Chapter 22: The Respiratory System – Structure, Function, and Physiology

Respiratory System Overview

Introduction

The respiratory system supplies the body with oxygen and removes carbon dioxide, a process known as respiration. Respiration involves several distinct processes that ensure efficient gas exchange between the body and the environment.

• Primary Function: Exchange of oxygen (O2) and carbon dioxide (CO2).

• Respiration: Encompasses ventilation, external respiration, transport of gases, and internal respiration.

Events of Respiration

Main Steps

• Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

• External Respiration: Exchange of O2 and CO2 between lungs and blood.

• Transport of Respiratory Gases: Movement of O2 and CO2 in the blood.

• Internal Respiration: Exchange of gases between blood and tissues.

Functional Anatomy of the Respiratory System

Major Structures

• Nose and Paranasal Sinuses

• Pharynx

• Larynx

• Trachea

• Bronchi and Subdivisions

• Lungs and Pleurae

The Bronchial Tree

• Primary Bronchi → Secondary Bronchi → Tertiary Bronchi → Bronchioles → Terminal Bronchioles

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveolar sacs (sites of gas exchange).

Alveoli

• Type I Alveolar Cells: Simple squamous epithelium; main site of gas exchange.

• Type II Alveolar Cells: Secrete surfactant to reduce surface tension.

• Alveolar Macrophages: Remove debris and pathogens from alveolar surfaces.

Mechanics of Breathing

Pressure Relationships

• Atmospheric Pressure (Patm): Pressure exerted by air surrounding the body (at sea level, 760 mm Hg).

• Intrapulmonary Pressure (Ppul): Pressure in the alveoli; fluctuates with breathing but always equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip): Pressure in the pleural cavity; always negative relative to Ppul (about -4 mm Hg).

Factors Holding the Lungs to the Thorax Wall

• Pleural Fluid: Provides surface tension between parietal and visceral pleura.

• Negative Intrapleural Pressure: Maintained by opposing forces (lung recoil vs. chest wall expansion).

• Transpulmonary Pressure: Difference between Ppul and Pip; keeps lungs inflated.

Pulmonary Ventilation: Inspiration and Expiration

Boyle's Law

At constant temperature, the pressure of a gas varies inversely with its volume:

$P_1V_1 = P_2V_2$

• If volume increases, pressure decreases; if volume decreases, pressure increases.

Inspiration

• Major Muscles: Diaphragm and external intercostals.

• Diaphragm contracts and flattens, increasing thoracic cavity volume.

• External intercostals contract, lifting the rib cage.

• Increased volume lowers intrapulmonary pressure, drawing air into lungs.

Expiration

• Usually a passive process due to lung recoil.

• Forced expiration involves abdominal and internal intercostal muscles.

Physical Factors Influencing Pulmonary Ventilation

• Airway Resistance: Determined by airway diameter; greatest in medium-sized bronchi.

• Alveolar Surface Tension: Surfactant reduces surface tension, preventing alveolar collapse.

• Lung Compliance: Measure of lung expandability; decreased by fibrosis, low surfactant, or reduced thoracic mobility.

Respiratory Volumes and Dead Space

Respiratory Volumes

• Tidal Volume (TV): Air inhaled or exhaled during normal breathing.

• Inspiratory Reserve Volume (IRV): Air that can be forcibly inhaled after normal inspiration.

• Expiratory Reserve Volume (ERV): Air that can be forcibly exhaled after normal expiration.

• Residual Volume (RV): Air remaining in lungs after forced expiration.

Dead Space

• Anatomical Dead Space: Air in conducting passages not involved in gas exchange.

• Alveolar Dead Space: Non-functional alveoli.

• Total Dead Space: Sum of anatomical and alveolar dead space.

Gas Exchange in the Body

Basic Properties of Gases

• Dalton's Law of Partial Pressures: Total pressure of a gas mixture equals the sum of partial pressures of each gas.

• Henry's Law: Amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

Composition of Alveolar Gas

• Alveolar gas differs from atmospheric air due to gas exchange, humidification, and mixing of inspired and residual air.

• Partial pressures of O2 and CO2 in alveoli are key for efficient gas exchange.

Gas Exchange Mechanisms

• External Respiration: O2 enters blood, CO2 leaves blood at alveoli.

• Internal Respiration: O2 leaves blood, CO2 enters blood at tissues.

• Driven by partial pressure gradients and diffusion across respiratory membranes.

Ventilation-Perfusion Coupling

• Efficient gas exchange requires matching of alveolar ventilation and pulmonary blood flow (perfusion).

• Local autoregulatory mechanisms adjust airflow and blood flow to optimize gas exchange.

Surface Area and Thickness of Respiratory Membrane

• Large surface area and thin membrane (0.5–1 μm) facilitate rapid gas exchange.

• Diseases that thicken the membrane or reduce surface area impair gas exchange.

Transport of Respiratory Gases by Blood

Oxygen Transport

• 98.5% of O2 is carried bound to hemoglobin (Hb) in red blood cells.

• O2 binds reversibly to iron in Hb: $\text{Hb} + \text{O}_2 \leftrightarrow \text{HbO}_2$

• Oxygen-hemoglobin dissociation curve shows relationship between O2 partial pressure and Hb saturation.

Carbon Dioxide Transport

• CO2 is transported in three forms:

  • Dissolved in plasma (7–10%)

  • Chemically bound to Hb as carbaminohemoglobin (20%)

  • As bicarbonate ion (HCO3-) in plasma (70%)

• CO2 conversion to bicarbonate: $\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

Haldane and Bohr Effects

• Haldane Effect: Deoxygenated blood can carry more CO2.

• Bohr Effect: Increased CO2 or decreased pH reduces Hb's affinity for O2, enhancing O2 release to tissues.

Control of Respiration

Neural Control

• Medullary Respiratory Centers: Ventral and dorsal respiratory groups generate basic rhythm.

• Pontine Respiratory Centers: Modify and fine-tune breathing rhythms.

Influence of Higher Brain Centers

• Hypothalamus: Emotions and pain can alter breathing patterns.

• Cerebral Cortex: Voluntary control over breathing (e.g., holding breath).

Chemical Factors

• Central and peripheral chemoreceptors monitor CO2, O2, and H+ levels.

• Increased CO2 (hypercapnia) is the most potent stimulus for increased ventilation.

Summary Table: Forms of CO2 Transport in Blood

Form	Percentage	Description
Dissolved in plasma	7–10%	CO2 directly dissolved in blood plasma
Carbaminohemoglobin	20%	CO2 bound to hemoglobin (HbCO2)
Bicarbonate ion (HCO3-)	70%	CO2 converted to bicarbonate in RBCs and transported in plasma

Key Equations

• Boyle's Law: $P_1V_1 = P_2V_2$

• CO2 to Bicarbonate: $\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

Example Application

• During exercise, increased CO2 production lowers blood pH, stimulating chemoreceptors to increase ventilation rate.

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.