The Respiratory System: Structure, Function, and Physiology

The Respiratory System: An Overview

Introduction to the Respiratory System

The respiratory system is essential for providing oxygen to body cells and removing carbon dioxide, a waste product of metabolism. This system works closely with the cardiovascular system to ensure efficient gas exchange and transport throughout the body.

• Oxygen is required for ATP production in cells.

• Carbon dioxide must be removed to prevent acid-base imbalance.

• Gas exchange occurs via diffusion across lung surfaces.

Anatomy of the Respiratory System

Major Organs and Divisions

The respiratory system is divided into upper and lower regions, each with specific structures and functions.

• Upper respiratory system: Nose, nasal cavity, paranasal sinuses, pharynx

• Lower respiratory system: Larynx, trachea, bronchi, bronchioles, alveoli

• Respiratory tract: Conducting portion (air movement) and respiratory portion (gas exchange)

Functions of the Respiratory System

• Provides surface area for gas exchange

• Moves air to and from exchange surfaces

• Protects respiratory surfaces from pathogens and debris

• Produces sounds (phonation)

• Facilitates olfaction (smell)

• Maintains acid-base balance

Histology of the Respiratory Tract

Respiratory Mucosa and Epithelium

The respiratory mucosa lines the conducting portion of the tract and consists of an epithelial layer and a deep areolar layer (lamina propria). The type of epithelium changes along the tract:

• Pseudostratified ciliated columnar epithelium with goblet cells: Nasal cavity, nasopharynx, superior lower respiratory tract

• Stratified squamous epithelium: Inferior pharynx, oropharynx, laryngopharynx

• Cuboidal epithelium: Smaller bronchioles

• Simple squamous epithelium: Alveoli (site of gas exchange)

Respiratory Defense System

• Filtration by nasal hairs removes large particles.

• Mucous/goblet cells and glands produce mucus to trap debris.

• Cilia sweep mucus toward the pharynx for expulsion or swallowing.

• Alveolar macrophages engulf small particles in the lungs.

Upper Respiratory Tract

Nose and Nasal Cavity

The nose and nasal cavity filter, warm, and humidify incoming air. They are lined with pseudostratified ciliated columnar epithelium and contain bony projections called nasal conchae that create turbulence, enhancing filtration.

Pharynx

The pharynx is divided into three regions:

• Nasopharynx: Pseudostratified ciliated columnar epithelium; contains pharyngeal tonsil

• Oropharynx: Stratified squamous epithelium; contains palatine and lingual tonsils

• Laryngopharynx: Stratified squamous epithelium

Larynx

The larynx is a cartilaginous structure that protects the glottis and vocal cords. It is involved in sound production and prevents food from entering the lower respiratory tract during swallowing.

• Main cartilages: Thyroid, cricoid, epiglottis

• During swallowing, the epiglottis folds over the glottis

Lower Respiratory Tract

Trachea

The trachea is supported by C-shaped rings of hyaline cartilage, keeping the airway open while allowing the esophagus to expand during swallowing. The carina at the base of the trachea triggers coughing if foreign material is detected.

Bronchial Tree

The bronchial tree consists of branching airways:

• Primary bronchi → Secondary (lobar) bronchi → Tertiary (segmental) bronchi

• Bronchioles: Smallest airways, lack cartilage, lined by simple cuboidal epithelium

• Terminal bronchioles: Final part of conducting airways

Alveoli and the Respiratory Membrane

Alveoli are tiny air sacs where gas exchange occurs. The respiratory membrane consists of alveolar epithelium, capillary endothelium, and fused basement membranes, allowing efficient diffusion of gases.

Lungs and Pleurae

The lungs are divided into lobes and further into pulmonary lobules by connective tissue partitions. Each terminal bronchiole supplies a lobule, which contains respiratory bronchioles, alveolar ducts, and alveolar sacs. The lungs are surrounded by pleural membranes (parietal and visceral) with pleural fluid reducing friction during breathing.

Alveolar Epithelium and Surfactant

• Type I alveolar cells: Simple squamous cells for gas diffusion

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

• Alveolar macrophages: Phagocytize debris and pathogens

Physiology of Respiration

Processes of Respiration

• Pulmonary ventilation: Movement of air in and out of lungs

• Pulmonary gas exchange: Exchange of gases between alveoli and blood

• Gas transport: Movement of gases in the blood

• Tissue gas exchange: Exchange of gases between blood and tissues

Pressure-Volume Relationships (Boyle's Law)

Boyle’s Law states that the pressure of a gas is inversely proportional to its volume, provided the number of gas molecules is constant:

• Decreasing container size increases pressure

• Increasing container size decreases pressure

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

Mechanics of Breathing

Breathing involves changes in thoracic cavity volume, creating pressure gradients that drive airflow:

• Inhalation: Diaphragm contracts, ribcage elevates, thoracic volume increases, pressure decreases, air flows in

• Exhalation: Diaphragm relaxes, ribcage lowers, thoracic volume decreases, pressure increases, air flows out

Respiratory Pressures

• Intrapulmonary pressure: Pressure within alveoli, fluctuates with breathing

• Intrapleural pressure: Pressure in pleural cavity, always less than atmospheric pressure

• Tidal volume: Amount of air moved per breath

Respiratory Muscles

• Diaphragm and external intercostals: Main muscles for quiet inhalation

• Accessory muscles: Used during forced breathing

• Normal exhalation is passive; forced exhalation uses accessory muscles

Physical Factors Influencing Ventilation

• Airway resistance: Increases with constriction or obstruction

• Alveolar surface tension: Reduced by surfactant to prevent collapse

• Pulmonary compliance: Ability of lungs and chest wall to stretch

Gas Exchange and Transport

Pulmonary and Tissue Gas Exchange

Gas exchange occurs across the respiratory membrane in the lungs (external respiration) and between blood and tissues (internal respiration). Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.

Oxygen Transport

• Most oxygen is transported bound to hemoglobin in erythrocytes

• Oxygen loading and unloading depend on partial pressures and affinity of hemoglobin

Carbon Dioxide Transport

• Dissolved in plasma (7–10%)

• Bound to hemoglobin as carbaminohemoglobin (20%)

• As bicarbonate ions in plasma (70%)

Control of Respiration

Neural Control

• Medullary respiratory centers: Dorsal (DRG) and ventral (VRG) respiratory groups

• Pontine respiratory group: Modulates medullary centers

• Regulate rate and depth of breathing based on blood gas levels

Chemoreceptor and Reflex Control

• Central and peripheral chemoreceptors: Monitor CO2, O2, and pH

• Stretch receptors: Respond to lung inflation

• Irritant receptors: Trigger protective reflexes

Noninfectious Respiratory Diseases

Restrictive Lung Diseases

• Decreased pulmonary compliance

• Reduced inspiratory capacity, vital capacity, and total lung capacity

Obstructive Lung Diseases

• Increased airway resistance, decreased efficiency of expiration

• Examples: Chronic Obstructive Pulmonary Disease (COPD), emphysema, asthma

• Asthma: Hyperresponsive airways, bronchoconstriction, inflammation, increased mucus