This higher tier covers Higher Carbon Dioxide Transport: Detailed Percentages within Gas Exchange in Humans for GCSE Biology. Lung structure, alveoli adaptations, breathing mechanism, gas transport in blood, and effects of smoking It is section 15 of 19 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Higher Carbon Dioxide Transport: Detailed Percentages
Carbon dioxide is transported in three forms in the blood:
- 70% as hydrogen carbonate ions (HCO₃⁻) dissolved in plasma — CO₂ reacts with water inside red blood cells to form carbonic acid (H₂CO₃), which rapidly dissociates into H⁺ and HCO₃⁻; the HCO₃⁻ diffuses out into plasma
- 23% dissolved directly in plasma as CO₂
- 7% bound to haemoglobin as carbaminohaemoglobin
In the lungs, the low CO₂ concentration of alveolar air reverses this process: HCO₃⁻ diffuses back into red blood cells, recombines with H⁺ to form carbonic acid, which breaks down into CO₂ and water; the CO₂ diffuses into alveolar air and is exhaled.
Practice questions for Gas Exchange in Humans
Which is the correct order of structures air passes through to reach the lungs?
Explain how the structure of alveoli is adapted for efficient gas exchange.