67 Diffusion

Gas exchange in the lungs is a vital physiological process essential for sustaining life. This intricate system involves the movement of oxygen and carbon dioxide between the alveoli and the pulmonary capillaries, a process critical for maintaining appropriate oxygen levels in the blood and removing carbon dioxide, a waste product of cellular respiration. In this comprehensive exploration, we will delve into the dynamic mechanisms underlying oxygen and carbon dioxide exchange, examining the influence of partial pressure gradients, surface area, permeability, and diffusion distance.

Oxygen and Carbon Dioxide Exchange in the Alveoli and Pulmonary Capillaries

At the heart of the respiratory system lies the alveoli, tiny air sacs nestled within the lungs where the exchange of gases takes place. The process begins with ventilation, the movement of air into and out of the lungs. Inspired air, rich in oxygen, reaches the alveoli during inhalation. Simultaneously, deoxygenated blood, loaded with carbon dioxide, is transported to the pulmonary capillaries surrounding the alveoli. This sets the stage for the exchange of gases to occur.

The concentration gradients of oxygen and carbon dioxide drive these exchanges. The partial pressure of oxygen (PO₂) in the alveoli is approximately 104 mm Hg, significantly higher than the partial pressure of oxygenated pulmonary venous blood, which hovers around 100 mm Hg. On the other hand, the partial pressure of carbon dioxide (PCO₂) in the alveoli is approximately 40 mm Hg, lower than that in the deoxygenated blood, which is ~45 mm Hg, creating a favorable gradient for carbon dioxide removal.

As oxygen-rich air fills the alveoli, oxygen diffuses across the thin respiratory membrane into the capillaries, binding with hemoglobin to form oxyhemoglobin. Simultaneously, carbon dioxide, carried by deoxygenated blood, diffuses from the capillaries into the alveoli. This exchange is crucial for maintaining the delicate balance required for cellular respiration, providing oxygen to tissues and removing waste carbon dioxide.

Factors Influencing Oxygen and Carbon Dioxide Movements

Several factors intricately regulate the movements of oxygen and carbon dioxide, ensuring efficient gas exchange in various physiological conditions.

Partial Pressure Gradients

Changes in partial pressure gradients significantly impact gas exchange. At high altitudes, where atmospheric pressure is lower, the partial pressure of oxygen decreases. This reduction in the driving force for oxygen diffusion may compromise gas exchange efficiency, affecting oxygen saturation levels in arterial blood. Individuals at high altitudes may experience hypoxia, prompting physiological adaptations to enhance oxygen-carrying capacity.

Conversely, changes in partial pressure gradients also influence carbon dioxide removal. During intense exercise, cellular respiration increases, elevating the partial pressure of carbon dioxide. This heightened gradient facilitates efficient removal of carbon dioxide during exhalation.

Surface Area and Permeability

The exchange surface area and permeability of the respiratory membrane are crucial determinants of gas exchange efficiency. Emphysema, a condition characterized by the destruction of alveolar walls, reduces the surface area available for gas exchange. This diminished surface area impedes oxygen diffusion, leading to hypoxemia and decreased oxygen saturation in arterial blood.

Diffusion Distance

The thickness of the respiratory membrane influences the distance gases must traverse during exchange. Diseases such as pulmonary fibrosis, which causes scarring and thickening of the alveolar walls, increase the diffusion distance. This heightened barrier impairs gas exchange, resulting in decreased oxygen uptake and inefficient carbon dioxide removal.

Local Changes in Gas Concentrations and Regulation of Pulmonary Arterioles and Bronchioles

Local adjustments in oxygen and carbon dioxide concentrations play a pivotal role in regulating the diameters of pulmonary arterioles and bronchioles, optimizing blood flow and ventilation in response to specific regional needs.

Pulmonary Arterioles

The diameter of pulmonary arterioles is sensitive to changes in the partial pressure of oxygen. In well-ventilated alveoli, where oxygen levels are high, pulmonary arterioles vasodilate, increasing blood flow to match the robust ventilation. This ensures that oxygen-rich blood is efficiently transported to tissues for cellular metabolism.

Conversely, in alveoli with reduced ventilation and consequently lower oxygen levels, pulmonary arterioles constrict. This redirection of blood flow away from poorly ventilated regions helps to optimize oxygen delivery, preventing systemic hypoxemia.

Bronchioles

The diameter of bronchioles, responsible for regulating airway resistance, responds to changes in the partial pressure of carbon dioxide. An elevated partial pressure of carbon dioxide in the alveoli induces bronchodilation, facilitating increased airflow. This adaptive response aids in efficient removal of excess carbon dioxide during exhalation, preventing its accumulation in the body.

Additionally, decreased oxygen levels in the blood supply prompt bronchodilation, enhancing the removal of carbon dioxide. This coordinated response ensures a balance between ventilation and perfusion, promoting optimal gas exchange.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 22