Erythrocytes (red blood cells) are the most numerous cells in the blood, about 5 million per mm3. Their function is to transport oxygen from the lungs to the tissues, and they also help to carry carbon dioxide.Erythrocytes are highly specialised for this role:• Biconcave disc shape: this gives a large surface area to volume ratio for rapid diffusion of oxygen in and out, and no part of the cytoplasm is far from the surface.
• No nucleus: mature mammalian red cells lose their nucleus, leaving more space for haemoglobin. Each cell contains about 280 million haemoglobin molecules.
• No mitochondria: they respire anaerobically, so they do not use up the oxygen they are carrying.
• Small and flexible: about 7 to 8 µm across, they can bend and squeeze through capillaries that are narrower than themselves. This slows them down and brings them close to the capillary wall, shortening the diffusion distance.
• Carbonic anhydrase: this enzyme in the cytoplasm speeds up the reaction of carbon dioxide with water, important in carbon dioxide transport.
Because they have no nucleus, erythrocytes cannot divide or repair themselves. They survive only about 120 days, after which they are broken down, mainly in the liver and spleen. The iron is recycled and the rest of the haem is converted to bile pigments.New erythrocytes are made continuously from stem cells in the bone marrow, at a rate of about 2 million per second in an adult.
• No nucleus: mature mammalian red cells lose their nucleus, leaving more space for haemoglobin. Each cell contains about 280 million haemoglobin molecules.
• No mitochondria: they respire anaerobically, so they do not use up the oxygen they are carrying.
• Small and flexible: about 7 to 8 µm across, they can bend and squeeze through capillaries that are narrower than themselves. This slows them down and brings them close to the capillary wall, shortening the diffusion distance.
• Carbonic anhydrase: this enzyme in the cytoplasm speeds up the reaction of carbon dioxide with water, important in carbon dioxide transport.
Because they have no nucleus, erythrocytes cannot divide or repair themselves. They survive only about 120 days, after which they are broken down, mainly in the liver and spleen. The iron is recycled and the rest of the haem is converted to bile pigments.New erythrocytes are made continuously from stem cells in the bone marrow, at a rate of about 2 million per second in an adult.