NURS 208 Ventilation and the Respiratory System

Overview of the Respiratory System

• The respiratory system is divided into two main categories of respiration:

  • External Respiration: Involves gas exchange between air in the lungs and the blood.

  • Internal Respiration: Refers to gas exchange between the blood and body tissues.

Lung Anatomy and Zones

• Components of the Lung:

  • Lungs are divided into two zones:

  • Conducting Zone:

    • Comprises airways without alveoli.

    • No gas exchange occurs here.

    • Includes passages from the trachea to terminal bronchioles (up to 16th generation).

  • Respiratory Zone:

    • Contains alveoli (air sacs).

    • Site of gas exchange.

    • Includes respiratory bronchioles, alveolar ducts, and alveolar sacs.

• The lungs contain approximately 300 million alveoli with an average diameter of about 250 µm.

Respiratory Muscles

• Muscles of Respiration:

  • Two major sets:

  • Inspiratory Muscles:

    1. Diaphragm:

    • Dome-shaped muscle separating thoracic and abdominal cavities.

    • Contraction increases thoracic volume by lowering the thoracic floor.

    • Innervated by the phrenic nerve (C3-C5).

    1. External Intercostal Muscles:

    • Elevate rib cage, increasing thoracic volume.

    1. Neck Muscles:

    • Scalenes: Elevate first two ribs.

    • Sternocleidomastoid: Elevates sternum.

    1. Nasal Muscles:

    • Help in the flaring of nostrils.

  • Expiratory Muscles:

    1. Internal Intercostal Muscles:

    • Depress rib cage.

    1. Abdominal Muscles:

    • Compress abdomen to push diaphragm up during forced expiration.

Mechanics of Breathing (Ventilation)

• Intrapleural Space:

  • Refers to the pleural cavity between visceral and parietal pleurae filled with pleural fluid.

  • Functions:

  1. Acts as a lubricant between pleural membranes during breathing.

  2. Helps keep the pleurae together, assisting lung inflation.

• Pressure Changes During Breathing:

  • Intrapleural Pressure (PIP): Negative pressure < barometric pressure, facilitating lung inflation.

  • Boyle's Law:
    P1 imes V1 = P2 imes V2

  • Any increase in lung volume results in a decrease in pressure during inhalation and vice versa during exhalation.

• Phases of Ventilation:

  1. End of Expiration:

  • Alveolar pressure (PA) equals atmospheric pressure (PB).

  1. Beginning of Inspiration:

  • Contraction of inspiratory muscles increases thoracic volume, lowering PA to below PB, causing air to flow in.

  1. End of Inspiration:

  • PA equals PB, stopping airflow into the lungs.

  1. During Quiet Expiration:

  • Relaxation of respiratory muscles decreases thoracic volume, raising PA above PB, allowing air to flow out.

Key Concepts

• Surfactant:

  • Secreted by Type II alveolar cells, helps to reduce surface tension and facilitates lung expansion during inhalation.

• Elastic Recoil:

  • Tendency of lungs to collapse due to inherent elasticity and surface tension of alveoli.

  • This elasticity interacts with the outward pull of the thoracic cage, maintaining appropriate lung volume.

Pneumothorax

• A condition where the pleural cavity is compromised, resulting in lung collapse (collapsed lung).

Sample Questions for Review

1. What main muscles facilitate inspiration?

2. Describe the role of surfactant in lung function.

3. Explain the significance of intrapleural pressure during quiet breathing.

Answers to Sample Questions

1. Diaphragm and External Intercostal Muscles are main inspiratory muscles.

2. Surfactant reduces surface tension in alveoli, preventing collapse and facilitating expansion.

3. Intrapleural pressure is subatmospheric, aiding lung inflation by promoting a pressure gradient for airflow.