Red blood cells. Coloured scanning electron micrograph of blood in tissue. Blood is composed mainly of red blood cells erythrocytes. Red blood cells are pumped to all parts of the body. They contain haemoglobin which bonds with oxygen in a reversible reaction in the lungs, and then releases that oxygen to cells in tissues. They then remove carbon dioxide from the cells and return it to the lungs. Tissues are collections of similar cells acting together to perform specific functions in the body eg. muscles.