Transport of Carbon dioxide

Carbon dioxide produced by the tissues has to be carried back to the lungs to be breathed out. Unlike oxygen, it is transported in more than one way, and its movement is closely tied to the same pressure conditions that govern oxygen transport.

Carried on haemoglobin

A portion of carbon dioxide — roughly 20 to 25 per cent — binds to haemoglobin and is carried as carbamino-haemoglobin. This binding depends on the partial pressure of carbon dioxide, and the partial pressure of oxygen is a major factor that influences it:

• In the tissues, where carbon dioxide pressure is high and oxygen pressure is low, more carbon dioxide binds to haemoglobin.
• In the alveoli, where carbon dioxide pressure is low and oxygen pressure is high, carbon dioxide dissociates from carbamino-haemoglobin. The carbon dioxide picked up at the tissues is therefore delivered at the alveoli.

Carried as bicarbonate — the role of carbonic anhydrase

Red blood cells contain a very high concentration of an enzyme called carbonic anhydrase, and small amounts of it are also present in the plasma. This enzyme speeds up the interconversion of carbon dioxide and water into bicarbonate and hydrogen ions, and it works in both directions depending on local conditions:

CO₂ + H₂O ⇌ (Carbonic anhydrase) H₂CO₃ ⇌ HCO₃⁻ + H⁺

• At the tissue site, where carbon dioxide pressure is high because of the breakdown of nutrients (catabolism), carbon dioxide diffuses into the blood — both into the red blood cells and the plasma — and the reaction runs forward, forming bicarbonate and hydrogen ions. …