The pulmonary ventilation is the action of the Poumon S which aspire the ambient Air (inspiration) and blow of the “vitiated” air (expiry). It is what one calls “breathing” in the usual vocabulary, but in medicine and biology, the term “breathing” indicates the energy production by the cells, in particular with dioxygene (usually called and by abuse language Oxygène) brought by ventilation (see the cellular article Respiration).
Modeling of pulmonary ventilation
One divides in a diagrammatic way the breathing apparatus into air Routes of conduction, which are the trachea, the bronchi, and the bronchioles (of the bronchus stock to final bronchiole) and in alveolar sector (i.e. very part of the breathing apparatus located after a final bronchiole: respiratory bronchioles, alveolar channels, and cells).
Pulmonary physiology uses in a permanent way a certain number of physical principles which underlie the clinical practice:
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resistance to the gas flow (on the level of the air routes):
with Pb pressure on the level of the mouth, PA pressure on the level of the cells, and V' gas flow crossing the air routes.
with L the length of the tube (here air routes), and R its ray.
The last formula is particularly important since it shows that the resistance of the air routes of conduction (this rule is also valid for vascular resistance) is controlled by the adaptation of their ray to the power fourth: a tiny modification of the ray thus involves a variation much more important of the resistance and thus of the capacity to convey the air in the cells.
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the law of Laplace (on the level of the cells):
with T the surface stress on the walls of the cell and R their ray.
For an equal tension in two cells of different rays, the pressure will be thus more important in the small cell than in the large one; consequently, the small cells should be emptied in the large ones by a phenomenon of balancing pressures, which is not the case in practice. This equation thus makes it possible to clarify the role of surfactant, liquid which papers the alveolar wall and lowers the surface tension of it, so that alveolar ventilation is homogeneous.
Respiratory mechanics
This homogeneous ventilation in addition justifies the mono-alveolar modeling which considers the lung as a giant cell which inflates and deflates with each ventilatory cycle. This representation makes it possible to include/understand the concept of died space, which is the whole of the elements of the respiratory tree filled up of air but not taking part actively in the gaseous exchange air/blood. Anatomically, they are the higher air routes of conduction, that is to say approximately 150 ml in the adult. But physiologically, it gathers in more the unit of spaces of gaseous exchange which does not provide their function by absence of perfusion.
One notes this last VD volume.
The spontaneous ventilatory movement is done by muscles which raise the rib cage, the increase in volume of the lungs causes a depression which aspires the air (one speaks about ventilation in negative Pression); at the time of the inspiration, the diaphragm drops and pushes the internal organs to make it possible the lungs to develop towards the feet (moreover, when a person sleeps, one sees her belly rising and bending down). The expiry is passive, it is the natural elasticity of the rib cage and the weight of the internal organs which makes decrease the volume of the lungs.
Ventilation is carried out in the thoracic cavity thanks to the respiratory functional units, with the air Routes, like the Plèvre S.
Several acini connected by the alveolar channels forms the pulmonary Lobule, the exchanges of gas between the air and blood is done in the bronchioles lobulaires.
Ventilation at rest in the adult in good health is from 12 to 20 movements per minute, it can vary according to several factors like the physical-activity or the emotions. The disorders of ventilation are called Dyspnée , ventilation can be for example faster ( Tachypnée ) or slower ( Bradypnée ) that the “normal” (normality depending on the individuals). When ventilation goes down in lower parts of 6 movement per minutes or stops ( Apnée ), it is estimated that it is ineffective and must be compensates by an artificial ventilation.
The disease of Ondine is a syndrome which appears by the complete absence of spontaneous breathing (no ventilatory reflex). The patient must “think” of breathing; the night, it must be placed under ventilator.
Gaseous exchange
At the time of the inspiration, the ambient air penetrates in the lungs, and the Dioxygène (O2, gas which composes 21 % of the air) passes in blood and is fixed at the red globules. The Carbon dioxide (CO2) dissolved in the Blood plasma passes him in the air contained in the lungs. It is this air impoverished of dioxygene and carbon dioxide nouveau riche which is expired.
Pulmonary capacity
The pulmonary capacity is the volume of air which can be inspired. It is measured with a Spiromètre. In general, three types of breathing are measured:
- the “normal” breathing, calms, which gives volume used at rest;
- the forced breathing, which gives the maximum capacity;
- a brutal expiry, which gives information on the Bronchiole S, in particular within the framework of search for Asthme.
Let us note that even when one expires completely, there remains air in the lungs.
Media
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Animations flash illustrating the active phenomenon of the inspiration and liability of the expiry by the contraction or the relaxation of the respiratory muscles.
See too
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