The Respiratory System: Structure, Function, and Physiology

Respiratory System Overview

Functions of the Respiratory System

The respiratory system is essential for gas exchange, regulation of blood pH, vocalization, and olfaction. It ensures the intake of oxygen (O2) and elimination of carbon dioxide (CO2), maintaining homeostasis and supporting cellular metabolism.

• Gas Exchange: Intake of O2 and elimination of CO2.

• Regulation of Blood pH: By controlling CO2 levels, the respiratory system helps maintain acid-base balance.

• Vocalization: Air movement through the larynx produces sound.

• Olfactory Receptors: Located in the nasal cavity, responsible for the sense of smell.

Gross Anatomy of the Respiratory System

Upper and Lower Respiratory Tracts

The respiratory system is divided into upper and lower tracts, each with distinct anatomical structures and functions.

• Upper Respiratory Tract: Nose and pharynx

• Lower Respiratory Tract: Larynx, trachea, bronchi, and lungs

Nose: Structure and Function

The nose provides an airway for respiration, moistens and warms incoming air, filters debris, and houses olfactory receptors. It also adds resonance to the voice.

• External Nose: Allows air entry through external nares (nostrils).

• Nasal Cavities: Internal nose lined with pseudostratified ciliated columnar epithelium, containing hair, blood vessels, and mucus glands for filtration, warming, and protection.

• Paranasal Sinuses: Located in frontal, maxillary, sphenoid, and ethmoid bones; warm and moisten air, trap debris, and add resonance to the voice.

Pharynx: Anatomy and Regions

The pharynx is a muscular tube that serves as a common pathway for air and food, divided into three regions:

• Nasopharynx: Air passageway only; lined with pseudostratified ciliated columnar epithelium.

• Oropharynx: Common passageway for food and air; lined with stratified squamous epithelium.

• Laryngopharynx: Common pathway for food and air; site of divergence of respiratory and digestive systems; lined with stratified squamous epithelium.

Larynx: Structure and Function

The larynx is composed of several cartilages and serves as a switching mechanism to separate food and air, provides a patent airway, and is essential for voice production.

• Thyroid Cartilage: Largest part; contains vocal cords.

• Epiglottis: Elastic cartilage flap that prevents food from entering the airway.

• Arytenoid and Cricoid Cartilage: Support vocal folds.

Trachea: Structure and Histology

The trachea is a tube supported by C-shaped hyaline cartilage rings, lined with pseudostratified ciliated columnar epithelium. It conducts air from the larynx into the thoracic cavity.

• Mucosa: Pseudostratified ciliated columnar epithelium with goblet cells producing mucus.

• Submucosa: Areolar connective tissue and mucus glands.

• Hyaline Cartilage: Maintains airway patency.

• Adventitia: Outermost connective tissue layer.

Bronchi and Bronchioles

The trachea divides into right and left primary bronchi, which branch repeatedly to form the bronchial tree, ending in terminal and respiratory bronchioles. The bronchioles are under autonomic control and can constrict or dilate.

• Conducting Zone: Transports air from trachea to terminal bronchioles.

• Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli where gas exchange occurs.

Alveoli and the Respiratory Membrane

Alveoli are the primary sites of gas exchange, consisting of a single layer of simple squamous epithelium fused to capillary endothelium, forming the respiratory membrane.

• Type I Cells: Form the alveolar wall.

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

• Macrophages: Remove dust and debris.

Changes in the Respiratory Tree

As airways branch and decrease in size, structural changes occur:

• Cartilage is replaced by smooth muscle in bronchioles.

• Epithelium transitions from pseudostratified ciliated columnar to simple cuboidal and finally to simple squamous in alveoli.

• Smooth muscle in bronchioles is regulated by autonomic nervous system (bronchoconstriction/bronchodilation).

Lungs and Pleura

Each lung is housed in its own pleural cavity, lined by a double-layered serous membrane (pleura):

• Parietal Pleura: Attaches lung to thoracic wall.

• Visceral Pleura: Covers the lung surface and secretes pleural fluid to reduce friction.

• Lobes: Right lung has 3 lobes; left lung has 2 lobes.

Respiratory Physiology

Processes of Respiration

Respiration involves four main processes:

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

• External Respiration: Gas exchange between alveoli and blood.

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

• Internal Respiration: Gas exchange between blood and tissues.

