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 between four and five litres (about ten units) of blood. One unit (475ml) is not easily missed and is quickly 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 constituent has its own function. Donated blood is divided into various products based on these functions, which means that patients can receive only the particular constituent part that they need.

The Function of the Constituent Parts

Red Blood Cells

These cells transport oxygen and carbon dioxide between the body’s tissue and lungs.

White Blood Cells

They are our body’s little fighters. They attack and eradicate viruses and infections.


This liquid component of blood accounts for about half of each unit of whole blood. Plasma contains nutrients and protein and is used primarily for its clotting abilities, replacement of blood volume and the production of albumin.


This blood component helps the blood clot.

Can Blood be Substituted?

There is no known substitute for blood and it cannot be replicated due to its complexity. Although lost blood volume can be temporarily replaced by synthetic solutions in trauma situations, these do not contain the necessary constituents to sustain the patient. Donor blood can (and often does) mean the difference between life and death.