The blood is not only a medium in which substances are transported around the body, but also a central part of the body’s immune system. Blood consists of a fluid component known as blood plasma and a solid component, the blood cells. These are divided into red and white blood cells and platelets. Blood plasma is a watery fluid containing many dissolved substances such as carbohydrates, amino acids, fats, minerals and vitamins, as well as larger proteins and substances for blood coagulation. Apart from nutrients, a major component of the substances dissolved in the blood plasma is metabolic waste – waste substances produced by the body as it works. These are transported by the blood to organs which are equipped to deal with their removal (e.g. the kidney and the liver).

The percentage of blood volume made up by the cellular component in proportion to the overall volume of blood is called haematocrit. In the horse this is between 32-46% at rest. Under exertion the horse’s haematocrit can rise to more than 60%, therefore almost doubling its oxygen-carrying capacity in a natural manner.