Mechanics of Breathing Study Notes

Chapter 17: Mechanics of Breathing

About this Chapter

• Overview of key topics:
    - The respiratory system
    - Gas laws
    - Ventilation

Functions of the Respiratory System

• 4 primary functions:
    1. Exchange of gases between the atmosphere and the blood.
    2. Homeostatic regulation of body pH:
       - Achieved by selectively retaining or excreting $CO_2$.
    3. Protection from inhaled pathogens and irritating substances.
    4. Vocalization: The respiratory system also enables sound production.

Principles of Bulk Flow of Air

• Airflow principles:
    - Air flows from regions of higher to lower pressure.
    - Muscular pumps such as the diaphragm create pressure gradients.
    - Resistance to airflow is influenced by tube diameter.

Respiration Processes

• External Respiration:
    - Movement of gases between the environment and the body’s cells.
• Internal/Cellular Respiration:
    - The intracellular reaction of oxygen with organic molecules to produce carbon dioxide, water, and ATP.

Structures of the Respiratory System

• Conducting system:
    - Airways leading from the external environment to the lungs.
• Exchange surface:
    - Alveoli, which are the sites for gas exchange.
• Supporting structures:
    - Bones and muscles of the thorax assist in ventilation.

Muscles Used for Ventilation

• Quiet breathing:
    - Inspiration: Muscles contract, primarily the diaphragm and external intercostals.
    - Expiration: Inspiratory muscles recoil.

Pleural Sac and Lung Function

• Pleural sac:
    - Maintains a moist, slippery surface for lung movement.
    - Holds the lungs tight against the thoracic wall, functioning like glue.

Branching of Airways

• Bronchial Tree:
    - Upon entering the lungs, the primary bronchi further divide, forming smaller diameter branches leading to respiratory bronchioles and alveolar ducts.
    - Structural change:
      - Less cartilage and more smooth muscle with branching may lead to muscle spasms, e.g., during asthma attacks.

Breakdown of Airways Structure and Function

Alveolar Structure and Function

• Alveoli:
    - Sac-like structures primarily for gas exchange with blood, with capillaries covering 80-90% of the surface.
• Types of alveolar cells:
    - Type I alveolar cells: Thin cells allowing gas exchange.
    - Type II alveolar cells: Secrete surfactant, which helps maintain lung function during breathing by reducing surface tension.
• Alveolar macrophages: Immune cells responsible for removing dust and pathogens.

Gas Laws Related to Respiration

• Gas Laws Overview:
    - The behavior of gases in the air is governed by specific laws.

Key Gas Laws

1. Dalton's Law: The total pressure of a mixture of gases is the sum of the pressures of the individual gases.
2. Gases, singly or in a mixture, move from areas of higher pressure to areas of lower pressure.
3. Boyle's Law: When the volume of a gas container changes, the pressure changes inversely.
      - V
   ightarrow P: If volume increases (V
   ightarrow), pressure decreases (P
   ightarrow).

Measurement of Lung Function

• Spirometer: Tool used to measure the volume of air moved with each breath.
• Lung Volume Measurement:
    - Key lung volumes include:
      - Tidal Volume (TV): Resistance volume of air during normal breathing, typically around 500 mL.
      - Inspiratory Reserve Volume (IRV): Extra volume of air that can be inhaled after normal inspiration.
      - Expiratory Reserve Volume (ERV): Extra volume of air that can be exhaled after normal expiration.
      - Residual Volume (RV): Volume of air remaining in the lungs after maximal forced expiration.
      - Vital Capacity (VC): Maximum amount of air a person can exhale after a maximum inhalation.
• Key Values for Men and Women:
    - Males: TV=500 mL, IRV=3000 mL, ERV=1100 mL, RV=1200 mL.
    - Females: TV=500 mL, IRV=2100 mL, ERV=700 mL, RV=1200 mL.

Conditioning of Inspired Air

• The respiratory system:
    - Warms incoming air to body temperature to protect alveoli.
    - Adds water vapor until air reaches 100% humidity, preventing drying of the epithelium.
    - Filters foreign materials to protect alveoli from pathogens and particles.

Mechanism of Air Flow

• Air flows due to pressure gradients:
    - The formula for flow is: Flow
  ightarrow rac{ riangle P}{R},
      - Where the flow is directly proportional to the change in pressure ( riangle P) and inversely proportional to resistance (R).
• Pressure Types:
    - Alveolar pressure (air pressure in the lungs).
    - Intrapleural pressure (pressure in the pleural fluid).

Phases of Breathing

• Inspiration: Alveolar pressure < atmospheric pressure.
• Expiration: Alveolar pressure > atmospheric pressure.

Diaphragm and Rib Cage Movement

• Diaphragm Contraction:
    - Responsible for 60-75% of inspiratory volume in normal breathing.

• Rib Cage Movement:
    - Contributes 25-40% of volume change through the contraction of external intercostals and scalene muscles, which lift the ribs upward and outward.

Pneumothorax Types

• Open Pneumothorax:
    - Caused by a penetrating thoracic injury allowing outside air into the pleural cavity.
    - Treatment: Sterile occlusive dressing on three sides to prevent further air entry.

• Closed Pneumothorax:
    - Occurs without an external wound, can be caused by trauma or spontaneously (e.g., ruptured bleb).

• Tension Pneumothorax:
    - A worsening condition where air cannot escape, increasing intrathoracic pressure, potentially shifting pressure onto the heart and good lung, leading to respiratory distress.

Compliance and Elastance

• Compliance: Indicates the lung's ability to stretch (high compliance = stretches easily; low compliance = requires more force).
• Elastance: Refers to the lung's ability to return to resting volume after stretching (referred to as