Red blood cells. Computer illustration of red blood cells, depicted as though they were travel- ling through a blood vessel. Red blood cells (erythrocytes) are flattened, circular, biconcave discs. They are the most abundant type of cell in human blood, accounting for 40% of blood volume. Each cubic millilitre of blood contains around five million of these tiny, flexible cells. Their red colour comes from a protein haemoglobin which they contain, and which picks up oxygen in the lungs and distributes it around the body. Because they are subjected to constant movement and physical pounding, red blood cells last only four months before being destroyed.