Memory tricks for airways and gas exchange

Air enters through the nasal cavity (filtered, warmed, humidified) → nasopharynx → oropharynx → laryngopharynx → larynx (contains vocal cords and epiglottis) → trachea (16-20 C-shaped cartilage rings) → primary bronchi (right and left) → secondary bronchi (lobar) → tertiary bronchi (segmental) → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli. Gas exchange only occurs in alveoli — everything above is conducting zone only.

The right lung has 3 lobes (upper, middle, lower) separated by the horizontal and oblique fissures. The left lung has only 2 lobes (upper and lower) separated by the oblique fissure — because the heart occupies space on the left side. The left lung has a cardiac notch and the lingula (corresponds to the right middle lobe). The right lung is larger and heavier. Memory trick: Right has 3 letters in "right" — 3 lobes. Left has 4 letters — but only 2 lobes (the heart took one!).

Gas exchange (external respiration) occurs at the alveolar-capillary membrane — only 0.5 micrometers thick. Gases move from high to low partial pressure. O2 partial pressure in alveoli (104 mmHg) is higher than in capillary blood (40 mmHg) — so O2 diffuses into blood. CO2 partial pressure in capillary blood (45 mmHg) is higher than in alveoli (40 mmHg) — so CO2 diffuses into alveoli. CO2 diffuses 20× faster than O2 across membranes.

The diaphragm does 75% of the work during quiet breathing — it contracts and flattens, increasing thoracic volume. External intercostals elevate ribs during inspiration. During forced inspiration, accessory muscles kick in: SCM (sternocleidomastoid), scalenes, pectoralis minor. Forced expiration uses internal intercostals and abdominals — quiet expiration is passive (elastic recoil). Using accessory muscles at rest is a sign of respiratory distress.

Tidal Volume (TV): air moved in one normal breath = ~500 mL. Inspiratory Reserve Volume (IRV): extra air you can inhale after normal breath = ~3000 mL. Expiratory Reserve Volume (ERV): extra air you can exhale after normal breath = ~1200 mL. Residual Volume (RV): air remaining after maximum exhalation = ~1200 mL (cannot be measured by spirometry). Vital Capacity = TV + IRV + ERV = ~4700 mL. Total Lung Capacity = VC + RV = ~5900 mL.

Obstructive diseases narrow the airways — air gets trapped. FEV1/FVC ratio is LOW (can't exhale fast enough). Examples: asthma, COPD, emphysema, chronic bronchitis. Restrictive diseases stiffen the lungs or chest wall — lungs can't expand fully. Total lung capacity is LOW. FEV1/FVC ratio is normal or HIGH. Examples: pulmonary fibrosis, sarcoidosis, pneumonia, obesity. Key test: FEV1/FVC ratio. Below 0.7 = obstructive.

Visceral pleura covers the lung surface directly. Parietal pleura lines the thoracic wall, diaphragm, and mediastinum. Between them is the pleural cavity containing a thin film of pleural fluid — reduces friction during breathing. Pneumothorax = air enters pleural cavity — lung collapses. Pleural effusion = excess fluid in pleural cavity. Tension pneumothorax = one-way valve effect — air accumulates, shifts mediastinum — medical emergency.

CO2 dissolves in blood to form carbonic acid (H2CO3), which dissociates into H+ and bicarbonate. More CO2 = more H+ = lower pH (acidosis). The respiratory system is the fastest pH regulator — changes in breathing rate change CO2 levels within minutes. Hyperventilation = blows off CO2 = respiratory alkalosis. Hypoventilation = retains CO2 = respiratory acidosis. Chemoreceptors detect CO2/H+ and signal the medulla to adjust breathing rate.

Thyroid cartilage: the largest, forms the Adam's apple (laryngeal prominence). Cricoid cartilage: only complete ring of cartilage in the airway — landmark for cricothyrotomy. Arytenoid cartilages: paired, control vocal cord tension and position. Epiglottis: elastic cartilage — flips down to cover larynx during swallowing, preventing aspiration. The cricothyroid membrane between thyroid and cricoid is where emergency airway access is obtained.