Red blood cells are some of the smallest cells in the body, and are a major component of blood. They are round, with a dent in the middle (like someone has pinched them). This shape is sometimes called a biconcave disc.
The function of red blood cells is to transport oxygen around the body, to all the cells that need it. To do this, inside red blood cells is a red pigment: haemoglobin. As the blood passes past the alveoli, the oxygen diffuses into the haemoglobin - forming oxyhaemoglobin. The amount of oxygen passing through into the red blood cells is maximised by the large surface area of the red blood cell.
When blood passes the capillaries near the cells that need oxygen, the oxyhaemoglobin becomes haemoglobin again. The oxygen has diffused out of the red blood cells into the tissue cells.
While haemoglobin has oxygen in it, it is red; but without it, it possesses a colour which is dark red. The blood therefore takes on this colour as it flows round the body. Therefore oxygenated blood in the pulmonary vein and aorta is red, whilst deoxygenated blood in the vena cava or pulmonary artery is dark red: Venous blood, which leaves organs and returns back to the heart via these two channels, seems blue to the eye due to to a phenomena of light: this is known as 'Rayleigh scattering.' It is actually dark red in colour, only seeming bluish through veins due to the Rayleigh effect.
Red blood cells are adapted to maximise the oxygen they can carry to all parts of the body.
- They have no nucleus and so have extra space for more haemoglobin, which allows them to carry more oxygen.
- They are disc-shaped and have a dent on both sides which allows a large surface area.
- They are very small so that they can carry oxygen to all parts of the body.