MECHANICS OF BREATHING

The diaphragm is the most important muscle of inspiration. Its contraction increases the vertical height of the thoracic cavity. The external intercostal muscles produces a movement of the ribs and also increase volume of the chest. 

Expiration is passive during quiet breathing but becomes active during exercise. The most important muscles of expiration are those of the abdominal wall. The internal intercostal muscles assist expiration by pulling the ribs downward and inward. 

When all the respiratory muscles are relaxed, the lung is at its resting lung volume, and there is equilibrium between the recoil of the lung and the recoil ofthe chest wall. At this point, the lung tends to collapse and the chest wall tends to expand, with the two being held together by a thin layer of pleural fluid. 

By acting to pull the lung and chest wall apart, the opposing recoil forces create negative pressure in the interpleuralspace. . 

The presence of negative pressure in the interpleural space create subatmospheric pressure within the chest accounts for a person’s ability to draw air into the airway during inspiration. 

Effect of pneumothorax on lung volume and thoracic volume 

If the chest wall is punctured, air will flow into the pleural space (pneumothorax) until interpleural pressure equals atmospheric pressure; the lung will then collapse and the chest wall will spring outward.

Surfactants

In the lungs there is one more mechanism to resist the tendency of alveoli to become smaller during expiration - it is the presence of detergent-like surfactant molecules at the airliquid interface of the alveoli. Surfactants are phospholipides secreted by pneumocytes and they reduce surface tension, thereby maintaining normal lung compliance and preventing alveolar collapse.

Airflow into and out of the lungs requires pressure gradients between the mouth and the alveolus, which are created by mechanical changes in lung volume.

Inspiration mechanism 

1. The respiratory center of the medulla oblongata sends impulses to the inspiratory muscles 
2. Inspiration muscles contract , this causes an expansion in the chest cavity. As a result, the volume of the chest increase 
3. Due to the negative pressure in the pleural cavity, the lung volume increases too 
4. As a result, air pressure in the alveoli decreases. Air moves from the area of higher pressure (in the mouth) to the region of lower pressure (in the alveoli) 
5. As air flows into the lung to occupy the increased volume, alveolar pressure returns to atmospheric pressure, ending inspiration. 

Exspiration mechanism 

1. Passive recoil of the lung and chest wall during quiet expiration causes the lung volume to decrease. 
2. As a result, the alveolar pressure increases and gas flows out of the lung.