The Respiratory System

Asthma

• Clinical: episodic wheeze, cough (may be the only symptom, "cough-variant asthma"), chest tightness, dyspnea, often nocturnal or exercise-triggered.
• Key histologic findings: mucus plugs containing whorled epithelial casts (Curschmann spirals), and crystalline eosinophil-derived debris (Charcot-Leyden crystals) in sputum.
• Hallmark: reversible airflow obstruction — FEV1 improves substantially after bronchodilator administration, distinguishing it from COPD.
• Status asthmaticus: a severe attack unresponsive to standard bronchodilator therapy — a medical emergency.
• Recognized clinical subtypes include extrinsic (allergen-driven, typically childhood-onset), intrinsic (non-allergic, typically adult-onset), exercise-induced, and cold air–induced asthma — different triggers converging on the same final bronchoconstrictive pathway.

α1-Antitrypsin Deficiency Mechanism

Bronchiectasis

• Permanent, irreversible dilation of the bronchi caused by a cycle of chronic infection and inflammation that destroys the bronchial wall.
• Presents with copious purulent sputum, recurrent infections, hemoptysis, digital clubbing.
• Common associations: cystic fibrosis, primary ciliary dyskinesia, chronic bronchial obstruction (tumor, foreign body), recurrent aspiration.

Systemic Corticosteroid Adverse Effects

Reserved for moderate-to-severe exacerbations because, unlike the inhaled route, systemic (oral/IV) corticosteroids carry a broader adverse effect profile: osteoporosis, Cushingoid features, psychiatric symptoms (including psychosis), impaired glucose tolerance, increased infection risk, hypertension, cataracts, and acne.

Regimens for Acute Exacerbations

Aspirin-Exacerbated Respiratory Disease Mechanism

Leukotriene Synthesis Pathway

• LTB4 is a potent neutrophil chemoattractant and promotes leukocyte adhesion — it drives inflammatory cell recruitment rather than bronchoconstriction directly.
• LTC4, LTD4, LTE4 (collectively the components of what was historically called slow-reacting substance of anaphylaxis) are the leukotrienes responsible for bronchoconstriction, increased vascular permeability, and mucus secretion — the targets of leukotriene receptor antagonists like montelukast.

β-Agonist Potency & Special Clinical Uses

• Relative potency at β-receptors (most → least potent): isoproterenol > epinephrine > norepinephrine.
• Because inhaled therapy limits systemic exposure, meaningful systemic β2 activation is uncommon — but when it occurs, it can cause vasodilation, a modest fall in peripheral vascular resistance, increased skeletal muscle and hepatic glycogenolysis, increased glucagon release, and relaxation of uterine smooth muscle.
• β2-selective agents have a niche use outside the lung: terbutaline can be used off-label to relax uterine smooth muscle and delay preterm labor (tocolysis), exploiting that same systemic effect.

Pathogenesis

• Most cases arise from microaspiration of oropharyngeal flora or inhalation of infectious droplets; less commonly via hematogenous spread, direct contiguous spread, or traumatic inoculation.
• Normal oral flora is dominated by gram-positive cocci; hospitalized patients are often colonized instead by gram-negative rods, which is why nosocomial pneumonia has a different typical organism profile than community-acquired disease.
• Risk is increased by anything that impairs airway protection or clearance — alcoholism, nasogastric tube placement, and any cause of obtundation.

Typical vs Atypical Presentation

Pattern of Lung Involvement

• Lobar: dense, confluent consolidation of an entire lobe — classic for S. pneumoniae.
• Bronchopneumonia: patchy, multifocal consolidation centered on airways — S. aureus, H. influenzae, gram-negative rods.
• Interstitial ("walking") pneumonia: inflammation confined largely to alveolar septa — atypical organisms and viruses.

