Red blood cells (erythrocytes) transport oxygen from the lungs to respiring tissues. They are the most numerous cells in the blood and are highly specialised for this job.
Adaptations:• Biconcave disc shape: this gives a large surface area to volume ratio, so oxygen diffuses in and out quickly.
• Small size (about 7-8 µm in diameter): every haemoglobin molecule is close to the cell surface, so the diffusion distance is short. The cells fit through capillaries in single file, bringing them close to tissue cells.
• No nucleus (in mammals), and no mitochondria or endoplasmic reticulum: this leaves more space for haemoglobin, so each cell carries more oxygen. Without mitochondria, the cell does not use the oxygen it carries.
• Flexible cytoskeleton: the cells can bend and twist to squeeze through capillaries narrower than their own diameter.
• They contain the enzyme carbonic anhydrase, which is involved in carrying carbon dioxide.Because they have no nucleus, red blood cells cannot divide or repair themselves, and they only survive about 120 days. They are broken down in the liver and spleen, and new ones are made continuously from stem cells in the bone marrow.When observed in a blood smear under a light microscope, red blood cells appear as pale-centred discs, because they are thinner in the middle. They greatly outnumber white blood cells, which are larger and have a stained nucleus.
Adaptations:• Biconcave disc shape: this gives a large surface area to volume ratio, so oxygen diffuses in and out quickly.• Small size (about 7-8 µm in diameter): every haemoglobin molecule is close to the cell surface, so the diffusion distance is short. The cells fit through capillaries in single file, bringing them close to tissue cells.
• No nucleus (in mammals), and no mitochondria or endoplasmic reticulum: this leaves more space for haemoglobin, so each cell carries more oxygen. Without mitochondria, the cell does not use the oxygen it carries.
• Flexible cytoskeleton: the cells can bend and twist to squeeze through capillaries narrower than their own diameter.
• They contain the enzyme carbonic anhydrase, which is involved in carrying carbon dioxide.Because they have no nucleus, red blood cells cannot divide or repair themselves, and they only survive about 120 days. They are broken down in the liver and spleen, and new ones are made continuously from stem cells in the bone marrow.When observed in a blood smear under a light microscope, red blood cells appear as pale-centred discs, because they are thinner in the middle. They greatly outnumber white blood cells, which are larger and have a stained nucleus.