Chapter 18: Pulmonary Mechanics – Study Notes

Pulmonary Mechanics

Major Functions of the Respiratory System

The respiratory system is essential for gas exchange and maintaining homeostasis. Its main functions include:

• Gas Exchange: Facilitates the exchange of oxygen and carbon dioxide between the atmosphere and the blood.

• Regulation of Blood pH: Adjusts the levels of CO2 in the blood, influencing acid-base balance.

• Protection: Filters, warms, and humidifies inhaled air; protects against inhaled pathogens and irritants.

• Vocalization: Enables speech and other vocal sounds by moving air past the vocal cords.

Anatomy of the Respiratory System

The respiratory system consists of a series of structures that conduct air from the external environment to the alveoli, where gas exchange occurs.

• Conducting Zone: Includes the nasal cavity, pharynx, larynx, trachea, bronchi, and terminal bronchioles. These structures filter, warm, and moisten air but do not participate in gas exchange.

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli. This is where gas exchange occurs.

• Pleura: The lungs are surrounded by two layers of pleura (visceral and parietal) with pleural fluid in between, reducing friction and helping maintain lung expansion.

• Thoracic Cage: Composed of the ribs, sternum, and diaphragm, it provides protection and assists in ventilation.

Pathway of Air: Nose/Mouth → Pharynx → Larynx → Trachea → Primary Bronchi → Secondary Bronchi → Tertiary Bronchi → Bronchioles → Terminal Bronchioles → Respiratory Bronchioles → Alveolar Ducts → Alveoli

Branching Structure and Functional Differences

• Trachea and Bronchi: Supported by cartilage rings; lined with ciliated epithelium and goblet cells.

• Bronchioles: Lack cartilage; contain more smooth muscle, allowing regulation of airway diameter.

• Alveoli: Thin-walled sacs specialized for gas exchange; surrounded by capillaries.

Comparison Table:

Structure	Cartilage	Smooth Muscle	Epithelium	Function
Bronchi	Present	Some	Ciliated, goblet cells	Conduct air, filter particles
Terminal Bronchioles	Absent	More	Ciliated, fewer goblet cells	Conduct air, regulate flow
Respiratory Bronchioles	Absent	Present	Simple cuboidal	Some gas exchange
Alveoli	Absent	Absent	Simple squamous	Gas exchange

Respiratory Epithelium and Cell Types

• Ciliated Epithelial Cells: Move mucus and trapped particles upward toward the pharynx (mucociliary escalator).

• Goblet Cells: Secrete mucus to trap dust and microbes.

• Submucosal Gland Cells: Produce additional mucus and serous fluid.

• Alveolar Cells (Pneumocytes):

  • Type I: Thin, flat cells for gas exchange.

  • Type II: Secrete surfactant to reduce surface tension and prevent alveolar collapse.

Gas Laws in Respiratory Physiology

Dalton’s Law of Partial Pressures

The total pressure of a gas mixture is the sum of the partial pressures of each individual gas.

• Partial Pressure: The pressure exerted by a single gas in a mixture.

$P_{total} = P_1 + P_2 + P_3 + ...$

$P_{gas} = (\text{Fraction of gas}) \times (\text{Total pressure})$

Boyle’s Law

Describes the inverse relationship between the pressure and volume of a gas at constant temperature.

$P_1 V_1 = P_2 V_2$

As lung volume increases during inspiration, alveolar pressure decreases, causing air to flow in. The reverse occurs during expiration.

Lung Volumes and Capacities

Lung volumes and capacities are measured to assess respiratory function.

• Tidal Volume (TV): Volume of air inhaled or exhaled in a normal breath (~500 mL).

• Inspiratory Reserve Volume (IRV): Maximum volume inhaled above TV.

• Expiratory Reserve Volume (ERV): Maximum volume exhaled below TV.

• Residual Volume (RV): Volume remaining after maximal exhalation (cannot be measured by spirometry).

• Inspiratory Capacity (IC): TV + IRV

• Functional Residual Capacity (FRC): ERV + RV

• Vital Capacity (VC): IRV + TV + ERV

• Total Lung Capacity (TLC): VC + RV

Note: RV and capacities containing RV (FRC, TLC) cannot be measured by spirometry.

Pressure and Volume Changes During Breathing

During inspiration, the diaphragm contracts, thoracic volume increases, and alveolar pressure drops below atmospheric pressure, drawing air in. During expiration, the diaphragm relaxes, thoracic volume decreases, and alveolar pressure rises above atmospheric pressure, pushing air out.

Intrapleural Pressure

Intrapleural pressure is always subatmospheric due to the opposing elastic recoil of the lungs and chest wall. This negative pressure keeps the lungs inflated.

Pressure and Volume Changes During the Respiratory Cycle

• At the start of inspiration: Alveolar pressure < atmospheric pressure; air flows in.

• At the end of inspiration: Alveolar pressure = atmospheric pressure; no airflow.

• During expiration: Alveolar pressure > atmospheric pressure; air flows out.

• At the end of expiration: Alveolar pressure = atmospheric pressure; no airflow.

Surface Tension and Surfactants

Surface tension in the alveoli tends to cause collapse. Surfactant, produced by Type II alveolar cells, reduces surface tension, increasing lung compliance and preventing collapse (Law of Laplace).

$P = \frac{2T}{r}$

Where P is the pressure required to keep an alveolus open, T is surface tension, and r is the radius of the alveolus.

Airway Resistance and Control Mechanisms

• Factors Influencing Resistance: Airway diameter (main factor), mucus, smooth muscle tone.

• Bronchoconstriction: Caused by parasympathetic stimulation, histamine, and irritants; increases resistance and decreases airflow.

• Bronchodilation: Caused by sympathetic stimulation (β2-adrenergic receptors), CO2, and epinephrine; decreases resistance and increases airflow.

Total Pulmonary Ventilation vs. Alveolar Ventilation

• Total Pulmonary (Minute) Ventilation: Total volume of air entering or leaving the lungs per minute. Formula: $\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$

• Alveolar Ventilation: Volume of fresh air reaching the alveoli per minute (accounts for dead space). Formula: $\text{Alveolar Ventilation} = (\text{Tidal Volume} - \text{Dead Space}) \times \text{Respiratory Rate}$

Alveolar ventilation is a better indicator of effective gas exchange than total ventilation.

Alveolar Gas Composition and Ventilation

• Alveolar gas composition remains relatively constant during normal breathing due to continuous exchange and mixing.

• Hyperventilation: Increases alveolar O2, decreases CO2.

• Hypoventilation: Decreases alveolar O2, increases CO2.

Ventilation-Perfusion Matching (V/Q Ratio)

Efficient gas exchange requires matching of alveolar ventilation (V) and pulmonary blood flow (Q). Local changes in O2 and CO2 concentrations regulate the diameter of bronchioles and arterioles to optimize V/Q matching.

• Low O2 in alveoli → constriction of pulmonary arterioles (diverts blood to better-ventilated areas).

• High CO2 in alveoli → dilation of bronchioles (increases airflow to remove CO2).

Example: In regions of the lung with poor ventilation, local vasoconstriction reduces blood flow to those areas, improving overall gas exchange efficiency.