Organism Pearls

Natural History of Lobar Pneumonia

Lung Abscess

• In a comatose or obtunded patient, the leading cause is aspiration pneumonia secondary to a depressed cough/gag reflex.
• Other causes: septic emboli (e.g., from infective endocarditis), direct spread from an adjacent infected structure, and obstruction/necrosis from an underlying malignant tumor.
• Typically due to anaerobes (e.g., Bacteroides, Fusobacterium, Peptostreptococcus) or S. aureus; look for an air-fluid level on imaging, which suggests cavitation.
• Location tracks patient positioning at the time of aspiration — right lower lobe if the aspiration happened while upright, right upper or middle lobe if recumbent, mirroring the general aspiration-pneumonia pattern.
• Managed with antibiotics; drainage or surgery for refractory cases.

Pneumococcal Vaccination

Pneumococcal conjugate vaccines generate a T-cell-dependent IgG response that reduces nasopharyngeal carriage of S. pneumoniae, lowering rates of invasive disease including lobar pneumonia. Newer 20-valent conjugate formulations extend serotype coverage beyond the earlier 13/15-valent versions.

Viral Pneumonia

• Respiratory syncytial virus (RSV): also a leading cause of bronchiolitis; peaks in winter months; can cause severe respiratory distress in infants.
• Influenza: frequently complicated by secondary bacterial pneumonia (classically S. aureus) as viral damage to the respiratory epithelium predisposes to bacterial superinfection.

Fungal Pneumonias by Geography

• Histoplasma capsulatum — Ohio/Mississippi River valleys; intracellular in macrophages.
• Coccidioides immitis — Southwestern US deserts ("valley fever"); spherules filled with endospores.
• Blastomyces dermatitidis — Great Lakes/Mississippi-Ohio basins; broad-based budding yeast.
• Paracoccidioides brasiliensis — Central and South America; budding yeast forms with a characteristic "captain's wheel" (multiple peripheral buds) appearance.
• Aspergillus — forms a fungus ball (aspergilloma) in pre-existing cavities, or invasive disease in the severely immunocompromised.
• Cryptococcus neoformans — encapsulated yeast; especially important as a cause of meningitis in immunocompromised hosts.
• Sporotrichosis — not primarily a pneumonia but a skin/lymphatic infection classically affecting rose gardeners after a thorn prick; included here as a fungal exposure pearl.

Most Common Pathogens by Age Group

Primary vs Secondary (Reactivation) TB

• Presentation: fever, night sweats, weight loss, hemoptysis, chronic cough.
• Acid-fast bacillus — the thick, lipid- and mycolic-acid-rich cell wall resists standard Gram staining and retains carbol-fuchsin dye despite acid-alcohol decolorization.
• Extrapulmonary reactivation sites to remember: vertebral bodies (Pott disease), meninges, kidneys, adrenal glands, and the psoas major muscle (often via direct spread from vertebral disease).

First-Line Anti-TB Therapy (RIPE)

Standard regimen: all four RIPE drugs together for an initial 2-month intensive phase, followed by isoniazid + rifampin alone for a continuation phase of roughly 4 to 7 more months, depending on clinical response.

Pulmonary Embolism

• Underlying risk framework — Virchow triad: venous stasis, endothelial injury, hypercoagulability.
• Most commonly embolizes from proximal deep vein thrombosis (femoral/iliac veins).
• Presentation: sudden dyspnea, pleuritic chest pain, tachycardia, tachypnea; large emboli can cause hypotension and sudden death.
• Physiology: creates dead space (high V/Q) in the affected segment → reflex hyperventilation → respiratory alkalosis; widened A–a gradient.
• A saddle embolus lodges at the main pulmonary artery bifurcation and can be rapidly fatal.
• Common clinical settings that predispose to PE: malignancy, multiple fractures/major orthopedic surgery, oral contraceptive or other estrogen use, prolonged immobilization/bed rest, and heart failure.
• When infarction does occur, it classically produces a hemorrhagic, wedge-shaped area of necrosis, because the dual (bronchial + pulmonary) blood supply to the lung makes true infarction less common than the vascular occlusion alone would suggest.
• Non-thrombotic emboli to remember: fat emboli (long bone fracture, orthopedic surgery), amniotic fluid emboli (peripartum), air emboli (central line placement, diving).
• Beyond the classic thrombus, an embolus can be composed of several other materials — worth keeping as a checklist when a patient doesn't fit the typical DVT-to-PE story: fat (long-bone fracture, liposuction; triad of hypoxemia, neurologic changes, petechial rash), air (decompression sickness in divers, or iatrogenic from line placement — treated with hyperbaric O₂), bacteria (septic emboli), amniotic fluid (labor/postpartum — can trigger DIC), and tumor fragments.
• On gross pathology, a thrombus formed before death shows alternating pale (platelet/fibrin) and red (red cell) laminations known as lines of Zahn — useful for distinguishing an antemortem thrombus from a postmortem clot, which lacks this layered pattern.
• Treatment: anticoagulation (heparin, or a direct thrombin/factor Xa inhibitor) is first-line; an IVC filter is reserved for patients in whom anticoagulation is contraindicated.

