The Respiratory System

Five Basic Functions

Two Divisions of the Respiratory Tract

The Respiratory Mucosa

The respiratory mucosa lines the conducting portion. It has two layers:

• Respiratory epithelium — ciliated columnar epithelium with many mucous cells. Cilia beat in coordinated waves, sweeping mucus and trapped debris toward the pharynx for swallowing.
• Lamina propria — underlying areolar connective tissue with mucous glands that secrete additional mucus onto the surface.

Once swallowed, trapped pathogens are destroyed by the acids and enzymes of the stomach.

Air Conditioning in the Conducting Passageways

By the time air reaches the alveoli, the conducting passageways have:

Role of the Nasal Conchae

The superior, middle, and inferior nasal conchae project from the lateral walls of the nasal cavity. They create turbulent airflow — air eddies and swirls between adjacent conchae, increasing contact time with the warm, moist, mucus-coated mucosa for better filtration, warming, and humidification.

Additional Mucus Sources

• Paranasal sinuses (frontal, sphenoid, ethmoid, maxillary, palatine) — produce mucus that helps flush the nasal cavity
• Tears flowing through the nasolacrimal duct into the nasal cavity

The Nose and Nasal Cavity

Air enters through the paired external nares (nostrils). The nasal vestibule is the entrance — coarse hairs screen out large particles. The nasal septum divides the cavity (anterior = hyaline cartilage; posterior = vomer + ethmoid bone). The hard palate (palatine + maxillary bones) forms the floor, separating oral and nasal cavities. The soft palate extends posteriorly, underlies the nasopharynx. The cavity opens into the nasopharynx at the internal nares.

The Pharynx — Three Subdivisions

The Larynx

Surrounds and protects the glottis (narrow opening to the airway). Nine cartilages total:

Vocal cords: Upper pair = false vocal cords (inelastic, protective). Lower pair = true vocal cords (elastic ligaments that vibrate to produce sound). Pitch depends on diameter, length, and tension. Male cords are thicker/longer after puberty → lower pitch. The coughing reflex is triggered when food/liquid touches the vocal cords — glottis closes, chest muscles compress lungs, glottis opens suddenly to blast material out.

The Trachea

Tough, flexible tube (~2.5 cm diameter, ~11 cm long). Begins at cricoid cartilage (C6) and ends at T5 where it branches into the right and left primary bronchi.

The Bronchi and Bronchial Tree

The trachea branches into right and left primary bronchi. The right bronchus is larger in diameter and descends at a steeper angle → aspirated foreign objects usually enter the right bronchus. Primary → secondary bronchi (enter lung lobes) → tertiary bronchi (9–10 per lung, each supplying a bronchopulmonary segment) → smaller bronchi. As branches get smaller: cartilage decreases, smooth muscle increases. At ~1 mm diameter with no cartilage = bronchiole.

Bronchioles

Walls dominated by smooth muscle under autonomic control. Sympathetic → bronchodilation. Parasympathetic → bronchoconstriction. Extreme bronchoconstriction = asthma attack. Bronchioles are to the respiratory system what arterioles are to the cardiovascular system — both regulate resistance and distribution.

Terminal bronchioles (0.3–0.5 mm) each supply a single lobule. Within a lobule: terminal bronchiole → respiratory bronchioles → alveolar ducts → alveolar sacs → individual alveoli.

Alveoli and the Respiratory Membrane

Each lung contains ~150 million alveoli, providing ~140 m² of exchange surface.

The respiratory membrane where gas exchange occurs has three layers: (1) squamous alveolar epithelial cells, (2) capillary endothelial cells, (3) their fused basement membranes. Total thickness averages only 0.5 μm. Both O₂ and CO₂ are lipid soluble → rapid diffusion.

The Lungs and Pleural Cavities

Parietal pleura lines the body wall, diaphragm, and mediastinum. Visceral pleura covers the lung surfaces. Pleural fluid between them reduces friction and creates a fluid bond holding the lung against the chest wall. Pneumothorax = air in pleural cavity → broken fluid bond → atelectasis (collapsed lung). Hemothorax = blood in pleural cavity.

