Blood is a biological fluid that circulates in the blood vessels (arteries, veins) and the heart. It is a vital biological fluid composed of a watery fluid, plasma, and billions of cells, the majority of which are red blood cells. It is these cells that give blood its colour. But blood is composed of 3 cell types: red blood cells, white blood cells and platelets. These last two types of cells represent only 1% of the volume of the blood, whereas the red blood cells represent 44%.

Blood is vital because it allows the transport of oxygen and nutrients necessary for the vital processes of all the tissues of the body. It also carries waste products such as carbon dioxide and nitrogenous waste to disposal sites such as the kidneys, lungs, liver and intestines. It also carries cells and molecules from the immune system to the tissues, and diffuses hormones throughout the body.