The Respiratory System: Pulmonary Ventilation – Structure and Function

Chapter 16a: The Respiratory System – Pulmonary Ventilation

Chapter Outline

• 16.1 Overview of Respiratory Function

• 16.2 Anatomy of the Respiratory System

Learning Outcomes

• Compare internal respiration to external respiration and describe the processes occurring in each.

• Describe the major structures of the respiratory system and list the functions of each.

• Describe the anatomy of the respiratory membrane and explain how its structure facilitates the exchange of gases between blood and air.

• Describe the anatomy of alveoli. Explain the roles of type I cells, type II cells, and alveolar macrophages in respiratory function.

16.1 Overview of Respiratory Function

Internal Respiration

Internal respiration refers to the metabolic processes within cells that utilize oxygen and produce carbon dioxide as a waste product.

• Aerobic cellular respiration: The process by which cells use oxygen to produce ATP via oxidative phosphorylation.

• Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

External Respiration

External respiration encompasses the exchange of gases between the atmosphere and the body, including ventilation and gas transport.

• Pulmonary ventilation: Movement of air into and out of the lungs (inspiration and expiration).

• Exchange between lungs and blood: Diffusion of O2 and CO2 across alveolar and capillary membranes.

• Transport in blood: Movement of gases via the circulatory system.

• Exchange between blood and body tissues: Diffusion of gases at the tissue level.

Additional Functions of the Respiratory System

• Acid-base balance: Regulation of blood pH by controlling CO2 levels.

• Vocalization: Production of sound via airflow through the larynx.

• Immune function: Defense against inhaled pathogens.

• Water & heat loss: Evaporation and warming of air during breathing.

• Respiratory pump action: Assists venous return to the heart.

• Activation of plasma proteins: Example: Angiotensin-converting enzyme (ACE) in pulmonary endothelium.

16.2 Anatomy of the Respiratory System

Upper Airways

The upper airways filter, warm, and humidify incoming air.

• Nasal cavities: Primary site for air filtration and conditioning.

• Oral cavity: Secondary route for air entry.

• Pharynx: Passageway for both air and food.

The Respiratory Tract

The respiratory tract conducts air from the pharynx to the lungs and is divided into conducting and respiratory zones.

Conducting Zone

• Larynx: Contains the glottis and epiglottis, which protect the airway during swallowing.

• Trachea: 2.5 cm diameter, 10 cm long, supported by C-shaped cartilage bands.

• Primary bronchi: Enter each lung, supported by cartilage rings.

• Secondary bronchi: Branch into lobes of the lungs (3 right, 2 left), supported by cartilage plates.

• Tertiary bronchi: 20–23 orders of branching, further subdividing the airways.

• Bronchioles: No cartilage, less than 1 mm in diameter, end in terminal bronchioles.

Epithelium of the Conducting Zone

• Mucus/respiratory escalator: Traps particles and moves them out of the airways.

• Goblet cells: Secrete mucus.

• Ciliated cells: Move mucus toward the pharynx.

• Smoker's cough: Result of damaged cilia, leading to impaired mucus clearance.

Functions of the Conducting Zone

1. Air passageway: 150 mL in volume (anatomical dead space).

2. Warms air: Adjusts temperature to body conditions.

3. Humidifies air: Prevents drying of respiratory surfaces.

Respiratory Zone

• Respiratory bronchioles

• Alveolar ducts

• Alveolar sacs

• Alveoli

Epithelium of the Respiratory Zone

• Epithelial cell layer of alveoli & basement membrane: Thin barrier for efficient gas exchange.

Functions of the Respiratory Zone

1. Exchange of gases between air and blood

2. Diffusion

Alveoli

Alveoli are the primary site of gas exchange in the lungs.

• Number: Approximately 300 million alveoli, providing a large surface area (size of a tennis court).

• Capillary network: Dense meshwork for efficient gas exchange.

• Alveolar pores: Allow pressure equilibration between adjacent alveoli.

Cell Types in Alveoli

• Type I alveolar cells: Simple squamous epithelium, responsible for gas exchange.

• Type II alveolar cells: Secrete surfactant, reducing surface tension and preventing alveolar collapse.

• Alveolar macrophages: Engulf and remove debris and pathogens.

Respiratory Membrane

The respiratory membrane is the thin barrier through which gases diffuse between alveolar air and blood.

• Structure: Composed of type I alveolar cells and their basement membrane, capillary endothelial cells and their basement membrane.

• Thickness: Approximately 0.2 μm, facilitating rapid diffusion.

Anatomical Features Table

The following table summarizes the anatomical features of the conducting and respiratory zones of the respiratory tract:

Structure	Inner Diameter (mm)	Cilia	Goblet Cells	Cartilage	Smooth Muscle
Larynx	15–20	+++	+++	++ (C-shaped)	+
Trachea	15–20	+++	+++	++ (C-shaped)	+
Primary bronchi	12–16	+++	+++	++ (rings)	+
Secondary bronchi	8–13	+++	+++	+ (plates)	++
Tertiary bronchi	2–8	+++	+++	+ (plates)	++
Smaller bronchi	1–2	+++	+++	+	+++
Bronchioles	0.5–1	++	++	–	+++
Terminal bronchioles	0.5	+	+	–	+++
Respiratory bronchioles	0.5	–	–	–	+++
Alveolar sacs	0.3	–	–	–	+
Alveoli	0.3	–	–	–	+

Structures of the Thoracic Cavity

The thoracic cavity houses and protects the lungs and is essential for ventilation.

• Chest wall: Airtight structure formed by bones (sternum, ribs, thoracic vertebrae) and muscles (internal & external intercostals, diaphragm).

• Pleura: Double-layered membrane lining the lungs and chest wall.

• Pleural sac: Contains visceral pleura (covers lungs), parietal pleura (lines chest wall), and intrapleural space with fluid (15 mL) to reduce friction.

Example: Gas Exchange Across the Respiratory Membrane

Oxygen diffuses from alveolar air into capillary blood, while carbon dioxide diffuses from blood into alveolar air, driven by partial pressure gradients.

• Equation for diffusion rate (Fick's Law): $\text{Rate of diffusion} = \frac{A \cdot D \cdot (P_1 - P_2)}{T}$ Where: $A$ = surface area, $D$ = diffusion coefficient, $(P_1 - P_2)$ = partial pressure difference, $T$ = thickness of membrane

Summary Table: Key Functions of Respiratory System Structures

Structure	Main Function
Nasal cavity	Filters, warms, humidifies air
Pharynx	Conducts air to larynx
Larynx	Protects airway, vocalization
Trachea	Conducts air to bronchi
Bronchi	Distributes air to lungs
Bronchioles	Regulates airflow, no cartilage
Alveoli	Site of gas exchange
Pleura	Reduces friction, maintains pressure

Additional info: The notes above expand on the original slides by providing definitions, equations, and context for each anatomical structure and physiological process, ensuring a comprehensive and self-contained study guide for college-level Anatomy & Physiology students.