Cell differentiation is the process by which a cell changes to become specialised for its function. As cells change they develop different sub-cellular structures, eventually turning into different types of cells. An example of this is red blood cells. The function of red blood cells is to carry oxygen from the lungs to the rest of the body. Red blood cells differentiate to become more efficient at this job. They do this by changing into a biconcave shape (curved on both sides, like a donut), providing a large surface area for absorbing oxygen. Additionally, they lose their nucleus to allow more room to carry oxygen. Finally, red blood cells will contain haemoglobin, a red pigment which binds to oxygen, so, the red blood cell can carry the oxygen around the body.