Respiratory System: Ventilation, Anatomy & Mechanics

Ventilation (Pulmonary Ventilation)
- Mass movement of air into or out of the lungs (measured only one direction at a time).
- Analogy: A fan moving air through a room.
Respiration (Gas Exchange)
- External / Pulmonary Respiration: exchange between alveoli & pulmonary capillaries.
- Internal / Tissue Respiration: exchange between systemic capillaries & body tissues (muscle, hepatocytes, etc.).
- Bottom line: respiration ≠ ventilation; the former is molecular diffusion, the latter is bulk flow.

Formula mirrors cardiac output: \dot VE = f{\text{breaths}} \times V_T where
- f_{\text{breaths}} ≈ 11 breaths·min⁻¹ at rest.
- V_T = tidal volume ≈ volume per single breath ("stroke volume" analogue).
Residual Volume: even a maximal exhalation leaves air behind; prevents lung collapse & aids re-inflation.

Air pathway (ventilation only):
1. Nostrils (external nares)
2. Nasal cavity → conchae & meatuses
3. Oral cavity (optional route; dries mouth)
4. Pharynx: naso-, oro-, laryngo- regions
5. Larynx (voice box, vocal folds)
6. Trachea → carina
7. Right & Left primary bronchi → secondary (lobar) → tertiary → …
8. Terminal bronchioles (last purely conducting segment)
Supportive structures:
- Rib cage & intercostal muscles
- Diaphragm (skeletal, voluntary)
- Pleural membranes (parietal on ribs, visceral on lungs, fluid-filled cavity in between)

Nose hairs (integumentary) filter large particulates—critical in dusty environments.
Mucous membrane covers all internal airways; always wet, but mucus content varies.
Conchae & meatuses create turbulent flow → ↑ contact with mucus → better filtration.
Conditioning of inspired air
- Warms to 98.6\,^\circ\text{F}
- Humidifies to 100 % relative humidity; prevents alveolar dehydration.
Paranasal sinuses lighten skull; blocked ostia → unequal pressure → sinus pain.
Pharyngotympanic (Eustachian) tube equalises middle-ear pressure; flatter in children → otitis media.

Tonsils
- Palatine (visible), lingual (posterior tongue), pharyngeal/adenoids (roof of nasopharynx).
- Chronic infection → tonsillectomy; branches of significant arteries so post-op bleeding is a risk.
Epiglottis (elastic cartilage)
- Depresses during swallowing; mis-timing → aspiration/choking.
Esophagus vs. Larynx split at laryngopharynx; dorsal esophagus vs. ventral airway.

Wall layers (lumen → out)
1. Mucosa: ciliated pseudostratified columnar epithelium + goblet cells.
2. Submucosa: seromucous glands (water) + areolar CT.
3. C-shaped hyaline cartilage rings (open posteriorly).
4. Adventitia.
Trachealis muscle (smooth) bridges open ends; functions
- Reflex contraction → ↓ lumen, ↓ inhaled irritants.
- Allows esophageal bolus to bulge during swallowing.
Primary bronchus differences
- Right bronchus shorter, wider, steeper (clinically more aspirated objects).
- Right lung = 3 lobes; left = 2 lobes + cardiac notch.

Bronchioles (no cartilage; complete ring of smooth muscle with \beta_2-adrenergic receptors).
- Sympathetic \uparrowE/NE → bronchodilation.
- Histamine, allergens → bronchoconstriction (asthma).
- Rescue inhalers = \beta_2 agonists → relaxation.
Terminal bronchiole = final conducting airway.
Respiratory bronchiole → alveolar ducts → alveolar sacs
- Begin "respiratory zone": ventilation and gas exchange.

Generalised equation: \text{Flow}=\dfrac{\Delta P}{R} (analogous to Ohm’s law).
Resistance profile
- Medium bronchi show highest fixed anatomical resistance.
- Bronchioles contribute most variable resistance; small change in radius → large R change.
Asthmatic attack: inflammation + bronchoconstriction → ↑R → ↓Flow → impaired ventilation.

Epithelial gradient
- Pseudostratified → columnar → cuboidal (small bronchioles) → simple squamous (alveoli).
Type I alveolar cells = simple squamous; minimal diffusion distance.
Capillary interface
- Fused basement membrane between alveolar epithelium & capillary endothelium further shortens path.
Elastic fibres provide passive recoil (exhalation aid); loss → emphysema (COPD).
Alveolar macrophages = final particulate defense.

Type II alveolar cells = surfactant-secreting cells.
- Surfactant = amphipathic detergent → disrupts H^+–O bonding between water molecules.
- ↓ Surface tension → prevents alveolar collapse → enables inflation/deflation cycle.
Newborn relevance: many delivery carts stock exogenous surfactant; premature infants risk Respiratory Distress Syndrome without it.

Rib articulations
- Posterior: synovial joints with thoracic vertebrae.
- Anterior: costal cartilage hinge to sternum; "bucket-handle" lift during inspiration.
Intercostal muscles
- External: inspiration (rib lift).
- Internal: forced expiration (rib depress).
Diaphragm
- Prime mover of inspiration; contraction flattens & lowers central tendon.
Pleura
- Parietal (thoracic wall) & visceral (lung surface).
- Intrapleural pressure < intrapulmonary & atmospheric pressure → keeps lungs inflated.
- Puncture → pneumothorax → lung collapse; if bilateral → severe ventilatory compromise.

Boyle’s statement: P1V1 = P2V2; pressure inversely proportional to volume.
Inspiration sequence
1. Diaphragm + external intercostals contract.
2. Thoracic cavity volume ↑.
3. Lung (intrapulmonary) volume ↑ → P{\text{pul}} ↓ (({\text{atm}})).
4. Air flows into lungs down pressure gradient.
Expiration (quiet)
1. Muscles relax.
2. Elastic recoil + rib descent → thoracic & lung volume ↓.
3. P{\text{pul}} rises ((>P{\text{atm}})).
4. Air flows out passively.
Forced expiration recruits internal intercostals, abdominal muscles, & small airway smooth muscle.