What is human blood made of?

Blood sustains our bodies by transporting oxygen and nutrients to cells, and getting rid of carbon dioxide and waste materials. It makes up 7 to 8% of our body weight; an adult’s body contains on average 5 -6 litres of blood. When the National Blood Service receives a donation of blood it is then separated into four components:

Red Cells

Red cells or erythrocytes make up about 40 – 50% of the total volume of our blood. They are large cells, with no nuclei. 95% of a red blood cell is made up of the protein haemoglobin. Each cell contains around 270 million haemoglobin molecules, whose tasks are to transport gases around the body, delivering oxygen from the lungs and removing carbon dioxide. A deficiency of red blood cells leads to anaemia; someone who is anaemic will feel tired due to a shortage of oxygen reaching the muscle cells. The red colour of blood is primarily due to the oxygenated haemoglobin molecules.

White Cells

White cells or leukocytes make up about 1% of the blood in healthy individuals. There are many different types and what follows is a simplification. White cells are the first responders of our immune system, seeking out and bonding with alien proteins on fungi, viruses and bacteria. Other white cells, such as macrophages and granulocytes, then arrive and remove the alien cells. White cells can also remove dead red blood cells and foreign matter such as asbestos and dust particles. White blood cells are also produced in other parts of the body i.e. the liver, spleen, lymph glands and bone marrow.


Platelets or thrombocytes are pieces of cells that help to form clots at wound sites. They bind with blood-clotting chemicals and seal the ruptured walls of blood vessels. For a clot to form 13 different clotting factors work in succession, each one triggering the next. Haemophiliacs lack Factors 8 and 9 and their blood, therefore, lacks the ability to form clots.


Plasma is a translucent, yellow liquid, 92% water but also containing sugar, salts, fat and proteins. The proteins include gamma globulin, proteins that make up anti-bodies, and fibrinogen that helps with clotting. Plasma transports the other components of the blood around the body.