The Respiratory System: Structure, Function, and Clinical Implications

The Respiratory System

Overview and Primary Functions

The respiratory system is essential for gas exchange, supplying oxygen (O2) to body cells and removing carbon dioxide (CO2). It also plays roles in olfaction and speech. The respiratory and cardiovascular systems work together to ensure efficient gas transport and exchange.

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

• Pulmonary gas exchange: Exchange of O2 and CO2 between lungs and blood.

• Tissue gas exchange: Exchange of O2 and CO2 between blood and tissues (handled by the cardiovascular system).

• Protection: Respiratory surfaces are protected by mucous membranes, cilia, and immune defenses against pathogens and debris.

Additional info: The respiratory system also helps regulate blood pH and produces sounds for communication.

Organs of the Respiratory Tract

Upper and Lower Respiratory Tract

The respiratory tract is divided into upper and lower regions, each with specialized structures and functions.

• Upper respiratory tract: Nose, paranasal sinuses, pharynx

• Lower respiratory tract: Larynx, trachea, bronchi (and branches), lungs, alveoli

Nose and Nasal Cavity

The nose is the only externally visible part of the respiratory system. It provides an airway, moistens and warms air, filters particles, serves as a resonating chamber for speech, and houses olfactory receptors. The nasal cavity is divided by the nasal septum and lined with mucosa that traps debris and pathogens.

• Nasal vestibule: Lined with vibrissae (hairs) to filter coarse particles.

• Olfactory mucosa: Contains olfactory epithelium for smell.

• Respiratory mucosa: Contains cilia and mucus-secreting cells with lysozyme and defensins.

• Sneeze reflex: Triggered by irritants to expel them from the nasal cavity.

Pharynx

The pharynx (throat) connects the nasal cavity to the larynx and esophagus and is divided into three regions:

• Nasopharynx: Airway posterior to the nasal cavity; closes during swallowing.

• Oropharynx: Passageway for food and air; contains palatine and lingual tonsils.

• Laryngopharynx: Passageway for food and air; continuous with the esophagus.

Larynx (Voice Box)

The larynx provides an open airway, routes air and food, and houses the vocal folds for sound production. It is composed of several cartilages, including the thyroid (Adam's apple), cricoid, and epiglottis. The epiglottis covers the laryngeal inlet during swallowing.

• Glottis: Opening between vocal folds.

• Valsalva's maneuver: Closing the glottis increases intra-abdominal pressure for activities like lifting.

Trachea and Bronchial Tree

The trachea (windpipe) descends from the larynx and divides into right and left main bronchi. Its wall contains C-shaped cartilage rings for support. The bronchial tree undergoes multiple branchings, ending in bronchioles and terminal bronchioles.

• Conducting zone: All airways from the nose to terminal bronchioles; transports, cleans, warms, and humidifies air.

• Respiratory zone: Includes respiratory bronchioles, alveolar ducts, and alveoli; site of gas exchange.

Lungs and Alveoli

The lungs are divided into lobes and contain millions of alveoli, which are the primary sites of gas exchange. The respiratory membrane is extremely thin, allowing efficient diffusion of gases. Type II alveolar cells secrete surfactant to reduce surface tension, and alveolar macrophages keep the surfaces clean.

• Alveolar pores: Connect adjacent alveoli to equalize air pressure and provide alternate routes for airflow.

• Pleurae: Double-layered serous membranes (parietal and visceral) that reduce friction and compartmentalize the lungs.

Pulmonary and Bronchial Circulation

The lungs are supplied by two circulations:

• Pulmonary circulation: Pulmonary arteries carry deoxygenated blood to the lungs; pulmonary veins return oxygenated blood to the heart.

• Bronchial circulation: Bronchial arteries supply oxygenated blood to lung tissue (except alveoli).

Innervation is by both sympathetic (bronchodilation) and parasympathetic (bronchoconstriction) fibers.

Respiratory Physiology

External vs. Internal Respiration

External respiration is the exchange of gases between alveoli and pulmonary capillaries. Internal respiration is the exchange of gases between systemic capillaries and tissues. Both processes rely on diffusion gradients and membrane properties.

• Pulmonary ventilation: Consists of inspiration (air in) and expiration (air out).

• Pressure relationships: Intrapulmonary pressure (Ppul) and intrapleural pressure (Pip) fluctuate during breathing. Transpulmonary pressure (Ppul – Pip) keeps lungs inflated.

Physical Principles of Gas Exchange

Gas exchange is governed by the principles of diffusion, partial pressure gradients, and membrane properties. Boyle's law describes the inverse relationship between pressure and volume of a gas:

$P_1 V_1 = P_2 V_2$

• Airway resistance: Greatest in medium-sized bronchi; can be increased by constriction (e.g., asthma).

• Alveolar surface tension: Reduced by surfactant; lack of surfactant leads to infant respiratory distress syndrome (IRDS).

• Lung compliance: Determined by lung tissue distensibility and alveolar surface tension; decreased by fibrosis or reduced surfactant.

Pulmonary Volumes and Capacities

Several lung volumes are measured to assess respiratory function:

• Tidal volume (TV): Air moved per breath (~500 mL).

• Inspiratory reserve volume (IRV): Extra air inhaled beyond TV (2100–3200 mL).

• Expiratory reserve volume (ERV): Extra air exhaled beyond TV (1000–1200 mL).

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

Gas Transport in Blood

Oxygen is transported in blood dissolved in plasma (1.5%) and bound to hemoglobin (98.5%). Hemoglobin's affinity for oxygen is influenced by partial pressure of O2, temperature, pH, and PCO2. Carbon dioxide is transported dissolved in plasma, as bicarbonate ions, and bound to hemoglobin.

• Hypoxia: Inadequate O2 delivery; can be caused by anemia, ischemia, or carbon monoxide poisoning.

Control of Respiration

Neural and Chemical Regulation

Respiratory rate and depth are controlled by centers in the medulla and pons, and are influenced by chemoreceptors (sensitive to CO2, O2, and pH), higher brain centers, and reflexes.

• Medullary respiratory centers: Ventral and dorsal groups generate and modify rhythm.

• Hyperventilation: Excessive ventilation lowers CO2 (hypocapnia), causing dizziness or fainting.

Clinical Implications

Common Disorders

• Rhinitis: Inflammation of nasal mucosa, often spreading to sinuses (sinusitis).

• Laryngitis: Inflammation of vocal folds, causing hoarseness.

• Pleurisy: Inflammation of pleurae, causing pain and possibly pleural effusion.

• Atelectasis: Lung collapse due to airway obstruction or pleural cavity compromise.

• Infant respiratory distress syndrome (IRDS): Due to lack of surfactant in premature infants.

• Asthma: Bronchoconstriction can severely limit airflow; treated with bronchodilators like epinephrine.

Summary Table: Key Structures and Functions of the Respiratory System

Structure	Function
Nose/Nasal cavity	Airway, filters, warms, moistens air, olfaction
Pharynx	Passage for air and food, immune defense (tonsils)
Larynx	Airway, routes food/air, voice production
Trachea	Conducts air to bronchi, filters particles
Bronchi/Bronchioles	Air distribution, resistance regulation
Alveoli	Site of gas exchange
Pleurae	Reduce friction, compartmentalize lungs