Red blood cells
Red blood cells are the most numerous: about 5 million in every mm3 of blood.
Figure to be added
Shape
A biconcave disc, which gives a large surface for taking up oxygen.
No nucleus
Leaving more room for haemoglobin.
Haemoglobin
An iron-containing red pigment that binds oxygen in the lungs and releases it in the tissues.
Life span
About four months; old cells are broken down in the liver and spleen, and new ones are made in the bone marrow.
Anaemia
Too few red cells, or too little haemoglobin, means the blood carries too little oxygen: tiredness and pale skin follow. A lack of iron in the diet is a common cause.