Which blood cells carry oxygen around the body?

Red blood cells handle oxygen transport, and almost everything about their design serves that one job. Each is packed with haemoglobin, a protein containing iron atoms that bind oxygen loosely in the lungs and release it in tissues where oxygen levels are low. Mature human red cells eject their nucleus, freeing up space for more haemoglobin, and take on a flattened disc shape dimpled on both sides, which increases surface area and lets them fold to squeeze through the narrowest capillaries. The iron is also what makes blood red, and oxygen-rich blood is a brighter red than the darker blood returning to the heart. Red cells last around four months before being broken down, mainly in the spleen and liver, with the iron recycled. Their production is controlled by a hormone from the kidneys that increases when oxygen is scarce, which is why training at altitude raises red cell counts.

Carbon monoxide is dangerous because it grips haemoglobin far more tightly than oxygen does, blocking the sites.