Respiratory Anatomy & Mechanics

Core concept

What you'll do

Move through six concept slides
Three slides have a drawable canvas
Toolbar: pen color, size, eraser, undo, add image, save
Drop in your own anatomy images to label them

Conducting Zone + Respiratory Zone

Upper airways

Nose + nasal cavity

Pharynx

Larynx

Lower airways

Trachea

Bronchi → bronchioles

Alveoli (~480 million)

Anatomy

Airway Map

Top to bottom

Nasal cavity: warms, filters, humidifies; turbinates ↑ surface area
Pharynx: shared with digestion (nasopharynx, oropharynx, laryngopharynx)
Larynx: voice box, has vocal folds; epiglottis closes during swallowing
Trachea: cartilage rings keep open
Primary bronchi → secondary → tertiary → bronchioles
Terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs

Takeaway: the right primary bronchus is wider, shorter, and more vertical than the left. Aspirated objects usually go right. Important for emergency intubation positioning.

Lungs

Lung Anatomy

Lobes + pleura

Right lung: 3 lobes (upper, middle, lower)
Left lung: 2 lobes (upper, lower) + cardiac notch (heart space)
Hilum: where vessels, bronchi, nerves enter
Visceral pleura hugs lung; parietal pleura lines thoracic wall
Pleural cavity with serous fluid: slick coupling, allows lung to follow chest wall
Negative intrapleural pressure keeps lungs inflated

Draw: an anterior view of both lungs in the thorax. R lung with 3 lobes labeled, L lung with 2 lobes + cardiac notch. Show pleural layers + cavity. Add diaphragm beneath.

Mechanics

Breathing Mechanics

Boyle's law in action

Inspiration (active): diaphragm + external intercostals contract
Thoracic volume ↑ → intrapulmonary pressure ↓ → air rushes in
Expiration at rest (passive): diaphragm + intercostals relax, elastic recoil
Forced expiration (active): internal intercostals + abdominal muscles
Boyle's law: P × V = constant, so when volume up, pressure down
Accessory muscles in distress: SCM, scalenes

Takeaway: tripoding (leaning on hands, shoulders elevated) is a body posture that recruits accessory muscles. Common in COPD exacerbation. Visible "work of breathing" tells you severity.

Volumes

Lung Volumes & Capacities

Spirometry tracing

Tidal volume (TV): normal breath, ~500 mL
Inspiratory reserve volume (IRV): extra you can inhale, ~3000 mL
Expiratory reserve volume (ERV): extra you can exhale, ~1100 mL
Residual volume (RV): can't exhale, ~1200 mL
Vital capacity (VC): TV + IRV + ERV
Total lung capacity (TLC): VC + RV
Functional residual capacity (FRC): ERV + RV

Draw: a spirometry tracing showing all volumes. Y-axis = volume, X-axis = time. Show normal tidal breaths in the middle, then a max inspiration (IRV), then max expiration (ERV). Mark RV at bottom as the unmeasurable floor.

Surfactant

Surfactant & Compliance

Why alveoli stay open

Surface tension in alveoli would collapse them
Surfactant (made by type II pneumocytes) lowers surface tension
Allows uniform inflation (small alveoli would otherwise collapse into big ones)
Compliance: how easily lungs stretch
Low compliance: stiff lungs (pulmonary fibrosis, ARDS)
High compliance: floppy lungs (emphysema)
Premature infants lack surfactant → respiratory distress syndrome

Takeaway: surfactant peaks at ~34 weeks gestation. Mothers at risk of preterm delivery get antenatal corticosteroids to boost fetal surfactant production. Big impact on neonatal mortality.

Clinical

Respiratory Mechanics Disorders

Common hits

Pneumothorax: air enters pleural space, lung collapses
Tension pneumothorax: one-way leak, mediastinal shift, emergency
Pleural effusion: fluid in pleural space
Asthma: reversible airway obstruction (bronchospasm)
COPD: chronic bronchitis + emphysema, irreversible
Pulmonary fibrosis: restrictive, low compliance
Atelectasis: alveolar collapse

Takeaway: obstructive (asthma, COPD): trouble getting air out (low FEV1/FVC ratio). Restrictive (fibrosis, ARDS): trouble getting air in (low VC, preserved ratio). The PFT pattern locates the problem.

Respiratory Anatomy & Mechanics.