Boyle's Law and Pulmonary Ventilation

Air flows from higher pressure to lower pressure. In a closed, flexible container: as volume increases, pressure decreases; as volume decreases, pressure increases. The lungs are held against the chest wall by the pleural fluid bond, so thoracic cavity volume changes directly affect lung volume.

At rest between breaths: Pᵢ = Pₒ → no air movement.

Compliance

Compliance = how easily the lungs expand. Higher compliance = easier to fill/empty (e.g., emphysema). Lower compliance = more force required (e.g., respiratory distress syndrome, arthritis of rib joints). Normal resting ventilation uses only 3–5% of resting energy. Reduced compliance dramatically increases this cost.

Partial Pressures

Each gas in a mixture contributes to total pressure proportionally to its abundance. The pressure from a single gas = its partial pressure (P). At sea level: total atmospheric pressure = 760 mm Hg.

Alveolar Air vs. Atmospheric Air

Alveolar air differs from atmospheric air because inhaled air is warmed, humidified, and mixed with residual air from the previous breath. Result: alveolar P_O₂ ≈ 100 mm Hg (vs 159 atmospheric); alveolar P_CO₂ ≈ 40 mm Hg (vs 0.3 atmospheric).

Gas Exchange — Partial Pressure Gradients

Quiet Breathing

Inhalation is active; exhalation is passive.

Forced Breathing

Both inhalation and exhalation are active.

Oxygen Transport

98.5% of O₂ is bound to hemoglobin (Hb) — specifically to the iron ions in heme units. Only 1.5% is dissolved in plasma. Reaction: Hb + O₂ ⇌ HbO₂ (completely reversible).

Three factors promote oxygen RELEASE from hemoglobin in active tissues:

Carbon Dioxide Transport — Three Mechanisms

The chloride shift: As HCO₃⁻ leaves the RBC, Cl⁻ enters from plasma to maintain electrical neutrality — one anion traded for another. All three mechanisms are completely reversible — at the lungs, reactions run backward to release CO₂ for exhalation.

CO₂ — The Primary Driver

Under normal conditions, arterial PCO₂ is the single most powerful stimulus for respiratory regulation. A small increase in PCO₂ powerfully stimulates chemoreceptors. Arterial PO₂ rarely drops enough to activate O₂ receptors under normal conditions.

All Factors That Influence Respiration

Respiratory Centers in the Brain

Three pairs of nuclei in the reticular formation of the pons and medulla oblongata:

Mechanoreceptor Reflexes

Baroreceptor effects: Carotid and aortic baroreceptors affect respiratory centers via CN IX and X. Falling BP → increased respiratory rate. Rising BP → decreased rate.

Chemoreceptor Reflexes

• Peripheral chemoreceptors — carotid bodies (adjacent to carotid sinus) and aortic bodies (near aortic arch). Sensitive to pH, PCO₂, and PO₂ in arterial blood. Signals via CN IX and X.
• Central chemoreceptors — in the medulla oblongata. Respond to pH and PCO₂ in cerebrospinal fluid. CO₂ crosses the blood-brain barrier rapidly.
• CO₂ levels dominate over O₂ levels under normal conditions. Also sensitive to pH — lactic acid after exercise drops pH, which stimulates respiratory activity.

Higher Center Control

• Cerebral cortex — voluntary control (talking, singing, deliberate breath-holding)
• Limbic system — involuntary changes during emotional states (rage, eating, sexual arousal)

Changes at Birth

Before delivery: pulmonary arterial resistance is high (vessels collapsed), rib cage is compressed, lungs contain fluid and no air.

Aging and the Respiratory System

Together, chest wall stiffening and emphysema contribute to reduced exercise performance with increasing age.

The respiratory system provides oxygen to, and removes carbon dioxide from, every other organ system. It has extensive structural and functional connections to the cardiovascular system.