Biomechanics of external breath

Gas exchange between an alveolar gas mix and the atmospheric air, providing effective diffusion of oxygen and carbonic gas through a membrane (a fig. 1.), it is carried out owing to work of the device of ventilation, which consists of two formations: a thorax with respiratory muscles and lungs with respiratory ways. The thorax represents a rigid mobile case for lungs, hearts and the vessels, possessing elasticity. The thorax actively changes the volume by means of reduction of a diaphragm and other respiratory muscles. At reduction of a diaphragm its dome becomes flat and is displaced aside a belly cavity that leads to increase in volume of a thorax, and after it and lungs. According to Boyle's law () the increase in volume of lungs is accompanied by pressure decline inside of them. It becomes below atmospheric and air is sucked in inside of lungs in alveoluses. So the breath is made.

At a breath air passes in a trachea and then on bronchial tubes reaches alveoluses. All respiratory ways in aggregate make the so-called branched out respiratory tube (fig. 1.). The trachea has the least area of cross-section section (on the average 2,54 sm²) at the adult person). Total section of two main things (left and right) bronchial tubes more than this size. The total section of each subsequent generation of bronchial tubes becomes more in a direction to alveoluses. So, the area of total section of all bronchial tubes of 16-th generation reaches 180 sm² and all alveolar saccules - 11 800 sm². For a continuous stream of air on aeriferous ways of animals and the person in physiological conditions the condition of indissolubility of a jet is satisfied: identical volumes of air are transferred at continuous current of the incompressible environment through any section of a jet in unit of time

With reference to the branched out respiratory tube it means, that Volumetric speed of an air stream) is identical in all it summary cross-section sections

Between volumetric (Q) and linear (v) speeds of an air stream establish following dependence:

S -the area of total section given generation of bronchial tubes. The conclusion: linear speed of an air stream different in different places of branched respiratory tube.

Fig. 5. The scheme of branched out respiratory tube.

Reduction of a diaphragm provides depth of a breath on 70-80 % at quiet breath. And on reduction of external intercostal muscles provides depth of a breath on 20-30 %. Strengthening of breath is connected with inclusion in work of auxiliary respiratory muscles (muscles of a neck and the humeral belt, attached to a thorax). They can promote speeding up as breath, and exhalation. The greatest contribution to the forced exhalation internal intercostal muscles and muscles bring a stomach (abdominal tension). At quiet breath reduction of respiratory muscles provides only a breath, whereas the exhalation is made passively - due to occurrence (at a breath) forces of elasticity both in lungs, and in fabrics of a thorax. Capacity of respiratory muscles at quiet breath makes 0,05 W, and at the forced breath increases on the order. In the first case for power supply of respiratory muscles it is spent only 2 %, whereas in the second - more than 20 % of the oxygen absorbed by an organism. Sequence of the biophysical processes providing ventilation of lungs, it is possible to present in the form of the following scheme:

The certificate of a breath: receipt of a nervous impulse to respiratory muscles synaptic (nervously-muscular) transfer reduction of respiratory muscles increase of volume of chest cavity increase of volume of lungs decrease of pressure in lungs (under Boyle's law) retraction of air from an atmosphere in lungs. The certificate of an exhalation: a relaxation of respiratory muscles (after reduction at a breath) Reduction of volume of a chest cavity reduction of volume of lungs Increase of pressure in lungs (under Boyle's law) Output of air from lungs in an atmosphere.

The basic contribution in elastic properties of a thorax bring elasticity of ribs, especially them gristle parts, and respiratory muscles.

Elastic resistance of a thorax depends on a degree of their stretching, and it, in turn, increases with increase in volume of lungs. At filling lungs approximately on 55 % of their maximal volume (V_max) elastic structures of a thorax are completely weakened. The increase in volume of lungs (concerning 55 % V_max) leads to a stretching of elastic components of a thorax, and reduction - to their compression. Both the stretching, and compression occur only at reduction of respiratory muscles (in the first case - muscles of a breath, in the second - muscles of the forced exhalation).

Force of elasticity in lungs which forces them to be fallen down on an exhalation, refer to as elastic draft of lungs (EDL). It has two basic components

First, to fabrics of lungs have elastic properties (they depend not only on factor of elasticity of their components and from a degree of filling of lungs blood).

Second component of elastic draft of lungs is force of a superficial tension which arises on border between an alveolar gas mix and internal surface of the alveoluses, the liquid covered by a layer. Pressure which c reates a superficial tension, calculate under the Laplace formula

r - radius of an alveolus, - factor of a superficial tension. Under action of this pressure the gases being an alveolus and compressed by it, aspire to leave it and to leave through respiratory ways outside. Average radius of an alveolus - 100—150 mcm, - on a breath - , the pressure caused by a superficial tension, reaches on a breath 800 Pа.

Energy of reduced respiratory muscles is spent for overcoming of elastic resistance of a thorax and lungs and on overcoming of forces of resistance to movement of air on respiratory ways. They depend on character of an air stream. At laminar movement of force of resistance are proportional to volume of moving air in unit of time, and at turbulent - almost to a square of this volume. At quiet breath in bronchial tubes the laminar air stream prevails. When pulmonary ventilation amplifies (for example, during physical work) or there is a spasm of bronchial tubes, movement of air can become turbulent. It conducts to strengthening the power expenses connected with breath.

The energy spent by respiratory muscles on realization of quiet breath, every minute makes 2-3,5 Дж, and 70 % of this energy are spent for overcoming of elastic resistance of a thorax and a pulmonary fabric, and the others of 30 % go on fulfilment of work on moving air against ­force of friction which are proportional to speed of moving of air weight. Therefore the second component of power expenses increases at increase of breath. Owing to work of respiratory muscles it is overcome so-called pulmonary resistance, resistance of aeriferous ways to fluctuations of a stream of air in them. It makes rather small size - The size, return pulmonary resistance, refers to as an extensibility.

Power expenses for overcoming of elastic resistance of bodies of breath do not depend almost on speed of movement of air, so, and from frequency of breath. They are defined by volume of air acting in lungs at a breath, depth of breath. Unequal dependence of each of components of work of the respiratory device on frequency and depths of breath leads to an establishment of an optimum parity between them at which work is supported on a minimum level at the given volume of ventilation. In conditions of physical rest depth of breath (respiratory volume) makes 0,5 л, and frequency 12-16 min^-1 Product of depth of breath (respiratory volume) on its frequency (counting upon 1 mines) defines minute volume of breath. At quiet breath it makes . At heavy physical activity increases more than up to owing to a deepening and increase of breath.

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