Distribution is the uneven distribution of ventilation (ventilation of the lungs), perfusion (perfusion of the lungs) and diffusion (gas exchange). As a result, the arterialization of the blood is reduced even in the healthy. Arterialization describes the setting of arterial breathing gas partial pressures.

What is the distribution?

Distribution is the uneven distribution of ventilation (ventilation of the lungs), perfusion (perfusion of the lungs) and diffusion (gas exchange).

Humans depend on constant oxygen supply. Equally important is the removal of metabolic products, especially carbon dioxide. This gas exchange takes place in the lungs, more precisely in the alveoli (alveoli), and is referred to as ventilation. The ventilation determines how much oxygen gets into the alveoli and how much carbon dioxide is taken from them.

Through the bloodstream, the oxygen reaches the tissue where it is needed. The carbon dioxide as a metabolic end product also passes through the bloodstream to the lungs, where it is exhaled. This perfusion is called perfusion. The ventilation-perfusion ratio is of key importance in adjusting the arterial partial pressures of the respiratory gases.

The third factor that does not affect the arterialization of the blood is diffusion. Diffusion is the passage of the 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 three factors result in the distribution.

Function & Task

The lung is not a homogeneous organ, which means that not all areas are equally well supplied with blood and ventilated. Physiologically, the lower lung areas are better ventilated and perfused than the upper ones. In addition, there is a small proportion (2%) of the blood volume bypassing the gas exchange areas. This blood is called shunt blood. It remains low in oxygen and enters the arterial system directly. As a result, the oxygen partial pressure is reduced here. Now if two lung areas are ventilated differently, the well-arterialized blood from the more ventilated area constantly mixed poorly arterialized blood from the less ventilated area. This results in a mixture in which the O2 partial pressure is smaller and the CO2 partial pressure slightly larger.

Due to the irregular distribution of ventilation, perfusion and diffusion and the additional addition of shunt blood, there is less oxygen in the arterial blood than in the alveoli. About the height of the arterial partial pressures one can make a statement about the total effect of the respiration.

The lung function is measured via these parameters. With age, arterial oxygen partial pressure decreases, due to increase in distributional non-uniformity. R

Values ​​with regard to the arterial oxygen partial pressure are approximately 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 slight effect on the actual O2 saturation of hemoglobin. Because the O2 binding curve has a very flat course in the higher partial pressure range. The result is that in adolescence, the O2 saturation at about 97% and this value is reduced in the elderly only about 94%. Thus, a sufficient oxygen loading of the blood is ensured even in old age.

Diseases & complaints

In lung diseases, arterialization is reduced all the more by deteriorated distribution. All diseases that affect ventilation, perfusion, and diffusion ultimately interfere with the setting of the arterial respiratory gas partial pressures. The result is almost always a decrease in the oxygen partial pressure with simultaneous increase in the 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 he is under it is a hypoventilation. All values ​​above are called hyperventilation.

In alveolar hypoventilation, the partial pressure of O2 drops and at the same time the CO2 partial pressure rises to the same extent. This change is also seen in the blood and there is a hypoxia. This greatly reduces hemoglobin loading with oxygen and cyanosis occurs. Cyanosis refers to the bluish discoloration of the skin.

Alveolar hyperventilation is associated with an increase in O2 and CO2. However, the organs are not supplied with oxygen because the hemoglobin is already fully saturated under normal conditions. However, by the carbon dioxide waste, the cerebral blood flow can be reduced.

One type of ventilation disorder is so-called atelectasis. There is a lack of ventilation of lung sections. This is evoked, for example, by laying a bronchus. The result is a deterioration of oxygenation. In addition, a pleural effusion or a pneumothorax can affect the ventilation and thus worsen the distribution. When pleural effusion is a fluid accumulation and pneumothorax an accumulation of air is the cause.

Obstructive ventilation disorders are associated with a restriction of the bronchi. This will reduce the ventilation of the lungs. Examples include bronchial asthma or chronic obstructive pulmonary disease.

The most common perfusion disorder is pulmonary embolism. Carrying out a thrombus leads to occlusion of a pulmonary artery and the lung is no longer perfused. The body tries to compensate for this by accelerating the heartbeat. In addition, a dyspnoea occurs.

Diffusion can also be disturbed, for example by pulmonary edema. The worsened distribution is noted by the patient mainly because of marked respiratory distress.

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