Comprehensive Study Notes: The Respiratory System (Ch. 22)

Ch. 22: The Respiratory System

Introduction

The respiratory system supplies the body with oxygen and disposes of carbon dioxide. The process of gas exchange, known as respiration, involves several distinct phases that ensure cellular metabolism and homeostasis.

• Primary Function: Oxygen delivery and carbon dioxide removal.

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

Events of Respiration

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

• External Respiration: Exchange of oxygen and carbon dioxide between lungs and blood.

• Transport of Respiratory Gases: Movement of gases between lungs and tissues via blood.

• Internal Respiration: Exchange of gases between blood and tissue cells.

Functional Anatomy of the Respiratory System

The respiratory system consists of several anatomical structures that facilitate air movement and gas exchange.

• Nose and Paranasal Sinuses

• Pharynx

• Larynx

• Trachea

• Bronchi and Subdivisions: The Bronchial Tree

• Lungs

• Pleurae

The Bronchial Tree

• Conducting Zone: Trachea → main bronchi → secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles

• Respiratory Zone: Terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli

Alveoli

• Type I Cells: Simple squamous epithelium; form the alveolar wall.

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

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

Mechanics of Breathing

Breathing involves changes in pressure and volume within the thoracic cavity, allowing air to flow in and out of the lungs.

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

• Intrapulmonary Pressure: Pressure within the alveoli; fluctuates with breathing but always equalizes with atmospheric pressure.

• Intrapleural Pressure: Pressure within the pleural cavity; always less than intrapulmonary pressure (about -4 mm Hg).

Factors Holding the Lungs to the Thoracic Wall

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

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

• Adhesive Force: Pleural fluid bonds visceral and parietal pleura together.

Pulmonary Ventilation: Inspiration and Expiration

Boyle's Law

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

• Equation: $P_1V_1 = P_2V_2$

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

Inspiration

• Major Muscles: Diaphragm and external intercostals.

• Diaphragm contracts and flattens, increasing thoracic volume.

• External intercostals contract, lifting the rib cage.

Expiration

• Usually a passive process due to lung elasticity.

• 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 tension, preventing alveolar collapse.

• Lung Compliance: Ease with which lungs can expand; decreased by fibrosis, reduced surfactant, or thoracic cage ossification.

Respiratory Volumes and Dead Space

• Tidal Volume (TV): Amount of air inhaled or exhaled during normal breathing.

• Dead Space: Air in respiratory passages that does not participate in gas exchange.

• 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 exerted by a mixture of gases is the sum of the pressures exerted independently by each gas.

• Equation: $P_{total} = P_1 + P_2 + P_3 + \ldots$

• Henry's Law: The amount of gas that dissolves 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 new and old air.

External Respiration: Pulmonary Gas Exchange

• Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.

• Efficiency depends on partial pressure gradients, gas solubility, ventilation-perfusion coupling, and thickness/surface area of the respiratory membrane.

Ventilation-Perfusion Coupling

• Optimal gas exchange requires matching of alveolar ventilation and pulmonary blood flow.

• Local autoregulatory mechanisms adjust bronchiolar diameter and pulmonary arteriolar diameter in response to CO2 and O2 levels.

Thickness and Surface Area of Respiratory Membrane

• Healthy lungs have a thin respiratory membrane (0.5–1 μm), facilitating efficient gas exchange.

• Greater surface area allows more gas diffusion.

Transport of Respiratory Gases by Blood

Oxygen Transport

• Most oxygen is carried bound to hemoglobin (Hb) in red blood cells.

• Equation: $\text{Hb} + \text{O}_2 \rightleftharpoons \text{HbO}_2$

• Small amount dissolved in plasma.

Carbon Dioxide Transport

• Transported in three forms: dissolved in plasma, chemically bound to hemoglobin, and as bicarbonate ion (HCO3-).

• Equation: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$

Haldane Effect and Bohr Effect

• Haldane Effect: Deoxygenated blood can carry more CO2.

• Bohr Effect: Increased CO2 and H+ lower hemoglobin's affinity for O2, facilitating oxygen release.

Factors Affecting Hemoglobin Saturation

• Partial pressure of O2 (PO2), temperature, blood pH, and PCO2.

• Increased temperature or decreased pH reduces O2 affinity.

Control of Respiration

Neural Control

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

• Pontine Respiratory Group: Modifies rhythm during activities like speaking or exercise.

Influence of Higher Brain Centers

• Hypothalamus: Emotional stimuli can alter breathing rate and depth.

• Cortical Controls: Voluntary control over breathing (e.g., holding breath).

Chemical Factors

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

• Increased CO2 (hypercapnia) stimulates increased ventilation.

• Decreased O2 (hypoxemia) stimulates ventilation only when very low.

Summary Table: Forms of Carbon Dioxide Transport

Form	Percentage	Description
Dissolved in plasma	~7-10%	CO2 directly dissolved in blood plasma
Chemically bound to hemoglobin	~20%	CO2 binds to globin part of hemoglobin (carbaminohemoglobin)
As bicarbonate ion (HCO3-)	~70%	CO2 converted to bicarbonate in RBCs and transported in plasma

Example: Application of Boyle's Law in Breathing

• During inspiration, thoracic volume increases, intrapulmonary pressure decreases, and air flows into the lungs.

• During expiration, thoracic volume decreases, intrapulmonary pressure increases, and air flows out of the lungs.

Additional info: Academic context and expanded explanations have been added to clarify and complete fragmented points from the original notes.