Mechanics of respiration, pneumothorax

Process of Inspiration:

Inspiration is the phase of ventilation in which air enters the lungs. It is initiated by the contraction of the *inspiratory muscles*, which expand the thoracic cavity and reduce intrapulmonary pressure, allowing air to flow into the lungs.

1. Diaphragm:

   - Flattens and descends, increasing the superior-inferior dimension of the thoracic cavity.

   - Primary muscle of inspiration.

   

2. External Intercostal Muscles:

   - Elevate the ribs and sternum, expanding the anterior-posterior dimension of the thoracic cavity.

   - Assist the diaphragm in increasing thoracic volume.

- As the volume of the thoracic cavity increases, the lungs expand due to the *pleural seal* (the tight adherence between the visceral and parietal pleurae).

- *Boyle's Law*: As the volume of the lungs increases, the pressure inside the lungs (intrapulmonary pressure) decreases. Atmospheric pressure, being higher than lung pressure, forces air to move into the lungs.

Process of Passive Expiration:

Expiration is the phase of ventilation in which air is expelled from the lungs. In quiet breathing, expiration is typically passive, involving relaxation of the inspiratory muscles.

1. Diaphragm:

   - Relaxes and moves upwards, reducing the superior-inferior dimension of the thoracic cavity.

2. External Intercostal Muscles:

   - Relax, allowing the ribs and sternum to return to their original position, reducing the anterior-posterior dimension.

- As the thoracic volume decreases, the lungs *recoil elastically*, decreasing in volume.

- *Boyle's Law*: A decrease in lung volume increases the intrapulmonary pressure. When this pressure exceeds atmospheric pressure, air is expelled from the lungs.

Forced Breathing (Hyperpnea):

In situations of increased respiratory demand (e.g., exercise), breathing becomes *active*, engaging accessory muscles to increase the volume of the thoracic cavity for inspiration and forcefully reduce it for expiration.

Active Inspiration (Increased Volume):

1. *Scalene Muscles*: Elevate the ribs.

2. *Sternocleidomastoid*: Elevates the sternum.

3. *Pectoralis Major and Minor*: Pull ribs anteriorly and outwards.

4. *Serratus Anterior*: Elevates ribs when the scapula is fixed.

5. *Latissimus Dorsi*: Elevates the lower ribs.

Active Expiration (Decreased Volume):

1. *Anterolateral Abdominal Wall Muscles*: Increase intra-abdominal pressure, pushing the diaphragm further into the thoracic cavity.

2. *Internal Intercostal Muscles*: Depress the ribs, reducing thoracic volume.

3. *Innermost Intercostal Muscles*: Also depress the ribs to assist in forced expiration.

Pneumothorax (Collapsed Lung):

A *pneumothorax* occurs when air enters the pleural cavity, the space between the visceral and parietal pleurae. This breaks the surface tension holding the lung against the thoracic wall, leading to lung collapse. The pneumothorax can be partial or complete.

Types of Pneumothorax:

1. Primary Pneumothorax:

   - Occurs spontaneously without any underlying lung condition.

   - Often seen in tall, thin individuals or those with a history of smoking.

2. Secondary Pneumothorax:

   - Occurs as a result of an underlying lung condition (e.g., COPD, asthma, cystic fibrosis).

   - May follow trauma, infection, or mechanical ventilation.

Tension Pneumothorax:

A life-threatening form of pneumothorax, in which air continues to enter the pleural space without being able to escape. This causes increased intrathoracic pressure, leading to:

- Compression of the lungs and *mediastinal shift*.

- Decreased venous return to the heart, which may result in *cardiovascular collapse* and death if not treated promptly.

Symptoms of Pneumothorax:

- Sudden onset of sharp chest pain.

- Shortness of breath.

- Difficulty taking deep breaths.

 

Signs of Pneumothorax:

- *Tachycardia*: Fast heart rate.

- *Tachypnea*: Rapid breathing.

- *Hypoxia*: Low oxygen levels in the blood.

- Reduced breath sounds on the affected side during auscultation.

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Summary:

The mechanics of respiration rely on changes in thoracic volume, governed by Boyle's Law, to facilitate airflow. Pneumothorax disrupts the pleural seal, leading to lung collapse, with tension pneumothorax being a critical emergency. Understanding these principles is essential for recognizing and managing respiratory dysfunctions effectively.