Distribution is the uneven distribution of ventilation (aeration of the lungs), perfusion (blood flow to the lungs), and diffusion (gas exchange). This reduces arterialization of the blood even in healthy individuals. Arterialization describes the setting of arterial respiratory gas partial pressures.
What is distribution?
Distribution is the uneven distribution of ventilation (aeration of the lungs), perfusion (blood flow to the lungs), and diffusion (gas exchange). Humans depend on a constant supply of oxygen. Equally important is the removal of metabolic products, especially carbon dioxide. This gas exchange takes place in the lungs, more specifically in the alveoli (air sacs), and is called ventilation. Ventilation determines how much oxygen enters the alveoli and how much carbon dioxide is removed from them. Oxygen travels through the bloodstream to the tissues where it is needed. Carbon dioxide, as a metabolic end product, also travels through the bloodstream to the lungs, where it is exhaled. This circulation of blood is called perfusion. The ventilation-perfusion ratio is central in setting the arterial partial pressures of the respiratory gases. The third factor, but one that does not affect arterialization of blood as much, is diffusion. Diffusion is the passage of respiratory gases through the alveolar wall. According to Fick’s law of diffusion, it depends on the partial pressures of the respiratory gases, the diffusion distance and the area available. These 3 factors result in the distribution.
Function and task
The lung is not a homogeneous organ, meaning that not all areas are equally well perfused and ventilated. Physiologically, it is the case that the lower lung areas are better ventilated and perfused than the upper ones. In addition, there is a small percentage (2%) of blood volume that bypasses the gas exchange areas. This blood is referred to as shunt blood. It remains deoxygenated and enters the arterial system directly. As a result, the partial pressure of oxygen is reduced here. If two lung areas are now differently ventilated, poorer arterialized blood from the less ventilated area is constantly mixed in with the well-arterialized blood from the more ventilated area. This results in a mixture in which the O2 partial pressure becomes smaller and the CO2 partial pressure somewhat larger. The irregular distribution of ventilation, perfusion, and diffusion and the additional admixture of shunt blood result in less oxygen being present in the arterial blood than in the alveoli. The level of arterial partial pressures can be used to make a statement about the overall effect of respiration. Lung function is measured by these parameters. With age, arterial partial pressure of oxygen decreases, which is due to increase in distribution inequalities. R
icht values regarding arterial partial pressure of oxygen are about 95 mmHg in healthy adolescents, 80 mmHg in a 40-year-old, and 70 mmHg in a 70-year-old. However, the partial pressure drop has only a minor influence on the actual O2 saturation of hemoglobin. This is because the O2-binding curve shows a very flat course in the higher partial pressure range. As a result, in adolescence, O2 saturation is about 97%, and in the elderly, this value is reduced only to about 94%. Thus, sufficient oxygen loading of the blood is ensured even in old age.
Diseases and ailments
In lung diseases, arterialization is reduced all the more by worsened distribution. All diseases that affect ventilation, perfusion, and diffusion ultimately affect the setting of arterial respiratory gas partial pressures. The result is almost always a decrease in oxygen partial pressure with a concomitant increase in carbon dioxide partial pressure. Most importantly, the arterialization effect is determined by the ratio of ventilation to perfusion. Physiologically, this value is 0.8-1. If it is lower, it is hypoventilation. All values above this are called hyperventilation. In the case of alveolar hypoventilation, the partial pressure of O2 decreases and at the same time the partial pressure of CO2 increases to the same extent. This change is also reflected in the blood and hypoxia occurs. As a result, hemoglobin loading with oxygen is greatly reduced and cyanosis occurs.Cyanosis refers to the bluish discoloration of the skin. Alveolar hyperventilation is accompanied by an increase in O2 and a decrease in CO2. However, the organs do not receive an improved supply of oxygen because the hemoglobin is already maximally saturated under normal conditions. However, the drop in carbon dioxide may reduce cerebral perfusion. One type of ventilation disorder is called atelectasis. It results in a reduced ventilation of sections of the lungs. This is caused, for example, by obstruction of a bronchus. The consequence is a deterioration of oxygenation. In addition, a pleural effusion or a pneumothorax can impair ventilation and thus worsen distribution. In pleural effusion, fluid accumulation is the cause, and in pneumothorax, air accumulation is the cause. Obstructive ventilation disorders are associated with bronchial constriction. As a result, ventilation of the lungs is reduced. Examples include bronchial asthma or chronic obstructive pulmonary disease. The most common perfusion disorder is pulmonary embolism. Carryover of a thrombus leads to occlusion of a pulmonary artery and the lung is no longer supplied with blood. The body tries to compensate by accelerating the heartbeat. In addition, dyspnea occurs. Diffusion can also be disturbed, for example, by pulmonary edema. The patient notices the worsened distribution primarily because pronounced shortness of breath occurs.
