Role of red blood cells in oxygen transport

· Red blood cells transport most oxygen from the lungs to respiring tissues.
· Oxygen binds reversibly to haemoglobin to form oxyhaemoglobin.
· Each haemoglobin molecule has 4 haem groups, each containing Fe²⁺, so each haemoglobin can carry 4 oxygen molecules.
· High partial pressure of oxygen (pO₂) in the lungs → haemoglobin loads oxygen.
· Low pO₂ in respiring tissues → oxyhaemoglobin unloads oxygen.
· Cooperative binding: after the first oxygen binds, haemoglobin changes shape so the next oxygen molecules bind more easily.

This diagram shows how haemoglobin inside red blood cells carries oxygen. The haem groups contain iron, which binds oxygen reversibly, allowing oxygen loading in the lungs and unloading in tissues. Source

Carbon dioxide transport in red blood cells

· Carbon dioxide (CO₂) from respiration diffuses from tissues into blood and then into red blood cells.
· Inside red blood cells, carbonic anhydrase catalyses: CO₂ + H₂O ⇌ H₂CO₃.
· Carbonic acid (H₂CO₃) dissociates into H⁺ and HCO₃⁻.
· H⁺ ions bind to haemoglobin to form haemoglobinic acid (HHb).
· This prevents large changes in blood pH, helping haemoglobin act as a buffer.
· Some CO₂ binds directly to haemoglobin to form carbaminohaemoglobin.
· At the lungs, these reactions reverse so CO₂ is released and exhaled.

Chloride shift

· Hydrogencarbonate ions (HCO₃⁻) diffuse out of red blood cells into the plasma.
· To maintain electrical neutrality, chloride ions (Cl⁻) diffuse into red blood cells.
· This exchange is called the chloride shift.
· The chloride shift allows continued conversion of CO₂ into HCO₃⁻, so more CO₂ can be transported from tissues to lungs.
· In the lungs, the process reverses: HCO₃⁻ enters red blood cells, Cl⁻ leaves, and CO₂ is regenerated for exhalation.

Role of plasma in carbon dioxide transport

· Plasma transports CO₂ mainly as hydrogencarbonate ions (HCO₃⁻).
· A smaller amount of CO₂ is transported dissolved directly in plasma.
· Plasma also carries some CO₂ after it binds to plasma proteins, forming carbamino compounds.
· Transport as HCO₃⁻ is the most important route because it allows much more CO₂ to be carried than dissolved CO₂ alone.
· In the lungs, HCO₃⁻ returns to red blood cells, where CO₂ is re-formed and diffuses into the alveoli.

Oxygen dissociation curve of adult haemoglobin

· The oxygen dissociation curve shows the relationship between pO₂ and percentage saturation of haemoglobin with oxygen.
· Adult haemoglobin has a sigmoid / S-shaped curve due to cooperative binding.
· At low pO₂, haemoglobin has low affinity for oxygen, so oxygen is released easily.
· At medium pO₂, small increases in pO₂ cause large increases in oxygen saturation.
· At high pO₂, haemoglobin becomes almost fully saturated, forming a plateau.
· In the lungs, high pO₂ gives high haemoglobin saturation, so oxygen loads efficiently.
· In respiring tissues, lower pO₂ causes oxygen to dissociate from haemoglobin and diffuse into cells.

Bohr shift

· The Bohr shift occurs when high CO₂ concentration lowers pH, reducing haemoglobin’s affinity for oxygen.
· In actively respiring tissues, CO₂ is high, so more H⁺ ions form.
· H⁺ ions bind to haemoglobin, causing haemoglobin to release oxygen more readily.
· This shifts the oxygen dissociation curve to the right.
· A right shift means haemoglobin has lower oxygen affinity and unloads more oxygen at the same pO₂.
· The Bohr shift is important because tissues with the highest respiration rate receive the most oxygen.
· In the lungs, CO₂ is removed, pH rises, haemoglobin affinity for oxygen increases, and oxygen loads again.

Key exam comparisons

· Chloride shift = movement of Cl⁻ into red blood cells as HCO₃⁻ leaves; maintains electrical neutrality.
· Bohr shift = high CO₂ / H⁺ reduces haemoglobin’s oxygen affinity; curve shifts right.
· Haemoglobinic acid = haemoglobin bound to H⁺, helping buffer blood pH.
· Carbaminohaemoglobin = CO₂ bound directly to haemoglobin.
· Oxyhaemoglobin = oxygen bound to haemoglobin.
· Carbonic anhydrase = enzyme in red blood cells that catalyses conversion of CO₂ + H₂O into H₂CO₃.

Common exam traps

· Do not say oxygen is carried mainly dissolved in plasma; most oxygen is carried by haemoglobin.
· Do not confuse chloride shift with Bohr shift.
· Do not say HCO₃⁻ is made in plasma first; most is formed inside red blood cells.
· Do not forget that carbonic anhydrase speeds up a reversible reaction.
· Do not describe the oxygen dissociation curve as linear; it is sigmoid because of cooperative binding.
· Do not say the Bohr shift helps oxygen load in tissues; it helps oxygen unload in tissues.