8.2.1—Red blood cell roles in O2 and CO2 transport

Red blood cells transport most of the body’s oxygen using haemoglobin. Oxygen bound to haemoglobin forms oxyhaemoglobin; this is distinct from the smaller amount of oxygen carried dissolved in plasma.

• Biconcave shape: the disc-shaped cell has a large surface relative to its volume → oxygen can reach haemoglobin across a short cell distance → loading and unloading are efficient.
• No nucleus: the mature red cell has no nucleus → more internal space is available for haemoglobin → more oxygen can be carried per cell.
• High haemoglobin content: haemoglobin contains four haem groups, each able to bind one oxygen molecule → one haemoglobin molecule can carry four oxygen molecules → red cells provide the main reversible oxygen-carrying capacity of blood.
• Lung–tissue boundary: in the lungs, oxygen binds to haemoglobin and oxyhaemoglobin forms; in respiring tissues, oxygen can dissociate and enter cells. The oxygen-dissociation curve and Bohr shift explain how conditions tune this exchange in later cards.

Do not equate oxygen transport with oxygen dissolved in plasma: most is haemoglobin-bound. Keep carbon-dioxide forms, the oxygen-dissociation curve and the Bohr shift for their dedicated cards. No image generated or bound.