Diffusion and Gas Exchange in the Lungs

Requirements for Effective Gas Exchange

For gas exchange between the alveoli and blood to occur efficiently, three key conditions are necessary:

• Continuous alveolar ventilation. This ensures that the gas composition within the alveolus remains relatively constant.
• Gas diffusion. The movement of molecules across the alveolar-capillary membrane. The driving force here is the partial pressure gradient of the gases.
• Adequate perfusion. The blood supply to the pulmonary capillaries must closely match their level of ventilation. This is known as the ventilation-perfusion ratio.

Mechanisms of Gas Diffusion

Diffusion of oxygen and carbon dioxide always follows the partial pressure gradient—moving from an area of high pressure to an area of low pressure.

In the Lungs

Here, gas exchange occurs between alveolar air and the deoxygenated blood arriving at the capillaries:

• Oxygen ($O_2$): Alveolar $O_2$ partial pressure is approximately $102\text{ mmHg}$, whereas in venous blood it is $40\text{ mmHg}$. Driven by this gradient ($102 \rightarrow 40$), oxygen diffuses into the blood.
• Carbon dioxide ($CO_2$): The partial pressure of $CO_2$ in incoming blood is $48\text{ mmHg}$, while in the alveoli it is $40\text{ mmHg}$. Carbon dioxide diffuses into the alveolus ($48 \rightarrow 40$).

In the Tissues

Gas exchange occurs between arterial blood and metabolically active cells:

• Oxygen: In tissue capillaries, $O_2$ partial pressure is about $90\text{ mmHg}$, whereas in actively metabolizing cells, it drops to $0–20\text{ mmHg}$. Oxygen moves down its gradient from the blood into the cells.
• Carbon dioxide: Cells continuously produce $CO_2$, raising local tissue partial pressure up to $60\text{ mmHg}$. In the incoming arterial blood, it is about $46\text{ mmHg}$. Carbon dioxide diffuses into the blood.

Ventilation-Perfusion Coupling

The lungs possess local autoregulatory mechanisms to optimize gas exchange efficiency. The body redistributes resources to avoid wasting blood flow on unventilated areas and vice versa.

1. Decreased perfusion (hypoperfusion). In poorly perfused regions, alveolar $CO_2$ drops, causing local bronchiolar constriction. Ventilation decreases, and airflow is redirected to better-perfused lung regions.
2. Decreased ventilation (hypoventilation). This leads to localized hypoxia and hypercapnia. This prompts local hypoxic pulmonary vasoconstriction. Blood flow decreases and is redirected toward well-ventilated alveoli.

Additionally, there is a vertical gradient in the lungs. Both ventilation and perfusion vary from apex to base, but the perfusion gradient is steeper. This creates regional differences in the ventilation-perfusion ratio ($V/Q$) across different parts of the lung.

Barrier Characteristics and Blood Flow

The blood-gas barrier across which gases diffuse has a total surface area of 70–80 m² and an extremely small thickness ranging from 0.3 to 2.0 µm.

Pulmonary surfactant plays a crucial role. Beyond regulating surface tension to prevent alveolar collapse, it helps modulate oxygen absorption kinetics across the gas-liquid interface.

The rate of oxygen diffusion into the blood heavily depends on the pressure gradient. A gradient of just $1\text{ mmHg}$ transfers $25–60\text{ mL/min}$ of $O_2$. With a gradient of $60\text{ mmHg}$, the rate reaches $3600\text{ mL/min}$ (mean resting oxygen consumption is about $300\text{ mL/min}$). Carbon dioxide has a much higher solubility and diffuses 25 times faster than oxygen.