One of the main functions of blood is to transport oxygen, cells, proteins, hormones, and other substances around the body to the organs and tissues. Oxygen is carried from the lungs to the body cells, and the waste product carbon dioxide is transported from the cells to the lungs. Blood also has a clotting mechanism that acts to seal damaged blood vessels and prevents internal and external blood loss.
Each red blood cell contains millions of molecules of haemoglobin, each made up of four protein chains (two alpha- and two beta-globin) and four haeme, an iron-bearing red pigment. Haemoglobin combines with oxygen from the lungs to give arterial blood its bright red colour. Once oxygen is released in the tissues, the blood becomes darker, a distinctive feature of venous blood.
When a blood vessel is cut or torn, the damage triggers a series of chemical reactions that lead to the formation of a blood clot to seal the injury. Clot formation depends on blood cells called platelets, which adhere at the site of injury. The platelets then clump together and release chemicals that activate proteins, called clotting factors.
When a blood vessel is damaged, it constricts at once. Platelets that come into contact with the damaged blood vessel walls are activated. They become sticky and start to adhere to the blood vessel walls near the site of the injury.
The platelets clump together. Damaged tissue and activated platelets release chemicals that start a “coagulation cascade”, a complex series of reactions involving clotting factors. At each stage, more clotting factors are activated.
The final stage is conversion of the protein fibrinogen, which is dissolved in the fluid part of the blood, to insoluble fibrin strands. The sticky fibrin threads form a tangled mesh that traps red cells and other blood cells, forming a clot.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.