Chapter 21: The Respiratory System - Anatomy, Physiology, and Clinical Concepts

Chapter 21: The Respiratory System

Module 21.1 Overview of the Respiratory System

The respiratory system is essential for gas exchange, providing oxygen to the body and removing carbon dioxide. Its organs are located in the head, neck, and thoracic cavity.

• Anatomy: Includes blood vessels of the pulmonary circuit, rib cage, respiratory muscles, lungs, and the respiratory tract.

• Respiratory Tract: Hollow passages for gas exchange: nose, nasal cavity, pharynx, larynx, trachea, bronchial tree (bronchi and bronchioles).

• Upper Respiratory Tract: Nasal cavity to larynx.

• Lower Respiratory Tract: Trachea to alveoli (site of gas exchange).

• Lungs: Located in the thoracic cavity, composed of alveoli, blood vessels, and elastic connective tissue.

Functional Zones:

• Conducting Zone: Air passageways from nose to bronchioles; air is filtered, warmed, and moistened.

• Respiratory Zone: Structures containing alveoli; site of gas exchange.

Primary Functions:

• Respiration (pulmonary ventilation, gas exchange, gas transport, tissue gas exchange)

• Speech and sound production

• Olfaction (sense of smell)

• Pressure changes for defecation, urination, childbirth

• Venous blood and lymph flow assistance

• Acid-base balance and homeostasis

• Production of angiotensin-II (blood pressure regulation)

Module 21.2 Anatomy of the Respiratory System

The Nose and Nasal Cavity

The nose and nasal cavity are the entryway for air. They warm, humidify, and filter air, house olfactory receptors, and enhance voice resonance.

• External Anatomy: Muscle, nasal bones, lateral and alar cartilages.

• Internal Anatomy: Bone and cartilage, divided by nasal septum; vestibule with hairs; conchae create turbulence for filtration.

• Paranasal Sinuses: Frontal, ethmoid, sphenoid, maxillary bones; warm/humidify air, enhance resonance, reduce skull weight.

• Histology: Vestibule lined with stratified squamous epithelium; rest lined with pseudostratified ciliated columnar epithelium and goblet cells (mucus production).

The Pharynx

The pharynx (throat) is divided into nasopharynx, oropharynx, and laryngopharynx, serving as a passage for air and food.

The Larynx

The larynx (voice box) prevents food/liquids from entering the respiratory tract and houses vocal cords for sound production.

The Trachea

The trachea (windpipe) extends from the neck to the mediastinum. C-shaped hyaline cartilage rings keep it open, allowing flexibility. The carina marks the division into bronchi. Lined with pseudostratified ciliated epithelium and goblet cells.

The Bronchial Tree

The bronchial tree consists of branching tubes from the primary bronchi to alveoli.

• Primary Bronchi: Left and right branches at carina.

• Secondary/Tertiary Bronchi: Further branching; histology changes with smaller size.

• Bronchioles: Smallest airways; simple cuboidal epithelium; terminal bronchioles branch into respiratory bronchioles, then alveolar ducts and sacs.

• Histological Changes: Cartilage decreases, smooth muscle increases, epithelium changes from columnar to cuboidal.

Alveoli and the Respiratory Membrane

Alveoli are thin-walled sacs for gas exchange, composed of three cell types:

• Type I Alveolar Cells: Squamous, 90% of wall, rapid gas diffusion.

• Type II Alveolar Cells: Cuboidal, produce surfactant to reduce surface tension.

• Alveolar Macrophages: Mobile phagocytes, clean debris.

The Lungs and Pleurae

The lungs are divided into lobes and segments, surrounded by pleural membranes.

• Right Lung: Three lobes; Left Lung: Two lobes (space for heart).

• Blood Supply: Pulmonary arteries (deoxygenated), pulmonary veins (oxygenated), bronchial arteries (nutrients).

• Pleural Cavity: Between parietal and visceral pleura; filled with pleural fluid for lubrication.

Module 21.3 Pulmonary Ventilation

Pulmonary ventilation is the process of moving air in and out of the lungs, driven by pressure gradients.

• Phases: Inspiration (inhalation), expiration (exhalation).

• Pressure-Volume Relationship: Described by Boyle's Law: at constant temperature, pressure and volume are inversely related.

Boyle's Law:

• As volume increases, pressure decreases; as volume decreases, pressure increases.

$P \propto \frac{1}{V}$

• Inspiratory Muscles: Diaphragm and external intercostals.

• Pressures: Atmospheric, intrapulmonary, intrapleural.

• Physical Factors: Airway resistance, alveolar surface tension, pulmonary compliance.

Pulmonary Volumes and Capacities

Measured by spirometry; important for assessing lung function.

• Tidal Volume (TV): Air inspired/expired during normal breathing (~500 ml).

• Inspiratory Reserve Volume (IRV): Air forcibly inspired after normal inspiration (2100–3000 ml).

• Expiratory Reserve Volume (ERV): Air forcibly expired after normal expiration (700–1200 ml).

• Residual Volume (RV): Air remaining after maximal expiration.

