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What is... blood?

written by

Leukaemia Care, Charity

  • What is blood?

The main role of blood is to carry nutrients and waste around the body, regulate salt levels and transport immune cells and hormones. In fact, anything that a cell needs to grow and live must be transported by the blood. When blood is left to settle in a tube, we’re able to see its make-up. Half of the blood is made up of blood cells (red cells, white cells and platelets), while the other half is plasma.


Plasma is mostly water but also contains:

Salts which help regulate water balance.

Clotting factors which cause the blood to clot when the blood vessel is damaged (serum is the watery substance left once the blood has clotted).

Sugars used for fuel.

Proteins and fats that are used for building cells.

Hormones which control many of the body’s processes.

Globulins (part of the immune system).

Carrier proteins which transport certain molecules around the blood.

Vitamins and iron.

• Chemicals that help regulate the acidity of the blood.

• Compliment system, a series of proteins that attack bacteria.

Waste products.

The plasma is regulated by the liver and kidneys. The plasma results from a blood test will show the doctor whether there is the correct balance of salts (sodium, potassium and calcium) in the blood and how well the kidneys and liver are working by looking at the waste levels. The lower the level of the waste product the better the kidneys (creatinine) or the liver (bilirubin and liver enzymes) are working. The constituents of the plasma vary with sex and age.

Blood cells

The number of actual blood cells in the blood is measured by the haematology laboratory and is referred to as the ‘Full Blood Count’ (FBC) or ‘Complete Blood Count’ (CBC).

Red cells

The number of red cells vary dependent on age and sex. In a male adult there are normally between 4-5 thousand billion red cells per litre of blood (4.0-5.0x1012/L) that means 20-30 trillion red cells in the entire body. Red cells are made in the bone marrow and live for about 120 days. Vitamin B12, folic acid and iron are all required to make red cells. The most important substance contained in a red cell is haemoglobin (Hb) which carries oxygen around the body. The total amount of haemoglobin, rather than the number of red cells in the blood, is usually used to determine how much oxygen the blood can carry and is measured as grams per litre (g/L).

The normal range for a middle aged man is usually 135-180 grams per litre. Your haemoglobin levels tend to fall as you get older and are higher in men than women. If your haemoglobin levels fall too low (anaemia) the blood’s ability to carry oxygen is reduced leading to tiredness, low energy, shortness of breath and looking pale. At what level people start noticing anaemia is dependent on their age, other medical problems (especially heart and lung problems) and the speed at which the anaemia developed. If someone is anaemic it is important to find the cause and if it cannot be treated, a transfusion of red blood cells is often required.

White cells

White cells are there to fight infection. They tend to use the blood as a way of getting from the bone marrow, (where they are made), to where the infection is. The total number of white cells in the blood is usually between 3.5 – 10 thousand million cells per litre (3.5-10x109/L), although this does vary with age and race. There are five main types of white cells normally found in the blood:

Neutrophils - the most common type, making up between 45-74% of the white cells. Their normal job is to fight infections due to bacteria and they do this by either engulfing the bacteria or releasing chemicals which kill it.

Lymphocytes - the most common white cell in children but decreases with age. Lymphocytes are either B-lymphocytes which produce antibodies (antibodies, or globulins, are sticky proteins that stick to bacteria highlighting them to the rest of the immune system) or T-lymphocytes that kill infected or abnormal cells.

Monocytes - leave the blood and become macrophages which engulf and destroy bacteria and any unwanted substances.

Basophils - least common type and are involved in allergic reactions.

Eosinophils - fight against parasites but are often the cause of an allergy.

Reductions in the number of white cells can increase the risk of infection. Infections are increasingly common when the neutrophils fall to less than 1.0x109/l - this is called neutropenia. At this level the body cannot fight infections itself and needs the help of antibiotics. The most common reason for low neutrophils is chemotherapy. Unlike platelets and red cells you cannot routinely transfuse white cells if they are low. Occasionally injections can be used to increase the bone marrow’s own production of white blood cells.


Platelets are very small blood cells that help the blood to clot and only live for a few days. There is normally between 150-400 thousand million per litre of blood (150-400x109/L), and there are considerable reserves of platelets in the blood. Bleeding doesn’t tend to happen until the platelets have fallen to less than 10 thousand million per litre (10x109/L) and most operations are safe as long as the platelet count is greater than 50 thousand million (50x109/L). Platelets can be transfused to increase the number in the blood and are usually given if the platelet count is less than 10x109/L or needed to be higher because of bleeding or an operation. Low platelets is called thrombocytopenia.

The bone marrow

The job of bone marrow is to make the blood move around the body. It is only found in the skull, spine, ribs, pelvis, breast bone and the top of the thigh bone and the humerus. The bone marrow contains stem cells, which can develop into any type of blood cell and this is dependent on hormone signals from the rest of the body telling the marrow what to do. The stem cell splits into two identical cells which in turn split. As they split they also develop and mature into different types of blood cells. It may take several splits, or divisions, to turn into a blood cell that is ready to be released into the blood. Some lymphocytes move to lymph nodes or the spleen to divide and mature further.

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