Hypoxic Pulmonary Vasoconstriction

• Unique to the pulmonary circulation: local alveolar hypoxia triggers vasoconstriction (opposite of systemic circulation, where hypoxia causes vasodilation).
• Physiologic purpose: diverts blood flow away from poorly ventilated lung units toward better-ventilated ones, optimizing overall V/Q matching.
• Global alveolar hypoxia (e.g., high altitude, severe COPD) causes diffuse pulmonary vasoconstriction → chronic pulmonary hypertension → right heart strain.

Pulmonary Hypertension

• Primary: idiopathic proliferation of pulmonary vascular smooth muscle.
• Secondary: far more common — driven by chronic hypoxia (COPD, high altitude), left heart disease, chronic thromboembolism, or chronically increased pulmonary blood flow, as seen with a left-to-right cardiac shunt.
• Chronic pressure overload of the right ventricle → right ventricular hypertrophy → eventual right heart failure, termed cor pulmonale.
• Exam findings reflect that right ventricular pressure overload: a loud P2 (pulmonic component of S2) and a left parasternal heave from the hypertrophied right ventricle.

Pulmonary Hypertension — Etiologic Groups

Targeted Pulmonary Arterial Hypertension Therapy

Group 1 disease reflects endothelial dysfunction along three vasoactive pathways — each is a distinct drug target.

Mediastinal Masses & Mediastinitis

• Mass location narrows the differential: anterior mediastinal masses trend toward thymoma, teratoma, and lymphoma (plus retrosternal thyroid); middle mediastinal masses include lymph node metastases, bronchogenic cysts, and hiatal hernia; posterior masses trend toward neurogenic tumors and esophageal pathology.
• Acute mediastinitis is most often postoperative (cardiothoracic surgery) or follows esophageal perforation; presents with fever, chest pain, and wound drainage.
• Pneumomediastinum: air tracks into the mediastinum, usually from a ruptured alveolus or bleb dissecting along the bronchovascular sheaths; presents with chest pain and often crepitus felt/heard over the precordium on exam.

Pleural Effusion

• Fluid accumulating between the visceral and parietal pleura restricts lung expansion; Light's criteria classify the fluid as an exudate if the pleural-to-serum protein ratio exceeds 0.5, the pleural-to-serum LDH ratio exceeds 0.6, or the pleural LDH exceeds two-thirds of the upper limit of normal serum LDH.
• Exudate (cellular, cloudy): driven by local processes that increase capillary permeability — infection, malignancy, connective tissue disease, or lymphatic obstruction (chylothorax); often needs drainage given the higher infection risk.
• Transudate (hypocellular, clear): driven by systemic pressure imbalances — ↑ hydrostatic pressure (heart failure) or ↓ oncotic pressure (nephrotic syndrome, cirrhosis).

Pulmonary Edema

Chronic cardiogenic pulmonary edema produces hemosiderin-laden alveolar macrophages ("heart failure cells") from breakdown of extravasated red cells.

Granulomatosis with Polyangiitis (formerly Wegener granulomatosis)

• Small-to-medium vessel necrotizing vasculitis of unclear (likely autoimmune) etiology.
• Classic triad: upper respiratory tract necrotizing granulomas (sinusitis, nasal ulceration/saddle-nose deformity), lower respiratory tract nodules/cavitation, and rapidly progressive (pauci-immune) glomerulonephritis.
• Serology: c-ANCA (anti-proteinase 3) positive in the great majority of cases.
• Untreated disease has a high mortality; immunosuppressive therapy is the mainstay of treatment.

