Mechanics of Breathing Study Notes

Chapter 17: Mechanics of Breathing

About this Chapter

• Overview of the respiratory system
• Introduction to gas laws
• Principles of ventilation

Functions of the Respiratory System

The respiratory system has four primary functions:

1. Exchange of Gases: This includes the exchange of oxygen and carbon dioxide between the atmosphere and the blood.
2. Homeostatic Regulation of Body pH: The respiratory system helps maintain the body's pH by selectively retaining or excreting carbon dioxide (CO2), influencing acid-base balance.
3. Protection from Pathogens and Irritants: It provides a defense mechanism against inhaled pathogens and irritating substances.
4. Vocalization: The respiratory system is essential for the production of sound through the vocal cords.

Principles of Bulk Flow of Air

• Air flows from regions of higher to lower pressure, facilitating respiration.
• A muscular pump (primarily the diaphragm and intercostal muscles) creates pressure gradients that enable air flow.
• Resistance to airflow is influenced by the diameter of the airways; narrower tubes increase resistance.

Respiration Types

• External Respiration: This term describes the movement of gases between the environment (atmosphere) and the body's cells.
• Internal/Cellular Respiration: Refers to the intracellular reaction of oxygen with organic molecules, producing carbon dioxide, water, and adenosine triphosphate (ATP).

Structures of the Respiratory System

• Conducting System: Includes all airways that lead from the external environment to the lungs.
• Exchange Surface: Comprises the alveoli, which are the sites of gas exchange.
• Bones and Muscles of Thorax: These structures assist in the process of ventilation (breathing).

Ventilation Muscles

• Quiet Breathing:
    - Inspiration: Involves contraction of the diaphragm and external intercostal muscles.
    - Expiration: Occurs when inspiratory muscles relax and recoil.

Pleural Sac and Lung Functionality

• The pleural fluid between the pleurae (membrane surrounding the lungs) creates a moist, slippery surface reducing friction during lung movement.
• It also holds the lungs tightly against the thoracic wall, functioning like glue.

Branching of Airways

• Bronchial Tree: Upon entering the lungs, the primary bronchi subdivide into smaller branches leading to respiratory bronchioles and alveolar ducts.
• As the bronchi branch, there is a decrease in cartilage and an increase in smooth muscle, which can constrict airways (e.g., asthma attack).
  

Detailed Branching Information

Alveoli and Gas Exchange

• Alveolar Structure:
    - Type I Alveolar Cells: Thin cells that facilitate gas exchange.
    - Type II Alveolar Cells: Secrete surfactant, mixing with alveolar fluid to prevent collapse during expiration and aid in lung expansion.
• Alveoli are sac-like structures clustered at the end of bronchioles and consist of epithelial cells. Surrounding blood capillaries cover 80-90% of their surface, facilitating efficient gas exchange.

Gas Laws

• Govern the behavior of gases in the atmosphere.
• Key Gas Laws:
    1. Dalton's Law: The total pressure of a mixture of gases equals the sum of the pressures of its individual gases.
    2. Boyle's Law: If the volume of a gas container changes, the pressure of the gas changes inversely (
    V
  ightarrow P). This means as volume increases, pressure decreases, and vice versa.

Spirometry and Lung Volumes

• Spirometry is used to measure the volume of air moved during breathing.
• Lung Volume Measurements:
    - Tidal Volume (VT): 500 mL, the amount inhaled/exhaled during normal breathing.
    - Inspiratory Reserve Volume (IRV): 3000 mL, the maximum volume that can be inhaled after a normal inhale.
    - Expiratory Reserve Volume (ERV): 1100 mL, the maximum volume that can be exhaled after a normal exhale.
    - Residual Volume (RV): 1200 mL, the volume of air remaining in the lungs after maximum exhalation.
    - Vital Capacity: $VC = IRV + VT + ERV$

Conditioning of Air

• The upper airways warm incoming air to body temperature and add humidity (100% relative humidity) to prevent drying out of the moist exchange epithelium.
• They also filter out foreign particles (viruses, bacteria, inorganic particles).

Ciliated Epithelium in Respiratory System

• Cilia in the respiratory tract move mucus to the pharynx, trapping inhaled dust and particles. Goblet cells secrete mucus, and a watery saline layer assists cilia in moving mucus effectively.

