Blood is a specialised body fluid made up of red blood cells, white blood cells, platelets and plasma.

Plasma is the straw-coloured liquid part of the blood that carries blood cells and other substances around your body. Blood is pumped by your heart and travels around your body through the blood vessels.

About seven to eight per cent of your body weight is blood. This means a person who weighs 70 kg has approximately 5 to 5.5 litres of blood. All blood cells have a limited lifespan and need to be replaced on a continual basis. A normal and healthy red blood cell, for example, has a lifespan of about 120 days, whereas platelets live for about 10 days. This means that every day, your body is producing billions of blood cells.

Blood has many functions, including:

• transporting oxygen and nutrients to the lungs and tissues
• transporting white cells and antibodies to fight infection
• forming blood clots to prevent bleeding
• carrying waste products to the liver and kidneys which assist in filtering and cleaning the blood
• helping to regulate body temperature.

Components of blood

Plasma

Plasma makes up about 55% of your blood. This straw-coloured liquid is made up of water, fat, protein, sugar and salts and helps your blood travel throughout the veins, arteries and capillaries that make up your blood vessels. Plasma also helps transport nutrients, waste products, proteins and chemical messengers such as hormones around your body.

Red blood cells

Red blood cells (also known as erythrocytes or RBCs) are by far the most abundant blood cells and make up about 40-45% of your blood. They are shaped a bit like a donut and contain a protein called haemoglobin (Hb), which carries oxygen from the lungs to all parts of the body and returns carbon dioxide to the lungs so it can be exhaled. Haemoglobin also gives red blood cells their colour.

The percentage of the blood (the space in the blood) that red blood cells take up is called the haematocrit.

Haemoglobin (Hb) ranges

The normal haemoglobin range for a man is approximately 130 – 170 g/L.

The normal haemoglobin range for a woman is approximately 120 -160 g/L.

Haematocrit ranges

The normal haematocrit range for an adult male in Australia is 40–54%

The normal haematocrit range for an adult female in Australia is 37–47%.

White blood cells

White blood cells fight infection and make up about 1% of your blood. There are different types of white blood cells (also known as leukocytes or WBCs) and they each have a specific function, although they all work together to protect your body:

Neutrophils
Neutrophils are the most common type of white blood cells and are considered the first responder to any infection. They have a very short life (less than one day) so your body is constantly making new cells.

T-lymphocytes
T-lymphocytes (also called T-cells) act as the controllers and help regulate the function of other immune cells. They also directly attack various infected cells and tumours. There are three categories for T-cells:

• Helper T-cells: These recognise the presence of a foreign antigen in the body and trigger antibody production. Helper T-cells trigger the immune systems by creating cytokines.
• Regulatory T-cells: These do the opposite to the Helper T-cells where they turn off the immune system once they threat has been eliminated.
• Cytotoxic T-cells: Also known as ‘killer T-Cells’, these are the soldiers who go out and destroy the foreign cells.

B-lymphocytes
B-lymphocytes (also called B-cells) make proteins called antibodies that can target viruses and bacteria. Rather than destroying invading pathogens, B-cells act as the alarm bells of the immune system. Once the B-cells encounter an antigen, they become activated and alert the other white blood cells of the presence of an invader and where to go after it.

Monocytes
While monocytes work with neutrophils and lymphocytes to assist in the fight against infections, they also help with antibody production and act as scavengers to remove dead tissue. These cells are known as ‘monocytes’ when they are found in the blood and ‘macrophages’ when they migrate into body tissue to help fight infection.

Eosinophils
These are types of white blood cells whose main job is to kill parasites.

Basophils
Basophils work with neutrophils to fight infection as they destroy parasitic infections. As basophils also contain a chemical called heparin, these cells also help the blood from forming unwanted clots in the circulatory system.

White blood cell ranges

An elevated number of white blood cells is called leukocytosis. A WBC count of 11.0 – 17.0 x 10⁹/L cells would be considered mild to moderate leukocytosis.

A decreased WBC count is called leukopenia. A count of 3.0 – 3.5 x 10⁹/L cells would be considered mild leukopenia.

Neutrophil ranges

A low number of neutrophils circulating in your blood is called neutropenia. The usual range of normal neutrophil numbers is 1.5 – 8.0 X 10^9/l (although this can vary according to age, heath and other factors).

Platelets

Platelets, also known as thrombocytes, are small pieces of cells. They help your blood clot or stick together, a process called coagulation. If you are bleeding – whether that be internally or externally – platelets rush to the gap in the blood vessel and bind to the edges of the gap as well as each other to form a clot (also known as a thrombus).

Platelet ranges

A normal platelet count ranges from 150,000 to 450,000 platelets per microlitre of blood.

A platelet count of more than 450,000 platelets is called thrombocytosis.

A platelet count of less than 150,000 is called thrombocytopenia.