Blood, without which the body cannot function, is a rather complex substance made up of fluid and different kinds of blood cells. All blood falls into one of eight blood types, which determine donor-recipient compatibility.

The average adult carries about five litres of blood so donating one unit (475ml) can quickly be replaced by the body of a healthy donor.

The Composition of Blood

Blood in its entirety (called ‘whole blood’) consists of four constituent parts – red blood cells, white blood cells, plasma and platelets – and each part has its own function. The unit of blood you donate is divided into various products, which means that patients receive only the part of your blood that they need and your one donation can help several different people.

Red Blood Cells

These cells transport oxygen and carbon dioxide between the organs and lungs to keep our tissues alive. Red blood cells can do this because they carry an iron-containing molecule called haemoglobin.

White Blood Cells

These are important cells in our immune system that equip our body to fight infection. White blood cells are not helpful when transfused to other people, so these are often removed through a filtering process when blood products are made.


This liquid component of blood accounts for about half of each unit of whole blood. Plasma contains important proteins that help our blood to clot, carry antibodies and maintain our blood volume.


These little cells form plugs at the site of bleeding in vessels to help people stop bleeding.

Can Blood be Substituted?

There is currently no known substitute for blood and it cannot be replicated due to its complexity. When patients bleed in emergency situations, blood can be temporarily replaced by synthetic fluids, but these do not contain the necessary cells to sustain the patient. Your donated blood can (and often does) mean the difference between life and death.