Lung cancer is the leading cause of cancer death for both sexes, despite not being the most commonly diagnosed cancer overall — it ranks second in incidence behind prostate cancer in men and breast cancer in women. Lung cancer mortality among women has been rising as a reflection of historical smoking trends in that population.

Central vs Peripheral Location — a Useful Organizing Principle

Metastatic lung lesions (from a primary tumor elsewhere) are actually more common overall than primary lung cancer, classically appearing as multiple "cannonball" lesions on imaging — most often seeded from breast, colon, prostate, and bladder primaries. When lung cancer itself metastasizes, the classic target organs are the liver (jaundice, hepatomegaly), adrenal glands, bone (pathologic fracture), and brain.

Paraneoplastic Syndromes

Pancoast Tumor & Superior Vena Cava Syndrome

• A Pancoast (superior sulcus) tumor arises at the lung apex and produces its effects by invading or compressing nearby structures rather than through distant metastasis: the recurrent laryngeal nerve (hoarseness), the sympathetic (stellate) ganglion (Horner syndrome), the brachial plexus (shoulder pain, hand muscle weakness/atrophy), and the phrenic nerve (hemidiaphragm elevation on imaging).
• Superior vena cava syndrome: obstruction of the SVC — by direct tumor invasion (Pancoast tumor, mediastinal mass) or by thrombus from an indwelling catheter — impairs venous drainage from the head, neck, and upper extremities. Presents with facial swelling/plethora, jugular venous distension, and upper-extremity edema; treated as a medical emergency because severe obstruction can raise intracranial pressure.

Risk Factors & Digital Clubbing

• Tobacco smoking is by far the dominant risk factor; also secondhand smoke, radiation exposure, environmental/occupational exposures (radon, asbestos), pre-existing pulmonary fibrosis, and family history.
• Digital clubbing (widened nail-bed angle >180°) reflects local release of growth factors (e.g., PDGF, VEGF) from platelets/megakaryocytes lodged in the distal digital vasculature, most often in the setting of an intrapulmonary shunt. Beyond lung cancer, it's seen with other chronic pulmonary disease (idiopathic pulmonary fibrosis, cystic fibrosis, bronchiectasis), cyanotic congenital heart disease, and chronic infection (lung abscess, TB) — notably, it is not a typical feature of plain COPD or asthma.

Other Respiratory Tract Carcinomas

Regional V/Q Gradient (Upright Lung)

• Whole-lung average V/Q ≈ 0.8 (ventilation slightly exceeds perfusion overall).
• A middle Zone 2 exists between apex and base, where arterial pressure exceeds alveolar pressure, which in turn exceeds venous pressure — capillaries stay open throughout the cardiac cycle and ventilation and perfusion are roughly matched (V/Q ≈ 1).
• During exercise, increased cardiac output raises pulmonary vascular pressures enough to recruit and distend capillaries throughout the lung, making the V/Q ratio much more uniform from apex to base.
• V/Q → 0 (perfusion without ventilation) = physiologic shunt — e.g., complete airway obstruction, atelectasis. Does not correct with supplemental oxygen because the blood never contacts ventilated alveoli.
• V/Q → ∞ (ventilation without perfusion) = physiologic dead space — e.g., pulmonary embolism. Wastes ventilation on alveoli with no blood flow to pick up the oxygen.

Anatomic Shunts

• Blood that passes from the venous to the arterial circulation without traversing ventilated alveoli — a normal, small contributor to the resting A–a gradient (roughly 2% of cardiac output, largely via bronchial and thebesian venous drainage).
• Pathologic right-to-left shunting can raise this fraction dramatically — up to roughly 50% of cardiac output in severe congenital lesions such as tetralogy of Fallot — and, like any true shunt, does not correct with supplemental oxygen.

