Blood makes up around 7% of the weight of a human body. Bone marrow contains stem cells which, through the process of haemopoiesis, generate the three main types of cells found in the blood: red blood cells, white blood cells and platelets. Red blood cells carry oxygen around the body. White blood cells are an important part of the body’s immune system. Platelets help blood clot in order to limit bleeding when your skin is cut. Take a close up look with our gallery.