, but the life-span varies widely.

Red blood cells live for about four months after they leave the marrow, white cells live for only a few hours, while platelets live for a few days. Because white cells and platelets have such short life-spans, they cannot easily be replaced by transfusion.

Red blood cells carry oxygen

Red blood cells contain haemoglobin (Hb) which transports oxygen from the lungs to all parts of the body. Haemoglobin also carries carbon dioxide to the lungs, where it can be breathed out.

If you do not have enough haemoglobin you will feel run down and weak. You may be pale and tire easily because your body is not getting enough oxygen. A reduction in the normal haemoglobin level is called anaemia.

White blood cells fight infection

White blood cells fight infection, ridding the body of disease-causing microbes (bugs) and defective cells.

There are a number of different types of white blood cell and each has a specific job to do:

• T-lymphocytes - control immunity, kill viruses and cancerous cells
• B-lymphocytes - make antibodies
• Neutrophils - fight infection, kill bacteria and remove damaged tissue
• Monocytes - work with lymphocytes to respond to infection by producing antibodies.

Neutropenia means a lower than normal neutrophil count. When someone is neutropenic they are at a high risk of frequent, and sometimes severe, infections.

Platelets help clotting

Platelets are disc-shaped fragments that help make clots and prevent bleeding.

If a blood vessel is damaged (for example, by a cut), the platelets rush to the site of injury, clump together and form a plug to help stop the bleeding. Other clotting factors join in to help make a proper clot.

A reduction in the normal platelet count is called thrombocytopenia. When a person's platelet count drops below normal they are at risk of bleeding and bruising easily.

Blood cell families

All blood cells come from the same original stem, or precursor, cell. However, early in their development these cells split into two main families - the myeloid and lymphoid families.

The myeloid family includes:

• all red blood cells,
• platelets, and
• certain types of white blood cells.

The white blood cells in the myeloid family include granulocytes (consisting mainly of neutrophils) or monocytes, depending on their function.

The lymphoid family includes:

• all other white blood cells. When mature, these cells are called lymphocytes.

How both families fight infection

Although white blood cells, especially the neutrophils, are the body’s immediate defence against infection, the lymphoid cells take longer to work but are more specific in their fight against invading organisms. One of their functions is to make antibodies.

Lymph system

Lymphoid cells are also found in the lymph tissue in all parts of the body. Special large collections of lymph tissue are found in the lymph glands, liver, spleen and along the gut and lung passages.

The lymph system is the body’s waste collection, filtration and drainage system. Just as the blood system circulates blood, the lymph system circulates a clear fluid called lymph that helps transport lymphocytes, which are found in the blood and lymph.

Blood-related diseases

Leukaemia, lymphoma, multiple myeloma, myelodysplastic disease, and other related blood conditions generally begin as a result of abnormalities occurring in the bone marrow, where blood cells are produced.

Original material provided by the Leukaemia & Blood Foundation of New Zealand, 2007. Edited by everybody, August 2010.