The Respiratory System

2Upper respiratory tract

Nose & nasal cavity

Nasal conchae increase surface turbulence so particles stick to mucus; respiratory mucosa (pseudostratified ciliated columnar epithelium with goblet cells) traps debris; rich capillaries warm the air; olfactory epithelium in the roof. Paranasal sinuses lighten the skull and warm/moisten air.

Nasopharynx

Air only; pharyngeal tonsil (adenoids); openings of the pharyngotympanic (auditory) tubes. The soft palate and uvula close it off during swallowing.

Oropharynx

Air and food; palatine and lingual tonsils; stratified squamous epithelium.

Laryngopharynx

Air and food; continuous with the esophagus and larynx.

The larynx (voice box) provides an open airway, routes air and food (the epiglottis covers the laryngeal inlet during swallowing) and produces voice. Its framework is nine cartilages, including the thyroid cartilage (“Adam's apple” — laryngeal prominence), cricoid cartilage (ring below it), and the arytenoid cartilages that anchor the vocal folds (true vocal cords) — vibrating folds that produce sound. The vestibular folds (false vocal cords) above them help close the glottis.

3Trachea, bronchi & lungs

The trachea runs from the larynx into the mediastinum. Its wall: mucosa (ciliated pseudostratified epithelium sweeping mucus toward the pharynx — the mucociliary escalator), submucosa with seromucous glands, and 16–20 C-shaped rings of hyaline cartilage (keep it open; the open back lets the esophagus expand). The trachealis smooth muscle closes the C. It ends at the carina, splitting into the right and left main (primary) bronchi.

The bronchial tree branches ~23 times: main bronchi → lobar (secondary) → segmental (tertiary) → bronchioles (< 1 mm) → terminal bronchioles. As branches get smaller, cartilage disappears (replaced by smooth muscle), epithelium becomes cuboidal, and cilia and mucus decrease.

Type I alveolar cells

Thin squamous cells forming most of the alveolar wall — with the capillary endothelium and fused basement membranes they make the respiratory membrane (~0.5 µm thick) where gas diffuses.

Type II alveolar cells

Cuboidal cells that secrete surfactant (reduces surface tension so alveoli don't collapse) and antimicrobial proteins.

Alveolar macrophages

“Dust cells” — remove debris and microbes.

Alveolar pores

Connect adjacent alveoli to equalize pressure and offer alternate routes.

Lungs: apex near the clavicle, base on the diaphragm, hilum where vessels and bronchi enter. The right lung has 3 lobes (superior, middle, inferior; horizontal and oblique fissures); the left lung has 2 lobes and a cardiac notch for the heart. Each lobe is divided into bronchopulmonary segments (can be removed surgically). Lungs receive two circulations: pulmonary (for gas exchange) and bronchial (nourishes lung tissue).

Pleurae: the parietal pleura lines the thoracic wall; the visceral pleura covers the lung. Pleural fluid in the pleural cavity lubricates and creates surface tension that keeps the lungs stuck to the thoracic wall.

4Mechanics of breathing

Boyle's law: at constant temperature, the pressure of a gas varies inversely with its volume (P₁V₁ = P₂V₂). Change the volume of the thoracic cavity, and pressure — then airflow — follows.

Quiet expiration is passive — inspiratory muscles relax and the elastic lungs recoil → thoracic volume decreases → pressure rises above atmospheric → air flows out. Forced expiration is active: abdominal wall muscles and internal intercostals.

Airway resistance

Mostly in medium-sized bronchi; bronchoconstriction (asthma, histamine) raises it; epinephrine dilates.

Alveolar surface tension

Water molecules pull inward; surfactant reduces it. Premature babies lacking surfactant develop infant respiratory distress syndrome (IRDS).

Lung compliance

Ease of lung expansion; reduced by fibrosis, lack of surfactant, decreased chest wall flexibility.

5Respiratory volumes & capacities

Anatomical dead space (~150 mL) — air that stays in the conducting zone and never reaches alveoli. Alveolar ventilation rate (AVR) = frequency × (TV − dead space) — the best measure of effective ventilation. Slow, deep breaths ventilate alveoli better than rapid, shallow ones.

6Gas exchange

Dalton's law: each gas in a mixture exerts its own partial pressure (atmospheric PO₂ ≈ 160 mmHg). Henry's law: the amount of gas dissolving in a liquid is proportional to its partial pressure and solubility (CO₂ is ~20× more soluble than O₂).

External respiration: O₂ diffuses alveoli → blood (104 → 40 gradient); CO₂ diffuses blood → alveoli (45 → 40, small gradient but CO₂ is very soluble). Efficiency depends on partial-pressure gradients, membrane thickness and surface area, and ventilation–perfusion coupling (airflow and blood flow matched: low O₂ in an area constricts its arterioles, directing blood to better-ventilated alveoli).

Internal respiration: in tissues, O₂ diffuses blood → cells and CO₂ diffuses cells → blood.

7Gas transport

Oxygen: 98.5% is bound to hemoglobin (oxyhemoglobin, HbO₂); 1.5% dissolved in plasma. Each Hb binds up to 4 O₂; binding is cooperative — once one O₂ binds, others bind more easily, which gives the S-shaped dissociation curve.

Hb is ~98% saturated in arterial blood and still ~75% saturated after passing through resting tissues — a large reserve for exercise. Factors that shift the curve right (Hb releases O₂ more readily): ↑ temperature, ↑ H⁺ (↓ pH) and ↑ PCO₂ (the Bohr effect), and ↑ BPG — all characteristics of active tissues.

In RBCs, carbonic anhydrase rapidly converts CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. HCO₃⁻ moves out into plasma in exchange for Cl⁻ (chloride shift); H⁺ binds hemoglobin. In the lungs the reaction runs in reverse to release CO₂. The Haldane effect: the less oxygenated the blood, the more CO₂ it can carry.

8Control of breathing

Ventral respiratory group (VRG) — medulla

Rhythm-generating center: sets the basic eupnea rhythm (12–15 breaths/min) by firing the phrenic and intercostal nerves.

Dorsal respiratory group (DRG) — medulla

Integrates input from stretch receptors and chemoreceptors and modifies the VRG.

Pontine respiratory centers — pons

Smooth the transitions between inspiration and expiration (fine-tune rhythm for speech, sleep, exercise).