Anatomic vs Physiologic Dead Space

• Anatomic dead space: the conducting airways (nose through terminal bronchioles) that carry air but never participate in gas exchange; measured by the Fowler method (tracking nitrogen washout during a single exhaled breath).
• Physiologic dead space: anatomic dead space plus any ventilated-but-underperfused alveoli that fail to eliminate CO2 effectively; measured by the Bohr method, using the relationship:

where Vd = physiologic dead space, Vt = tidal volume, PaCO2 = arterial CO2, and PECO2 = mixed expired CO2. In a healthy lung the two dead-space values are nearly equal; a widening gap between them signals V/Q mismatch from poorly perfused (but ventilated) alveolar units.

Approach to Hypoxemia

Diffusion-Limited vs Perfusion-Limited Gas Exchange

• Carbon monoxide is deliberately diffusion-limited under normal conditions (its capillary partial pressure essentially never rises high enough to equilibrate) which is exactly why it makes a convenient tracer gas for measuring diffusing capacity (DLCO) clinically.
• Diseases that thicken or destroy the alveolar-capillary membrane (pulmonary fibrosis, emphysema) push oxygen transfer from the perfusion-limited into the diffusion-limited regime — this is the mechanistic reason those diseases tend to cause hypoxia on exertion before they cause resting hypoxia.

Alveolar Gas Equation

• At sea level, atmospheric pressure ≈ 760 mmHg and the water vapor pressure of fully humidified alveolar air ≈ 47 mmHg.
• FiO2 is the fraction of inspired air that is oxygen (0.21 on room air); 0.8 is the respiratory quotient (CO2 produced per O2 consumed).
• Used to calculate the expected alveolar oxygen tension, which is then compared to the measured arterial oxygen tension to compute the A–a gradient.

Ventilation Equation

Rearranged, this means alveolar PCO2 varies inversely with alveolar ventilation for a fixed rate of CO2 production — for example, if CO2 production is unchanged, a threefold rise in expired PCO2 implies alveolar ventilation has fallen to a third of its prior value.

Hemoglobin Structure & Cooperative Binding

• Adult hemoglobin (HbA) is a tetramer of 2 α- and 2 β-globin chains, each carrying a heme group that binds one O₂ molecule (4 O₂ per Hb).
• Positive cooperativity: binding of the first O₂ molecule increases the affinity of the remaining subunits for O₂, producing the classic sigmoidal oxygen-hemoglobin dissociation curve. Myoglobin, by contrast, is a single-chain protein with no cooperativity — its dissociation curve is a simple hyperbola, and its higher, fixed affinity for O₂ suits its role as an O₂ reservoir in muscle rather than a transport molecule.

Oxygen Content of Blood

The first term (Hb bound) dominates total content; the second term (O₂ dissolved in plasma) is tiny by comparison. Total O₂ delivery to tissue = cardiac output × O₂ content — so a fall in cardiac output can cause tissue hypoxia even with completely normal blood gas values.

Oxyhemoglobin Dissociation Curve Shifts

CO₂ Transport

• ~90% travels as bicarbonate — CO₂ enters the red cell, carbonic anhydrase converts it (with water) to carbonic acid, which dissociates to H⁺ and HCO₃⁻; the bicarbonate is shuttled into plasma in exchange for chloride (the "chloride shift").
• ~5% binds directly to the N-terminus of deoxygenated globin as carbaminohemoglobin; ~5% dissolves directly in plasma.
• Haldane effect: oxygenation of Hb in the lungs promotes release of H⁺, which drives the equilibrium back toward CO₂ formation — this is what allows CO₂ to be released from red cells as blood passes through the pulmonary capillaries.

Response to High Altitude & Exercise

Methemoglobinemia

• Iron in Hb is normally kept in the reduced, ferrous (Fe²⁺) state. Oxidizing agents — dapsone, local anesthetics such as benzocaine, and nitrites — convert it to the ferric (Fe³⁺) state, which cannot bind O₂ effectively and also increases the O₂ affinity of the remaining normal subunits (a pathologic left shift).
• Presents with cyanosis unresponsive to supplemental oxygen and characteristically brown/chocolate-colored blood.
• Treatment: methylene blue (reduces Fe³⁺ back to Fe²⁺) plus vitamin C.

