Respiratory System: Structure, Function, and Physiology Study Guide

Respiratory System Overview

Respiratory Epithelium and Zones

The respiratory system is divided into conducting and respiratory zones, each lined by specialized epithelium that changes along the tract to support different functions.

• Conducting Zone: Includes nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. Lined primarily by pseudostratified ciliated columnar epithelium with goblet cells for mucus production.

• Respiratory Zone: Composed of respiratory bronchioles, alveolar ducts, and alveoli. Epithelium transitions to simple squamous cells (type I pneumocytes) for efficient gas exchange.

• Function Relationship: The epithelium in the conducting zone filters, warms, and humidifies air, while the respiratory zone facilitates gas exchange.

Respiratory Cell Types

Different cell types are found throughout the respiratory tract, each with specialized roles.

• Type I Alveolar Cells: Simple squamous cells forming the alveolar wall, allowing gas diffusion.

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

• Goblet Cells: Produce mucus to trap particles and pathogens.

• Ciliated Cells: Move mucus and trapped debris out of the airways.

Respiratory Defense System

The respiratory system employs several defense mechanisms to protect against pathogens and particulates.

• Mucociliary Escalator: Cilia move mucus toward the pharynx for swallowing or expectoration.

• Alveolar Macrophages: Engulf and digest foreign particles in the alveoli.

Structural and Functional Organization

Surface Area and Gas Exchange

The extensive surface area of the alveoli is crucial for efficient gas exchange.

• Alveolar Surface Area: Approximately 70-100 m2 in adults, maximizing contact with capillaries.

• Warming and Moistening: Nasal cavity and upper airways warm and humidify incoming air to protect delicate alveolar surfaces.

Trachea and Bronchial Tree

The trachea branches into bronchi and bronchioles, forming the bronchial tree that conducts air to the alveoli.

• Cartilage: Maintains airway patency in trachea and bronchi.

• Smooth Muscle: Regulates airway diameter in bronchioles.

Respiratory Membrane

The respiratory membrane is the site of gas exchange between alveolar air and blood.

• Structure: Composed of alveolar epithelium, capillary endothelium, and their fused basement membranes.

• Thickness: Extremely thin (~0.5 μm) to facilitate rapid diffusion.

Gas Exchange and Transport

Mechanisms of Gas Exchange

Gas exchange occurs by diffusion across the respiratory membrane, driven by partial pressure gradients.

• Oxygen: Moves from alveoli (high PO2) to blood (low PO2).

• Carbon Dioxide: Moves from blood (high PCO2) to alveoli (low PCO2).

Equation:

$\text{Rate of diffusion} \propto \frac{A \cdot D \cdot (P_1 - P_2)}{T}$

Where A = surface area, D = diffusion coefficient, (P1 - P2) = partial pressure difference, T = thickness of membrane.

Oxygen and Carbon Dioxide Transport

• Oxygen: Transported mainly bound to hemoglobin; a small amount is dissolved in plasma.

• Carbon Dioxide: Transported as dissolved CO2, bicarbonate ions, and carbaminohemoglobin.

• Oxygen Dissociation: Factors such as pH, temperature, and CO2 levels affect hemoglobin's affinity for oxygen (Bohr effect).

Gas Exchange Efficiency

• Diffusion Limitation: Thickening of the respiratory membrane (e.g., fibrosis) impairs gas exchange.

• Perfusion Limitation: Reduced blood flow limits gas exchange.

Respiratory Mechanics

Ventilation and Lung Volumes

Ventilation is the movement of air into and out of the lungs, driven by pressure changes resulting from thoracic volume changes.

• Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume and decreasing intrapulmonary pressure.

• Expiration: Usually passive; muscles relax, thoracic volume decreases, and air is expelled.

• Lung Compliance: The ease with which lungs expand; affected by surfactant, elasticity, and disease.

Equation:

$\text{Compliance} = \frac{\Delta V}{\Delta P}$

Where $\Delta V$ = change in lung volume, $\Delta P$ = change in transpulmonary pressure.

Dead Space

• Anatomic Dead Space: Air in conducting airways not involved in gas exchange.

• Alveolar Dead Space: Alveoli that are ventilated but not perfused.

Control of Respiration

Neural Respiratory Centers

Respiratory rhythm is controlled by centers in the brainstem.

• Medullary Respiratory Centers: Control basic rhythm of breathing.

• Pontine Centers: Modulate rhythm and transition between inspiration and expiration.

Pathophysiology and Clinical Considerations

Pneumothorax

Pneumothorax is the presence of air in the pleural cavity, leading to lung collapse.

• Symptoms: Sudden chest pain, dyspnea, decreased breath sounds.

• Causes: Trauma, spontaneous rupture, medical procedures.

Hypoxemia and Hypercapnia

• Hypoxemia: Low oxygen levels in the blood; can result from impaired gas exchange, ventilation-perfusion mismatch, or shunt.

• Hypercapnia: Elevated carbon dioxide levels; often due to hypoventilation or severe lung disease.

Factors Affecting Gas Exchange

• Fibrosis: Thickens the respiratory membrane, reducing diffusion.

• Emphysema: Destroys alveolar walls, decreasing surface area.

• Edema: Fluid accumulation impairs diffusion.

Hemoglobin and Gas Binding

• Oxygen Binding: Cooperative binding; affinity affected by pH, CO2, temperature, and 2,3-BPG.

• Carbon Monoxide: Binds hemoglobin with higher affinity than oxygen, reducing oxygen transport.

• Reversibility: Oxygen binding is reversible; CO binding is much less reversible and toxic.

Summary Table: Factors Affecting Oxygen Dissociation from Hemoglobin

Factor	Effect on O2 Dissociation	Mechanism
Increased CO2	Increases	Bohr effect (decreased pH)
Decreased pH	Increases	Bohr effect
Increased Temperature	Increases	Alters hemoglobin structure
Increased 2,3-BPG	Increases	Stabilizes deoxyhemoglobin
Carbon Monoxide	Decreases	Binds hemoglobin, blocks O2 binding

Additional info:

• Some questions reference clinical conditions (e.g., pneumothorax, hypoxemia) and physiological mechanisms (e.g., Bohr effect, compliance) that are essential for understanding respiratory system function.

• For exam preparation, students should be able to describe structural features, physiological processes, and pathological changes in the respiratory system.