39.2 Gas Exchange across Respiratory Surfaces

Lung Structure and Gas Exchange

Structure for gas exchange efficiency

The lungs are built to move gases between air and blood as efficiently as possible. Every structural feature of the alveoli serves this goal.

• Large surface area: The lungs contain roughly 300 million alveoli, creating a total surface area about the size of a tennis court (~70 m²). Each alveolus is surrounded by a dense network of capillaries, maximizing the contact between air and blood.
• Thin diffusion barrier: Alveolar walls are extremely thin (about 0.2 μm). Capillary walls are similarly thin. Together, these two layers form the respiratory membrane, the barrier gases must cross. Thinner barrier = faster diffusion.
• Moist surfaces: A thin layer of fluid lines the inside of each alveolus. Gases must dissolve in this fluid before they can diffuse across the membrane, so moisture is essential for exchange to occur.
• Ventilation-perfusion matching: In healthy lungs, airflow (ventilation) and blood flow (perfusion) are closely matched. Local mechanisms adjust bronchiole diameter and arteriole diameter so that well-ventilated alveoli receive adequate blood supply. If an area of the lung isn't getting much air, blood flow to that region decreases to avoid wasting perfusion.

Partial pressures of respiratory gases

Gas exchange depends on partial pressures, not total gas concentrations. Partial pressure is the pressure exerted by a single gas within a mixture.

You can calculate it using Dalton's Law:

$P_i = P_\text{total} \times F_i$

• $P_i$ = partial pressure of gas $i$
• $P_\text{total}$ = total pressure of the gas mixture (e.g., atmospheric pressure, ~760 mmHg at sea level)
• $F_i$ = fractional concentration of gas $i$ (oxygen is about 21% of air, so $F_\text{O2} = 0.21$)

Typical values at sea level:

Notice that alveolar $P_\text{O2}$ (104 mmHg) is lower than inspired air (150 mmHg). That's because fresh air mixes with air already in the lungs and because oxygen is constantly being absorbed into the blood.

Movement of gases between alveoli and capillaries

Gases move by diffusion: from regions of high partial pressure to regions of low partial pressure. The steeper the pressure gradient, the faster the diffusion.

Oxygen transport:

1. Alveolar $P_\text{O2}$ (~104 mmHg) is much higher than the $P_\text{O2}$ of incoming venous blood (~40 mmHg).
2. This gradient drives oxygen across the respiratory membrane into the capillary blood.
3. Once in the blood, most oxygen binds to hemoglobin in red blood cells, forming oxyhemoglobin. By the time blood leaves the pulmonary capillaries, $P_\text{O2}$ has risen to ~100 mmHg.

Carbon dioxide transport:

1. Venous blood arriving at the lungs has a $P_\text{CO2}$ of ~45 mmHg, while alveolar $P_\text{CO2}$ is ~40 mmHg.
2. This gradient (smaller than the oxygen gradient, but $\text{CO}_2$ is about 20× more soluble than $\text{O}_2$) drives $\text{CO}_2$ out of the blood and into the alveoli to be exhaled.
3. In the blood, $\text{CO}_2$ travels in three forms:
   • Dissolved in plasma (~7–10%)
   • As bicarbonate ions ($\text{HCO}_3^-$) (~60–70%), the dominant form
   • Bound to hemoglobin as carbaminohemoglobin (~20–30%)

Factors Affecting Gas Exchange

Several variables determine how quickly gases diffuse across the respiratory membrane. Fick's Law of Diffusion ties them together: diffusion rate increases with a larger surface area, a greater concentration (partial pressure) gradient, and higher gas solubility, but decreases with greater membrane thickness.

• Gas solubility: $\text{CO}_2$ is far more soluble in blood than $\text{O}_2$. This is why $\text{CO}_2$ exchanges efficiently even though its partial pressure gradient across the membrane is relatively small (only ~5 mmHg).
• Surface area and membrane thickness: Diseases like emphysema destroy alveoli (reducing surface area), while pulmonary edema increases fluid thickness in the membrane. Both impair gas exchange.
• Countercurrent exchange: Some animals (notably fish) use a countercurrent system where blood and water flow in opposite directions across the gas exchange surface. This maintains a partial pressure gradient along the entire length of the exchange surface, making it more efficient than the mammalian system at extracting oxygen from low-oxygen environments.