Respiratory Physiology

Respiration is the process of supplying oxygen to the body and eliminating CO2 to the environment. This process consists of four main stages:

1. Pulmonary ventilation: Gas exchange between the lungs and the environment.

2. Gas exchange in the lungs: Gas exchange between alveoli and blood capillaries.

3. Gas transport in the blood: Oxygen is transported from the lungs to the cells and CO2 is transported from the cells to the lungs.

4. Gas exchange between capillaries and tissues: Gas exchange between blood and cells.

Thoracic cage: The thoracic cage is a closed, elastic structure, with the diaphragm as its base. It can change size in three dimensions: anteroposterior, superior-inferior, and transverse. The thoracic cage consists of the following parts:

• Fixed: Spine

• Movable: Ribs, sternum

• Movement: Respiratory muscles

Inspiration muscles:

• Normal inspiration: Includes the diaphragm and external intercostal muscles. These muscles actively contract, primarily changing the size of the thoracic cage in the superior-inferior and transverse directions.

• Forced inspiration: Includes the scalene muscles, anterior serratus muscle, sternocleidomastoid muscle, cheek muscles, tongue muscles, and alar muscles. These muscles activate after normal inspiration, helping to enhance the effectiveness of inhalation.

Expiration muscles:

• Normal expiration: All muscles relax, requiring no energy, so no work is done. This is a passive process.

• Forced expiration: Includes internal intercostal muscles and anterior abdominal wall muscles. These muscles contract, helping to push air out of the lungs.

Pressure in the pleural cavity:

• Characteristics: The pressure in the pleural cavity is always lower than atmospheric pressure (negative P), around -5 mmHg.

• Function: Helps the lungs move with the movement of the thoracic cage, creating conditions for effective gas exchange in the lungs.

• Changes: The pressure in the pleural cavity is most negative (-6 mmHg) during forced inspiration and least negative (-1 mmHg or positive) during forced expiration.

Effect of the diaphragm:

• When the diaphragm descends 4 cm: Thoracic cage volume increases by 1 liter (1000 cm3).

• During forced inspiration: The diaphragm descends by 7-8 cm.

• Diaphragmatic paralysis: Leads to decreased ventilation due to the lack of increase in the vertical dimension of the thoracic cage during inspiration.

Note: Negative pressure in the pleural cavity is created by the lungs tending to collapse towards the hilum of the lungs.