The Respiratory System: Structure, Function, and Physiology

The Respiratory System: An Overview

Introduction to the Respiratory System

The respiratory system is essential for supplying 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 cellular ATP production.

• Carbon dioxide must be removed to prevent toxic accumulation.

• Gas exchange occurs via diffusion across lung surfaces and is transported by the blood.

Major Organs and Divisions

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

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

• Function: Upper system filters, warms, and humidifies air; lower system conducts air, produces sound, and enables gas exchange.

Functional Divisions

• Conducting portion: Nasal cavity to terminal bronchioles (air conduction only)

• Respiratory portion: Respiratory bronchioles and alveoli (site of gas exchange)

Histology of the Respiratory Tract

Respiratory Mucosa

The respiratory mucosa lines the conducting portion and consists of an epithelial layer and a deeper lamina propria. It contains mucous glands and, in some regions, smooth muscle.

Types of Respiratory Epithelium

• Pseudostratified ciliated columnar epithelium: 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

• Nasal hairs: Filter large particles

• Mucous/goblet cells: Trap debris and pathogens

• Cilia: Sweep mucus toward the pharynx

• Alveolar macrophages: Engulf small particles in the lungs

Anatomy of the Upper Respiratory Tract

Nose and Nasal Cavity

• Lined with pseudostratified ciliated columnar epithelium

• Functions: Warm, humidify, filter air; house olfactory receptors

• Bony projections: Superior, middle, and inferior nasal conchae create turbulence

Pharynx

• Nasopharynx: Pseudostratified ciliated columnar epithelium; contains pharyngeal tonsil

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

• Laryngopharynx: Stratified squamous epithelium

Anatomy of the Lower Respiratory Tract

Larynx

• Cartilaginous structure surrounding the glottis

• Main cartilages: Thyroid, cricoid, epiglottis

• Functions: Protect airway, produce sound, prevent food entry during swallowing

Trachea

• Conducts air to the bronchi

• Supported by C-shaped hyaline cartilage rings

• Carina: Contains sensory receptors for cough reflex

Bronchial Tree

• Primary bronchi branch into secondary (lobar) and 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: Air-filled sacs where gas exchange occurs

• Respiratory membrane: Formed by alveolar and capillary walls and their fused basement membranes

• Type I alveolar cells: Simple squamous cells for diffusion

• Type II alveolar cells: Produce surfactant to reduce surface tension

• Alveolar macrophages: Phagocytose debris

The Lungs and Pleurae

Lung Structure

• Lungs are divided into lobes and further into pulmonary lobules by connective tissue partitions (trabeculae and interlobular septa)

• Each terminal bronchiole supplies a pulmonary lobule

• Respiratory bronchioles connect to alveoli via alveolar ducts and sacs

Pleural Cavities and Membranes

• Each lung is enclosed in a pleural cavity lined by a double-layered serous membrane (pleura)

• Parietal pleura: Lines thoracic wall

• Visceral pleura: Covers lung surface

• Pleural fluid lubricates and reduces friction, maintaining surface tension

Mechanics of Breathing

Pressure-Volume Relationship (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:

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

As the thoracic cavity volume increases, pressure decreases, allowing air to flow in (inhalation). As volume decreases, pressure increases, pushing air out (exhalation).

Pulmonary Ventilation

• Driven by pressure gradients between atmospheric and intrapulmonary pressures

• Intrapleural pressure remains below atmospheric pressure, keeping lungs inflated

• Tidal volume: Amount of air moved in and out during a single respiratory cycle

Respiratory Muscles

• Diaphragm: Main muscle for inhalation

• External intercostals: Assist in elevating the ribcage

• Accessory muscles: Used during forced breathing

• Normal exhalation is passive (elastic recoil); forced exhalation uses accessory muscles

Gas Exchange and Transport

Pulmonary Gas Exchange (External Respiration)

• Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli

• Occurs across the thin respiratory membrane

Tissue Gas Exchange (Internal Respiration)

• Oxygen diffuses from blood to tissues; carbon dioxide diffuses from tissues to blood

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

• Transported in three forms:

  • Dissolved in plasma (7–10%)

  • Bound to hemoglobin as carbaminohemoglobin (20%)

  • As bicarbonate ions in plasma (70%)

• Conversion to bicarbonate is catalyzed by carbonic anhydrase:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+$

Control of Respiration

Neural Control Centers

• Medulla oblongata: Dorsal (DRG) and ventral (VRG) respiratory groups

• Pons: Pontine respiratory group (pneumotaxic center)

• Regulate rate and depth of breathing in response to blood gas levels and pH

Chemoreceptor Reflexes

• Central and peripheral chemoreceptors monitor CO2, O2, and pH

• Adjust ventilation to maintain homeostasis

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