Mechanics of Breathing (Ventilation)

Breathing is driven by changes in thoracic cavity volume and pressure gradients:

• Inhalation: Diaphragm contracts and moves downward, external intercostals elevate the rib cage, increasing thoracic volume and decreasing pressure, causing air to flow in.

• Exhalation: Diaphragm and intercostals relax, thoracic volume decreases, pressure increases, and air is expelled passively due to lung elasticity.

Boyle’s Law

Boyle’s Law states that at a constant temperature, the pressure of a gas varies inversely with its volume:

$P_1V_1 = P_2V_2$

As pressure increases, volume decreases, and vice versa.

Factors Affecting Ventilation

• Airway Resistance: Increased resistance decreases airflow.

• Lung Compliance: The ease with which lungs can expand.

• Lung Elasticity: The ability of lungs to recoil after stretching.

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

Gas Exchange and Diffusion

Gas exchange occurs across the thin respiratory membrane by simple diffusion, driven by partial pressure gradients of O2 and CO2. Efficient diffusion is favored by increased surface area, thin membranes, and steep gradients.

• Ventilation-Perfusion Coupling: Matching of air flow and blood flow in the lungs optimizes gas exchange.

Oxygen Transport and the Bohr Effect

Oxygen is primarily transported bound to hemoglobin as oxyhemoglobin. The affinity of hemoglobin for O2 decreases with increased CO2, lower pH, higher temperature, and increased 2,3-BPG, facilitating O2 release to tissues (Bohr Effect).

• Bohr Effect: O2 binding affinity to hemoglobin is inversely related to acidity and CO2 concentration.

Carbon Dioxide Transport

CO2 is transported in three forms:

• Dissolved in plasma (7%)

• Bound to hemoglobin as carbaminohemoglobin (23%)

• As bicarbonate ion (HCO3-) (70%)

The conversion of CO2 to bicarbonate is catalyzed by carbonic anhydrase in red blood cells:

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

Control of Respiration

Respiratory rate and depth are regulated by neural mechanisms in the medulla and pons, higher brain centers, and chemoreceptors sensitive to CO2, O2, and pH levels.

• Central Chemoreceptors: Located in the brainstem.

• Peripheral Chemoreceptors: Located in the aortic arch and carotid arteries.

• CO2 is the most potent regulator; increased CO2 triggers hyperventilation, while decreased CO2 triggers hypoventilation.

Respiratory Volumes and Capacities

Spirometry and Lung Volumes

Spirometry measures different volumes of air moved during breathing:

• Tidal Volume (TV): Volume of air inhaled or exhaled during normal breathing (~500 mL).

• Inspiratory Reserve Volume (IRV): Maximum volume inhaled above tidal volume.

• Expiratory Reserve Volume (ERV): Maximum volume exhaled beyond tidal volume.

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

• Total Lung Capacity (TLC): Maximum air in lungs after maximal inspiration (TLC = TV + IRV + ERV + RV).

• Vital Capacity (VC): Maximum air expired after maximal inspiration (VC = TV + IRV + ERV).

• Inspiratory Capacity (IC): Maximum air inspired after normal expiration (IC = TV + IRV).

• Functional Residual Capacity (FRC): Air remaining after normal expiration (FRC = ERV + RV).

Measurement	Adult Male Avg	Adult Female Avg	Description
Tidal Volume (TV)	500 mL	500 mL	Air inhaled/exhaled with each breath at rest
Inspiratory Reserve Volume (IRV)	3100 mL	1900 mL	Max air forcefully inhaled after normal inspiration
Expiratory Reserve Volume (ERV)	1200 mL	700 mL	Max air forcefully exhaled after normal expiration
Residual Volume (RV)	1200 mL	1100 mL	Air remaining after forced expiration
Total Lung Capacity (TLC)	6000 mL	4200 mL	Max air in lungs after max inspiration
Vital Capacity (VC)	4800 mL	3100 mL	Max air expired after max inspiration
Inspiratory Capacity (IC)	3600 mL	2400 mL	Max air inspired after normal expiration
Functional Residual Capacity (FRC)	2400 mL	1800 mL	Air remaining after normal expiration

Example: During exercise, tidal volume and respiratory rate increase to meet the body's higher oxygen demand and CO2 removal.

Additional info: These values are averages and can vary based on age, sex, body size, and physical conditioning.