Cyanide vs Carbon Monoxide Poisoning

Central Pattern Generation

• Medullary respiratory centers generate the automatic rhythm of breathing; pontine centers fine-tune the rate/depth of that rhythm.
• The medulla receives sensory input via the vagus and glossopharyngeal nerves and sends motor output via the phrenic nerve to the diaphragm and via spinal (intercostal) nerves to the intercostal and abdominal wall muscles.
• Voluntary override (breath-holding, speech) originates in the cerebral cortex and acts on the medullary pattern generator.

Muscles of Respiration

• The diaphragm does the majority of the work during quiet breathing.
• Accessory muscles — the sternocleidomastoid and the internal/external intercostals — are recruited only when ventilatory demand rises, as in exercise or respiratory disease; their use on exam is a visible sign of increased work of breathing.

Chemoreceptor Control

Reflex Inhibition of Inspiration

• Irritant receptors in the large airways and stretch receptors in the small airways, when activated, send inhibitory signals that terminate inspiration — a protective reflex limiting excessive lung distension.

Abnormal Breathing Patterns

• Cheyne–Stokes breathing: cyclic crescendo–decrescendo tidal volumes separated by apnea; reflects pontine dysfunction and is associated with heart failure, drug overdose, CNS depression, and elevated intracranial pressure.
• Kussmaul breathing: deep, rapid, labored breathing that is the respiratory compensation for severe metabolic acidosis (e.g., DKA) — "blowing off" CO₂ to raise pH.
• Biot's (ataxic) breathing: irregular breaths with unpredictable pauses; associated with medullary damage — poor prognostic sign.

Sleep-Disordered Breathing

Multiple pharmacologic agents have been tried for sleep apnea, but none rival CPAP in effectiveness.

• Diagnosis of either type is confirmed by a formal sleep study (polysomnography).
• Repeated nocturnal hypoxia from either type of sleep apnea drives systemic and pulmonary hypertension, atrial arrhythmias, and an increased risk of sudden death; the chronic hypoxic stimulus also raises erythropoietin release, producing secondary erythrocytosis over time.
• Additional OSA treatment options beyond weight loss/CPAP: oral appliances that reposition the jaw, hypoglossal nerve stimulation, and upper airway surgery for refractory cases.

Obesity Hypoventilation Syndrome

• Also called Pickwickian syndrome; defined by obesity (BMI ≥ 30) plus chronic daytime hypoventilation — an elevated waking PaCO₂ (with a compensatory drop in PaO₂) that isn't explained by another lung or neuromuscular cause.
• Gas exchange worsens further during sleep, and the syndrome frequently coexists with obstructive sleep apnea.
• Treatment mirrors OSA: weight loss and positive airway pressure support.

Particle Deposition by Size

Breathing pattern shifts where particles land: slow, deep breaths favor sedimentation and diffusion (more distal deposition), while the higher airflow rates of exercise favor inertial impaction (more proximal deposition).

Mechanical & Cellular Defenses

• Mucociliary escalator: ciliated pseudostratified epithelium sweeps trapped particles in mucus toward the pharynx to be swallowed or expectorated.
• Cough reflex: rapid, forceful expiration triggered by irritant receptors; can be suppressed by opioid antitussives.
• Alveolar macrophages: phagocytose particulates and organisms that reach the alveoli; "dust cells" filled with carbon pigment reflect chronic particulate exposure.
• Secretory IgA and complement: humoral defenses within airway secretions that provide first-line mucosal protection against inhaled pathogens.

Anatomic Predisposition to Aspiration

• The right main bronchus is wider, shorter, and more vertically oriented than the left — it branches off the trachea at a shallower angle.
• This makes it the preferential path for aspirated foreign material, whether liquid, food, or a dislodged object.
• In a supine patient, aspirated material tends to settle into the right lower lobe specifically, following gravity down the more direct right-sided airway.

