Gas exchange and haemoglobin

What you’ll learn

• Apply Fick’s relationship to gas-exchange surfaces.
• Interpret oxygen dissociation curves, the Bohr effect and fetal haemoglobin.
• Explain the roles of haemoglobin in oxygen and carbon dioxide transport.

Understand the idea

The lungs provide many alveoli with a large total surface area. Alveolar epithelium and capillary endothelium are thin, keeping the diffusion path short, while the moist surface allows gases to dissolve. Ventilation refreshes alveolar air and blood flow renews the capillary blood, maintaining oxygen and carbon dioxide gradients. Fick’s relationship predicts faster diffusion with larger area and concentration difference, and slower diffusion through a thicker barrier, when other factors are constant.

Haemoglobin contains four haem groups, each with an iron ion that can bind one oxygen molecule reversibly. Binding of the first oxygen changes the molecule’s shape and increases its affinity for subsequent oxygen, producing the steep middle region of the sigmoid dissociation curve. At the lungs, high oxygen partial pressure favours loading; in tissues, lower partial pressure favours unloading. Percentage saturation describes occupied oxygen-binding sites, rather than the oxygen concentration of plasma.

Respiring tissue releases carbon dioxide. Much of it is transported as hydrogencarbonate after conversion in red blood cells; haemoglobin buffers some of the hydrogen ions, and some carbon dioxide binds to haemoglobin. Raised carbon dioxide and the associated fall in pH reduce haemoglobin’s oxygen affinity: the Bohr effect shifts the curve to the right. Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin, supporting transfer from maternal blood at the placenta. Their blood supplies remain separate.

Key formulae

$$\text{diffusion rate}\propto\frac{\text{surface area}\times\text{concentration difference}}{\text{diffusion distance}}$$

$$\mathrm{Hb}+4\mathrm{O_2}\rightleftharpoons\mathrm{Hb(O_2)_4}$$

$$\mathrm{CO_2+H_2O\rightleftharpoons H_2CO_3\rightleftharpoons H^++HCO_3^-}$$

Worked example

A model gas-exchange surface has twice the original area and three times the original thickness. The concentration difference is unchanged. Calculate the diffusion rate as a fraction of its original value.

1. The area change multiplies rate by 2.
2. The thickness change divides rate by 3.
3. Combine the factors: new rate ÷ old rate = 2/3.

Answer: $2/3$ of the original rate, about $66.7\%$.