Pneumothorax

The presence of small volume of air within the pleural cavity results in partial collapse of the lung. This is closed pneumothorax in which ventila­tion still continues. Some time later air is absorbed from the pleural cavity, and the lung is expanded.

In open pneumothorax the pleural cavity has a direct communication with atmospheric air. This happens when the thorax is open by wound or during intrathoracic operations. The pressure around the lung becomes equal to that of atmosphere, and the lung almost fully collapses. Its ven­tilation is disrupted, although contraction of respiratory muscles con­tinues. Bilateral open pneumothorax results in death in the absence of urgent help. To prevent it, artificial respiration must be immediately resorted to by pumping air rhythmically into the lungs through the trachea or by making pleural cavity airtight.

Changes in Alveolar Pressure

The alveolar pressure is equal to the atmospheric pressure when the air­ways are open, and there is no flow of air through them. A decrease in the alveolar pressure causes inspiration. The degree of pressure decrease depends on the force of inspiratory muscle contraction and airway resistance to flow of air (aerodynamic resistance). Passage of air via air­ways requires energy expenditure to overcome air friction in the airway walls and frictional forces between the layers of air. As a result, pressure along the airways decreases. During expiration pressure in the alveoli becomes higher than that of atmosphere.

If airways are closed (at maximal resistance), an attempt at inspiration is attended by the fall of pressure in the lungs to — 70 mm Hg. An attempt at deep expiration in such conditions may cause a rise in the alveolar pressure to 100 mm Hg.

Airway resistance to air flow is determined in accordance with the Poiseuille formula. Aerodynamic resistance is changed during respiratory cycle: in inspiration it decreases and in expiration increases due to changes in the glottidis width and bronchial calibre.

Lung Volumes

The volume of air breathed in and out during quiet breathing is about 500 ml (from 300 to 800 ml) and is known as the tidal volume. The maximal volume of air that can be additionally inspired in deep inspiration is about 3000 ml. This is the inspiratory reserve volume. The maximal volume of air that can be expired after quiet expiration is about 1300 ml and is called the expiratory reserve volume.

The sum of these volumes is the vital capacity of the lunqs (VC): 500 + 3000 + 1300 = 4800 ml (Fig. 148). The tidal volume is quantitative expres­sion of respiratory depth. VC determines the maximal volume of air that can be breathed in or out from the lungs during one inspiration or expira­tion. These volumes are measured by means of spirometers of various design or pneumotachographs.

The VC is slightly higher in the male (4000-5500 ml) than in the female (3000-4500 ml). It is altered by posture: it is greater in the upright than in a sitting or lying position. Physical exercise is attended by VC increase.

A considerable volume of air (about 1200 ml) remains in the lungs after maximal expiration. It is called the residual volume. Most of it can be removed from the lungs only in open pneumothorax. The collapsed lungs contain a certain volume of air known as the minimal voume. This air is retained in the 'air traps' which are formed because part of bronchioles col­lapse prior to alveoli. That is why the lungs of adults and of newborns who had made inspiration do not sink in water.

The maximal volume of air that can be retained in the lungs is called the total lung capacity and is equal to the sum of the residual volume and VC (1200 + 4800 = 6000 ml in our example).

The volume of air remaining in the lungs at the end of quiet expiration (with relaxed respiratory musculature) is called functional reserve capacity (FRC). It is the sum of residual volume and expiratory reserve volume (1200 + 1300 = 2500 ml). FRC approaches the volume of alveolar air before initiation of inspiration.

Dead space. Air fills not only the alveoli but also airways that include the nasal cavity (or oral cavity in breathing through the mouth), nasopharynx, larynx, trachea, and bronchi. Air in the airways (except for respiratory bronchioles) takes no part in exchange of gases. Therefore, the airway lumen is called the anatomical dead space. During inspiration the last portions of atmospheric air enter the dead space and without changing theircomposition leave it in expiration. The anatomical dead space volume is about 150 ml or nearly one-third of the tidal volume during quiet breathing. So, out of 500 ml of inspired air only 350 ml enter alveoli. The ^ alveoli contain about 2500 ml of air (FRC) by the end of quiet expiration so that only one-seventh of the alveolar air is renewed in each quiet inspira­tion.

