Transport of oxygen and carbon dioxide

Blood carries both gases. About 97% of oxygen travels inside the red blood cells, and only 3% dissolved in plasma. Of CO₂, 20-25% travels in red blood cells, about 70% as bicarbonate ions, and about 7% dissolved in plasma.

Haemoglobin, the red, iron-containing pigment of red blood cells, picks up oxygen loosely and can release it again, forming oxyhaemoglobin. Each haemoglobin molecule is like a car with four seats: it can carry up to four O₂ molecules. One aboard means it is 25% full (saturated); four aboard, 100%.

How much oxygen it picks up depends mainly on pO₂. pCO₂, H⁺ (acidity) and temperature also affect it. Plotting how full haemoglobin is against pO₂ gives an S-shaped (sigmoid) curve, the oxygen dissociation curve.

• In the lungs (alveoli): high pO₂, low pCO₂, fewer H⁺, lower temperature → haemoglobin picks up oxygen.
• In the tissues: low pO₂, high pCO₂, more H⁺, higher temperature → the car drops its passengers off: oxyhaemoglobin releases oxygen.

Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues.

CO₂ also rides on haemoglobin, as carbamino-haemoglobin. It binds where pCO₂ is high and pO₂ low (the tissues) and is released in the alveoli.

Most CO₂, though, travels as bicarbonate. Red blood cells are rich in the enzyme carbonic anhydrase, which drives: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. In the tissues, where CO₂ is high, the reaction runs forwards, turning CO₂ into bicarbonate. In the alveoli, it runs backwards, releasing CO₂ to be breathed out. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.