Static Lung Volumes (building blocks)

Derived Capacities (sums of ≥2 volumes)

• Vital capacity (VC) = IRV + TV + ERV — the maximum air that can be moved in a single breath.
• Inspiratory capacity (IC) = TV + IRV.
• Functional residual capacity (FRC) = ERV + RV — the lung volume at the end of a normal, passive exhalation.
• Total lung capacity (TLC) = VC + RV — everything.
• Any capacity that includes RV (FRC, TLC) cannot be measured by spirometry alone — requires helium dilution, nitrogen washout, or body plethysmography.

Compliance

• Describes how easily the lung–chest wall system expands for a given change in pressure; the slope of the pressure–volume curve.
• At FRC, the lung's inward elastic recoil (tendency to collapse) exactly balances the chest wall's outward recoil (tendency to spring open) → net system pressure = atmospheric, no airflow.
• Breathing above FRC (inhaling) generates positive airway pressure relative to that baseline, while breathing below FRC (forced exhalation) generates negative airway pressure — FRC is the true zero-pressure reference point for the respiratory system, not empty lungs.

Poiseuille Relationship

A small reduction in airway radius produces a dramatic rise in resistance — the physiologic basis for why bronchoconstriction (asthma) causes such disproportionate symptoms.

• Parasympathetic (vagal) tone → bronchoconstriction + increased mucus secretion (muscarinic receptors); target of anticholinergic bronchodilators. Mediated by direct vagal stimulation, airway irritation, and mast-cell-derived mediators such as histamine and the leukotriene mixture historically termed slow-reacting substance of anaphylaxis (SRS-A).
• Histamine released during allergic reactions and allergic asthma is itself a potent constrictor of airway smooth muscle and a direct contributor to airway resistance.
• Sympathetic tone (β₂ receptors) → bronchodilation; target of β₂-agonist bronchodilators.
• Airway caliber is also passively maintained by radial traction from surrounding lung parenchyma — this is why fibrotic (stiff) lungs paradoxically have low airway resistance, while emphysematous lungs (loss of parenchymal tethering) have high resistance despite large airways.

Obstructive vs Restrictive Pattern — Interpreting Spirometry

On a flow-volume loop, obstructive disease characteristically scoops out the expiratory limb and shifts the whole loop leftward (toward higher lung volumes, reflecting air trapping), while restrictive disease produces a narrower loop of normal shape shifted rightward (toward lower lung volumes).

Structure of the Blood–Air Barrier

• Gas must cross three thin layers to move between alveolar air and pulmonary capillary blood:
  1. Type I pneumocyte membrane and cytoplasm (the thin, gas-permeable alveolar lining cell).
  2. A fused basement membrane shared by the type I pneumocyte and the capillary endothelial cell.
  3. Capillary endothelial cell membrane and cytoplasm.
• Any process that thickens this barrier (interstitial fibrosis, pulmonary edema) impairs diffusion and is the physiologic basis of a reduced DLCO.

• Diagnostic hallmarks: acute onset, bilateral infiltrates on imaging, hypoxemia not explained by cardiac cause, and a PaO₂/FiO₂ ratio below a defined threshold.
• Gross pathology: heavy, fluid-laden ("wet") lungs from the protein-rich edema fluid — a finding shared with NRDS.
• Barotrauma-related pneumothorax is a recognized complication that can develop rapidly and become fatal, adding to the rationale for lung-protective (low-pressure, low-volume) ventilation strategies.
• Histology: diffuse alveolar damage with hyaline membranes lining alveoli — the adult counterpart to NRDS's hyaline membranes, but driven by neutrophilic injury rather than surfactant deficiency. Hyaline membranes themselves are eosinophilic, protein-rich membranous deposits that line the alveolar walls, formed from leaked plasma proteins and cellular debris.
• Management centers on treating the underlying cause plus lung-protective mechanical ventilation (low tidal volumes to avoid ventilator-induced injury) and conservative fluid management.

Tracheal Deviation Rules of Thumb

• Tension pneumothorax (a space-occupying process) pushes the trachea away from the lesion.
• Volume loss (atelectasis, complete bronchial obstruction, lobectomy) pulls the trachea toward the affected side — and because ventilation to that lung is reduced, tactile fremitus and resonance are both decreased over it, distinguishing it from the hyperresonant, decreased-fremitus exam of a pneumothorax.