Importance of the airways. Though exchange of gases does not take place in most airways, they are indispensable for breathing. While passing along them, the inspired air is moistened, warmed and freed of dust and microorganisms. Air is most thoroughly freed of dust when breathing through the nose: passage of air through the narrow nasal passages with a complex structure is attended by turbulent movements which facilitate con­tact of dust particles with nasal mucosa. The walls of airways are covered with mucous secretion that entraps particles contained in the air. Mucus gradually moves (7-19 mm/min) toward the nasopharynx by the action of ciliated epithelium of the nasal cavity, trachea and bronchi. Mucus con­tains a bactericidal substance lysozyme.

Stimulation of the receptors of nasopharynx, larynx and trachea by dust particles and accumulated mucus causes cough', stimulation of the receptors of nasal cavity sneezing (protective respiratory reflexes). The coughing and sneezing centres are located in the medulla.

The size of bronchial lumen depends on many factors. The elastic recoil force of alveolar tissue has an influence on the walls of intrapulmonary bronchi, while extrapulmonary bronchi are under the influence of negative pressure in the pleural cavity. These forces enlarge the bronchial lumen.

The bronchial walls have smooth circular muscles that constrict the bronchial lumen. Bronchial muscles maintain a state of tonic activity which in­creases in expiration. Bronchial muscles contract due to activation of parasympathetic influences, under the action of histamine, serotonin and prostaglandins. They relax in activation of sympathetic influences b-adrenoreceptors predominate in bronchial muscle fibres) and under the action of adrenaline. Nerve fibres of non-adrenergic nature responsible for bronchial dilatation have also been discovered.

Pulmonary ventilation is determined by the volume of air inspired or expired per unit of time. The minute volume of respiration (V) is usually measured: its value during quiet breathing is 6-9 1/min. Pulmonary ven­tilation depends on respiratory depth and frequency which at rest is 16 per minute (from 12 to 18). The V is equal to the product of tidal volume and frequency of respiration per minute.

Visual material

Slide cards:

1. The scheme of functional system of maintenance of optimum sizes of respiratory indicators of an organism.

2. The basic stages of breath.

3. The mechanism of respiratory movements.

4. Donders’s model for demonstration of mechanics of a breath and an exhalation.

5. Pulmonary volumes and capacities.

Tables:

1. A structure of a pulmonary membrane.

2. Intrapulmonary and intrapleural pressure at a breath and an exhalation.

3. Structure of inhaled, exhaled, alveolar air

Rerferences

1. Физиология человека: Учебник /В двух томах. Т. I /Под ред. В.М.Покровского, Г.Ф.Коротько. –М.:Медицина, 2003. – С. 98-108.

2. Физиология человека (курс лекций в 2-х книгах). Учебник /Под ред. Н.А.Агаджанян, Л.З.Тель, В.И. Циркин, С.А.Чеснокова. – Алма-Ата: Казахстан, 1992. –С. 58-65.

3. Основы физиологии человека /Под ред. Б.И.Ткаченко. – С.-Петербург, 1994. Т. I. – С. 124-125.

4. Физиология. Основы и функциональные системы: Курс лекций. /Под ред. К.В.Судакова. –М.:Медицина, 2000. – С. 12-27.

5. Теория функциональных систем организма. Под ред. К.В. Судакова. М., 1996. – 95 с.

Control questions (feedback)

1. What is the breath?

2. Name breath stages.

3. What structure of lungs.

4. What is surfactant system of lungs.

5. What origin of negative pressure in a chest cavity

6. Value of negative pressure for breath and blood circulation.

7. How there is a breath?

8. How there is an exhalation mechanism?

9. Value of "dead" air space.

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