Many people baulk at the sight of blood, but without it, humans literally wouldn’t exist. That red liquid flowing through the body is a surprisingly complex substance that performs several different – and extremely vital – functions.

It isn’t possible to see all different elements that make up blood – not without a very powerful microscope – but there are actually four main components.

Red blood cells

Red blood cells (RBCs), also known as erythrocytes, make up around 40 per cent of the blood volume. They are produced in the bone marrow – specialised soft tissue found inside bones – and released into the bloodstream as they mature.

RBCs contain haemoglobin, a protein containing iron that binds oxygen and enables it to be carried from the lungs to every cell of the body via the bloodstream. RBCs are disc shaped and slightly indented in the centre – a bit like a doughnut – meaning there is a bigger surface area for oxygen to diffuse into, and also allowing the cells to be squeezed when passing through very narrow blood vessels.

Because RBCs are continuously moving through the circulatory system, they gradually get more and more damaged and eventually are broken down in the liver, spleen and bone marrow. The lifespan of an RBC is around 120 days.

White blood cells

White blood cells (WBCs), also known as leucocytes, are incredibly important to the body’s defence system, fighting infections, inflammation and allergies. There are several different types of WBCs:

• Neutrophils comprise over half of the white blood cell count and target bacteria and fungi
• Lymphocytes make up just under a third of WBCs and recognise foreign substances, including cancer cells and viruses, and produce antibodies to attack them
• Monocytes are similar to neutrophils but are more associated with chronic rather than acute infections
• Eosinophils help fight infections cause by parasites and play a part in allergic reactions, inflammation and the immune response
• Basophils are involved in the management of allergies.

The bone marrow produces more WBCs when the body needs them, for example if there is an infection or allergic reaction, or if there is a condition such as leukaemia. There are also some circumstances that can cause the WBC count to drop, for example, cancer treatments and immune disorders, which means that individuals are more prone to getting an infection.

The reactive nature of WBC production is the reason that the life cycle of WBCs vary, from hours to years, depending on the WBC type and what else is going on in the body.

Platelets

Also known as thrombocytes, platelets are fragments of cells that clump together and adhere to injury sites to stop bleeding. They have a short life cycle – just a week to 10 days – so the bone marrow is continually producing new ones to replace those used up, lost through bleeding or that have broken down.

If there aren’t enough platelets in the bloodstream, clotting may not occur, which puts the individual at risk of excessive bleeding. Too many platelets puts someone in danger of experiencing a blood clot that stops blood flowing around the body as it should do.

Plasma

Plasma is a straw-coloured fluid within which the other components of blood are suspended. It makes up around half the total blood volume, and is around 90 per cent water with the remainder made up of proteins such as albumin (important in the management of kidney and liver disease), immunoglobins (antibodies that protect against infections) and clotting factors, plus other substances that are being transported round the body, for example, hormones, gases, nutrients and waste products.