Pulmonary Capacities:

• Inspiratory Capacity: $TV + IRV$

• Functional Residual Capacity: $ERV + RV$

• Vital Capacity: $TV + IRV + ERV$

• Total Lung Capacity (TLC): $IRV + TV + ERV + RV$

Module 21.4 Gas Exchange

Gas exchange moves oxygen into blood and cells, and carbon dioxide out. Occurs in two processes:

• Pulmonary Gas Exchange: Between alveoli and blood.

• Tissue Gas Exchange: Between blood and body cells.

Gas Laws:

• Dalton's Law: Each gas in a mixture exerts its own partial pressure; total pressure is sum of partial pressures.

• Henry's Law: Gas solubility in water is proportional to its partial pressure.

Pulmonary Gas Exchange:

• Oxygen diffuses from alveoli (PO2 = 104 mm Hg) to blood (PO2 = 40 mm Hg).

• CO2 diffuses from blood (PCO2 = 45 mm Hg) to alveoli (PCO2 = 40 mm Hg).

Factors Affecting Efficiency:

• Surface area of respiratory membrane

• Thickness of membrane

• Ventilation-perfusion matching

Tissue Gas Exchange: Driven by partial pressure gradients; efficiency depends on surface area, diffusion distance, and perfusion.

Module 21.5 Gas Transport Through the Blood

Oxygen and carbon dioxide are transported in the blood via chemical reactions.

Oxygen Transport

• Hemoglobin (Hb): Protein in erythrocytes; four subunits, each binds one O2 molecule.

• Loading: O2 binds Hb in pulmonary capillaries (forms oxyhemoglobin, HbO2).

• Unloading: Hb releases O2 in tissues.

• Saturation: Depends on PO2 and Hb affinity for O2.

Oxygen-Hemoglobin Dissociation Curve: S-shaped; shows relationship between PO2 and Hb saturation.

Carbon Dioxide Transport

• 7–10% dissolved in plasma

• 20% bound to Hb (carbaminohemoglobin)

• 70% as bicarbonate ions (HCO3-)

Carbonic Anhydrase Reaction:

$\mathrm{CO_2} + \mathrm{H_2O} \leftrightarrow \mathrm{H_2CO_3} \leftrightarrow \mathrm{HCO_3^-} + \mathrm{H^+}$

• HCO3- diffuses into plasma; H+ binds Hb (buffering pH).

• Reverse reaction occurs in lungs to release CO2.

Blood pH Regulation: Carbonic acid-bicarbonate buffer system maintains pH.

• Hyperventilation: Decreases CO2, increases pH (alkalosis).

• Hypoventilation: Increases CO2, decreases pH (acidosis).

Module 21.7 Neural Control of Ventilation

Breathing is controlled by the brainstem, mainly the medulla oblongata.

• Respiratory Rhythm Generator (RRG): Sets basic breathing rhythm.

• Ventral Respiratory Group (VRG): Controls inspiration and expiration via phrenic and intercostal nerves.

• Dorsal Respiratory Group (DRG): Primarily involved in inspiration.

Control Mechanisms:

• Central Chemoreceptors: Detect CO2 and H+ in cerebrospinal fluid; adjust ventilation rate.

• Peripheral Chemoreceptors: Located in carotid and aortic bodies; sensitive to PO2, PCO2, and H+.

• Negative Feedback Loops: High CO2/H+ triggers hyperventilation; low CO2/H+ triggers hypoventilation.

Module 21.8 Diseases of the Respiratory System

Respiratory diseases affect lung function and gas exchange.

Restrictive Lung Diseases

• Decrease pulmonary compliance, reduce inspiration effectiveness.

• Decrease inspiratory capacity, vital capacity, and total lung capacity.

• Examples: Idiopathic pulmonary fibrosis, pneumoconiosis, neuromuscular diseases, chest wall deformities.

Obstructive Lung Diseases

• Increase airway resistance, decrease expiration efficiency.

• Increase residual volume, decrease vital capacity.

• Examples: Emphysema, small airway disease, chronic bronchitis.

• Asthma: Airways hyperresponsive to triggers.

• Lung Cancer: Tumors in bronchi, bronchioles, alveoli epithelium.

Table: Pulmonary Volumes and Capacities

Volume/Capacity	Definition	Typical Value (ml)
Tidal Volume (TV)	Air inspired/expired during normal breathing	500
Inspiratory Reserve Volume (IRV)	Air forcibly inspired after normal inspiration	2100–3000
Expiratory Reserve Volume (ERV)	Air forcibly expired after normal expiration	700–1200
Residual Volume (RV)	Air remaining after maximal expiration	~1200
Inspiratory Capacity	TV + IRV	~2600–3500
Functional Residual Capacity	ERV + RV	~1900–2400
Vital Capacity	TV + IRV + ERV	~3400–4800
Total Lung Capacity (TLC)	IRV + TV + ERV + RV	~4200–6000

Additional info: Values for pulmonary volumes and capacities vary by gender, age, and body size. The table above provides typical adult values.