68 Ventilation-perfusion matching

Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. However, factors such as regional gravity effects on blood, blocked alveolar ducts, or disease can cause ventilation and perfusion to be imbalanced.

While ventilation and perfusion coupling is a fundamental aspect of efficient gas exchange in the lungs, the influence of regional gravity effects introduces an additional layer of complexity to these processes. The gravitational forces acting on the lung contribute to variations in blood flow and ventilation, particularly when an individual transitions between upright and supine positions. When an individual is in an upright position, as commonly experienced during daily activities, gravity exerts a greater effect on the lower lung regions. Consequently, blood flow tends to be more pronounced in the bases of the lungs, and ventilation is relatively higher in the upper lung regions. This regional distribution helps optimize gas exchange by matching well-ventilated areas with a proportionate blood supply, enhancing overall pulmonary efficiency. Conversely, when an individual assumes a supine (lying down) position, the influence of gravity on lung perfusion changes. In this orientation, blood flow becomes more evenly distributed across the lung regions, and ventilation is also distributed more uniformly. While this redistribution ensures adequate gas exchange in different lung areas, it can also lead to potential mismatches in ventilation and perfusion, particularly if there are pre-existing conditions affecting these processes.

The partial pressure of oxygen in alveolar air is about 104 mm Hg, whereas the partial pressure of the oxygenated pulmonary venous blood is about 100 mm Hg. When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane. The body has mechanisms that counteract this problem. In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serve the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation. At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow. Factors such as carbon dioxide, oxygen, and pH levels can all serve as stimuli for adjusting blood flow in the capillary networks associated with the alveoli.

Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter, allowing carbon dioxide to be exhaled from the body at a greater rate. Similarly, a decreased level of oxygen in the blood supply also causes bronchodilation. As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow.

Oxygen and Carbon Dioxide Concentration Gradients and Net Gas Movements

In the systemic capillaries, the partial pressure of oxygen (PO2) is higher than in the body tissues, causing oxygen to diffuse from the blood into the tissues. Conversely, the partial pressure of carbon dioxide (PCO2) is higher in the tissues than in the systemic capillaries, leading to carbon dioxide diffusion from the tissues into the blood. This movement of gases is driven by the concentration gradients established by cellular respiration.

Influence of Cellular Respiration on Gas Gradients

Cellular respiration continuously consumes oxygen and produces carbon dioxide, thereby maintaining the concentration gradients that facilitate gas exchange between the blood and body tissues. The ongoing metabolic activity in tissues ensures that oxygen is consistently drawn from the blood and that carbon dioxide is released into the blood, maintaining the necessary gradients for efficient 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