Pressure and Airflow

• Air flow is governed by pressure gradients defined by the equation:
  $ext{Flow} \frac{ riangle P}{R}$,
  where
• change in pressure is directly proportional to flow and inversely proportional to resistance.
• Measurable pressures include:
    - Alveolar pressure: The pressure of the air inside the lungs.
    - Intrapleural pressure: The pressure within the pleural cavity.
• In a single respiratory cycle, inspiration is indicated by alveolar pressure being less than atmospheric pressure, whereas expiration involves alveolar pressure being greater than atmospheric pressure.

Mechanics of Inspiration and Expiration

• Diaphragm Movement: Diaphragm contraction contributes 60-75% of the volume change during normal inhalation.
• Rib Cage Movement: The rib cage, through the contraction of external intercostals and scalene muscles, contributes the remaining 25-40% to volume changes.
• Pneumothorax: This condition results from air entering the pleural cavity, leading to lung collapse.

Types of Pneumothorax

• Open Pneumothorax: Occurs due to penetrating thoracic injury causing external air to enter.
• Closed Pneumothorax: Occurs without an external wound, often due to trauma or spontaneous rupture.
• Tension Pneumothorax: A worsening condition where trapped air increases intrathoracic pressure, displacing mediastinal structures leading to acute symptoms such as chest pain and dyspnea. Symptoms of all types include cyanosis, absent breath sounds, and changes in vitals (tachycardia, hypotension).

Compliance vs. Elastance

• Compliance: The ability of the lung to stretch; high compliance means it stretches easily, while low compliance requires more force to stretch.
• Compliances are reduced in restrictive lung diseases (e.g., fibrotic lung diseases, inadequate surfactant).
• Elastance: Describes the lung's ability to return to its resting volume after stretching, essentially the elasticity of the lungs.

Law of Laplace

• This law states that tension (T) in the wall of a bubble is directly proportional to the internal pressure (P) and inversely proportional to the radius (r).
• Surfactant plays a crucial role by reducing surface tension, allowing alveoli of all sizes to inflate equally, thereby preventing collapse.

Surfactant

• A complex mixture of proteins and phospholipids, surfactant is crucial in the alveoli to lower surface tension and prevent collapse, especially in newborns who may suffer from "stiff" lungs due to inadequate surfactant leading to respiratory distress syndrome.

Airway Resistance

• Factors affecting airway resistance:
    - Diameter of Airways: Central to airflow, larger diameters reduce resistance.
    - Bronchoconstriction/Bronchodilation: Mediated by neural and hormonal factors (e.g., parasympathetic stimulation causes bronchoconstriction, while CO2 and epinephrine promote bronchodilation).

Ventilation

• Total Pulmonary Ventilation:
  $ext{Total Pulmonary Ventilation} = ext{ventilation rate} imes ext{tidal volume}$,
• Alveolar ventilation is defined as:
  $ext{Alveolar Ventilation} = ext{ventilation rate} imes ( ext{tidal volume} - ext{dead space volume})$.
• In a respiratory cycle, only a portion of inhaled air reaches alveoli and participates in gas exchange. The importance of understanding these ventilatory mechanics is highlighted in clinical settings, as they may influence the management of conditions such as hypoventilation or hyperventilation.

Summary of Ventilation Patterns

• Eupnea: Normal quiet breathing.
• Hyperpnea: Breathing faster or deeper in response to increased metabolism (e.g., exercise).
• Hyperventilation: Increased breathing without metabolic increase, often caused by anxiety or emotional states.
• Hypoventilation: Decreased ventilation leading to increased CO2 levels in the body.
• Tachypnea: Rapidly increased respiratory rates often without depth, like in panic or anxiety.
• Apnea: Temporary cessation of breathing, potentially due to unconsciousness or physiological signals.

Implications of Ventilation Changes

• Changing patterns of ventilation impact alveolar levels of oxygen (PO2) and carbon dioxide (PCO2), with direct consequences on the body's gas exchange capacity and overall homeostasis.
• Local control mechanisms function to match ventilation with perfusion to optimize gas exchange at the alveolar level, thereby ensuring effective oxygen transfer to the circulatory system and carbon dioxide removal.

Summary

• Comprehensive understanding of respiratory system functions, mechanics of breathing, gas laws, lung volumes, and types of pneumothorax.
• Distinction between total pulmonary ventilation and alveolar ventilation emphasizing the impact of breathing patterns on effective gas exchange.
• Recognition of the importance of compliance and elastance in understanding pulmonary conditions and treatment considerations, as well as the critical role of surfactant in maintaining lung function.