The Respiratory System: Structure, Function, and Mechanics of Breathing 23-1

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The Respiratory System

Introduction to the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen (O2) to body cells for aerobic ATP production and removing carbon dioxide (CO2), a metabolic waste product. It consists of passageways in the head, neck, trunk, and the lungs, and is organized both structurally and functionally.

Passageway for air: Air moves between the atmosphere and alveoli during breathing.
Site for gas exchange: O2 diffuses from alveoli into blood; CO2 diffuses from blood into alveoli.
Olfaction: Olfactory receptors in the superior nasal cavity detect odors.
Sound production: Air movement across the vocal cords produces sound.

General anatomy and organization of the respiratory system

Structural and Functional Organization

Upper respiratory tract: Nose, nasal cavity, pharynx, larynx
Lower respiratory tract: Trachea, bronchi, bronchioles, alveolar ducts, alveoli
Conducting zone: Nose to terminal bronchioles (air transport only)
Respiratory zone: Respiratory bronchioles, alveolar ducts, alveoli (site of gas exchange)

Overview of Respiration

Processes of Respiration

Respiration involves the movement of gases between the atmosphere and the body’s cells through four main processes:

Pulmonary ventilation: Movement of air between atmosphere and alveoli
Pulmonary gas exchange: Exchange of gases between alveoli and blood
Gas transport: Transport of gases in blood between lungs and systemic cells
Tissue gas exchange: Exchange of gases between blood and systemic cells

Overview of respiration: pulmonary ventilation, gas exchange, transport, and tissue exchange

Anatomy of the Bronchial Tree

Bronchial Tree Structure

The bronchial tree is a system of highly branched air passages that originate at the main bronchi and branch into narrower tubes, ending in bronchioles.

Main bronchi (primary): Trachea splits into right and left main bronchi, each entering a lung.
Lobar bronchi (secondary): Each main bronchus branches into lobar bronchi, one for each lung lobe.
Segmental bronchi (tertiary): Further division into smaller passageways.
Bronchioles: Tubes <1 mm in diameter, leading to terminal bronchioles (end of conducting zone) and respiratory bronchioles (start of respiratory zone).

Anatomy of the bronchial tree

Histology and Functional Changes in the Bronchial Tree

Main bronchi: Supported by incomplete rings of hyaline cartilage.
Bronchioles: No cartilage, but a thicker layer of smooth muscle.
Bronchoconstriction: Muscle contraction narrows bronchiole diameter, reducing airflow.
Bronchodilation: Muscle relaxation increases bronchiole diameter, increasing airflow.

Changes in wall structure of the bronchial tree and cross-sections of bronchioles

Respiratory Zone: Bronchioles, Alveolar Ducts, and Alveoli

Microscopic Anatomy

The respiratory zone includes respiratory bronchioles, alveolar ducts, and alveoli, where gas exchange occurs.

Respiratory bronchioles: Lined with simple cuboidal epithelium.
Alveoli and alveolar ducts: Lined by simple squamous epithelium for efficient gas exchange.
Alveolar pores: Openings that provide collateral ventilation between alveoli.
Elastic fibers: Allow for expansion during inspiration and recoil during expiration.

Bronchioles and alveoli of the lung

Cell Types of the Alveolar Wall

Alveolar type I cells: Squamous cells forming most of the alveolar surface and the respiratory membrane.
Alveolar type II cells (septal cells): Secrete pulmonary surfactant to reduce surface tension and prevent alveolar collapse.
Alveolar macrophages (dust cells): Engulf microorganisms and debris, either fixed in the wall or free to migrate.

Alveoli, surfactant, and the respiratory membrane

The Respiratory Membrane

Structure and Function

The respiratory membrane is a thin barrier (about 0.5 microns) that separates air in the alveoli from blood in pulmonary capillaries, facilitating rapid gas exchange.

Components: Plasma membrane of alveolar type I cell, plasma membrane of capillary endothelial cell, and their fused basement membranes.
Gas exchange: O2 diffuses from alveolus into capillaries; CO2 diffuses from blood to alveolus.

Pleural Membranes and Pressure Relationships

Pleural Structure and Pressure

Parietal pleura: Lines the thoracic wall.
Visceral pleura: Covers the lungs.
Pleural cavity: Space between pleurae containing pleural fluid.
Intrapleural pressure (Pip): Normally negative relative to intrapulmonary pressure, preventing lung collapse.
Transpulmonary pressure: Difference between intrapulmonary and intrapleural pressures; keeps lungs inflated.

Intrapulmonary and intrapleural pressure relationships

Pneumothorax

Pneumothorax is the presence of air in the pleural cavity, causing intrapleural pressure to equalize with atmospheric pressure, leading to lung collapse (atelectasis).

Pneumothorax: air in pleural cavity and lung collapse

Mechanics of Breathing (Pulmonary Ventilation)

Muscles of Breathing

Breathing involves cyclic phases of inspiration (inhalation) and expiration (exhalation), regulated by skeletal muscles and neural control centers in the brain.

Quiet breathing: Diaphragm and external intercostals contract for inspiration; relax for expiration.
Forced breathing: Additional muscles contract for greater changes in thoracic volume and pressure.

Muscles of breathing: quiet and forced inspiration/expiration

Thoracic Cavity Volume Changes

Vertical changes: Diaphragm contraction increases vertical dimension; relaxation decreases it.
Lateral changes: Rib elevation widens, depression narrows the thoracic cavity.
Anterior-posterior changes: Sternum moves anteriorly in inspiration, posteriorly in expiration.

Thoracic cavity dimensional changes: vertical changes

Boyle’s Law and Pressure Gradients

Boyle’s Law describes the inverse relationship between the pressure and volume of a gas at constant temperature:

As thoracic volume increases, intrapulmonary pressure decreases, causing air to flow into the lungs.
As thoracic volume decreases, intrapulmonary pressure increases, causing air to flow out.

Boyle's Law: relationship between pressure and volume Pressure gradients and airflow

Volumes and Pressures Associated with Breathing

Atmospheric pressure: Pressure of air in the environment (760 mm Hg at sea level).
Intrapulmonary pressure: Pressure within alveoli; fluctuates with breathing.
Intrapleural pressure: Pressure within pleural cavity; always lower than intrapulmonary pressure.
Air flows down its pressure gradient (from high to low pressure).

Areas and pressures associated with breathing

Mechanics of Quiet Breathing

Inspiration: Diaphragm and external intercostals contract, thoracic volume increases, intrapulmonary pressure drops below atmospheric pressure, air flows in (about 0.5 L as tidal volume).
Expiration: Muscles relax, thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure, air flows out (about 0.5 L).

Mechanics of quiet inspiration Mechanics of quiet expiration

Clinical Views

Bronchitis

Acute bronchitis: Short-term inflammation, often following infection.
Chronic bronchitis: Long-term irritation, persistent cough, and mucus production; increases risk of infection.

Asthma

Episodes of bronchoconstriction, wheezing, coughing, and excess mucus due to localized immune reactions.
Treatment includes inhaled steroids and bronchodilators.

Pneumonia

Infection of the lung, causing alveoli to fill with fluid or pus, leading to impaired gas exchange.
Symptoms include cough, fever, difficulty breathing, and chest pain.

Summary Table: Key Structures and Functions of the Respiratory System

Structure	Main Function
Nose/Nasal cavity	Warms, moistens, and filters air; olfaction
Pharynx	Passageway for air and food
Larynx	Voice production; airway protection
Trachea	Conducts air to bronchi
Bronchi/Bronchioles	Air passage; regulation of airflow
Alveoli	Site of gas exchange