Atelectasis

• Obstructive: airway blockage (mucus plug, tumor, foreign body) prevents air from reaching distal lung; trapped air is gradually resorbed and the lung collapses — this is the type that pulls the trachea toward the affected side.
• Compressive: an external mass, effusion, or air compresses adjacent lung tissue.
• Contraction (cicatrization): scarring distorts and pulls in surrounding parenchyma (e.g., sarcoidosis, radiation fibrosis).
• Adhesive: surfactant deficiency prevents alveoli from staying open (classically neonatal respiratory distress syndrome).
• Incentive spirometry and positive end-expiratory pressure both work by keeping alveoli inflated, directly counteracting the tendency to collapse.

Physical Exam Findings Across Pleural/Airway Pathology

Open (Sucking) Chest Wound

• Penetrating chest wall trauma that leaves an open communication between the pleural space and the atmosphere.
• Air preferentially enters through the wound rather than the trachea whenever the defect diameter approaches or exceeds the tracheal diameter — the path of least resistance.
• Impairs effective ventilation on the affected side and can evolve into a tension physiology if the wound is incompletely occluded, so initial management typically involves a three-sided occlusive dressing that vents air out but not in.

Shared physiology: ↓ lung volumes, normal-to-increased FEV1/FVC ratio, and (in parenchymal disease) reduced DLCO from thickened or destroyed alveolar-capillary interface.

Interstitial Lung Disease

Sarcoidosis Systemic Manifestations

• Pulmonary: dyspnea, dry cough, bilateral hilar adenopathy.
• Ophthalmologic: uveitis.
• Dermatologic: erythema nodosum.
• Metabolic: hypercalcemia from macrophage-mediated activation of vitamin D within granulomas.
• Cardiac: restrictive cardiomyopathy, conduction abnormalities.
• Characteristic anergy on tuberculin skin testing despite a hyperactive immune system overall (polyclonal hypergammaglobulinemia).

Occupational Lung Disease (Pneumoconiosis)

• Silica and coal dust both come from the ground and characteristically produce upper lobe disease, while asbestos — historically used overhead as insulation — characteristically produces lower lobe disease; a handy way to keep the lobar distributions straight.
• Silicosis classically shows "eggshell" calcification of the hilar lymph nodes on chest imaging.
• Caplan syndrome: rheumatoid arthritis occurring together with a pneumoconiosis (most classically coal worker's pneumoconiosis or silicosis), producing well-defined intrapulmonary nodules.

Mesothelioma

• Malignancy of the pleura strongly associated with asbestos exposure; can produce a hemorrhagic pleural effusion and diffuse pleural thickening.
• Histology classically shows psammoma bodies (concentric, laminated calcifications); on electron microscopy, tumor cells show long, slender microvilli.
• Immunohistochemistry positive for calretinin and cytokeratin 5/6 in nearly all cases — a useful way to distinguish mesothelioma from a metastatic adenocarcinoma involving the pleura, which is typically negative for these markers.
• Unlike bronchogenic carcinoma, tobacco smoking is not a risk factor for mesothelioma — asbestos exposure alone drives the risk.

• Autosomal recessive; the most common lethal genetic disease in individuals of European descent.
• Caused by mutations in the CFTR gene (chromosome 7), a chloride channel; the most common mutation is a three-base-pair deletion causing loss of a phenylalanine residue (ΔF508), which impairs protein folding and trafficking.

Organ System Manifestations

Treatment Principles

• Airway clearance techniques + mucolytics (N-acetylcysteine, dornase alfa) to reduce mucus viscosity.
• Pancreatic enzyme replacement and fat-soluble vitamin supplementation.
• CFTR modulator drugs (e.g., ivacaftor-class agents) directly target specific mutation classes — a major advance in disease-modifying therapy.
• Aggressive antibiotic therapy for pulmonary exacerbations.

Timeline of Lower Airway Formation

• Epithelial lining of the larynx, trachea, bronchi, and alveoli — endodermal origin.
• Cartilage, smooth muscle, and connective tissue of the airway wall — splanchnic mesoderm origin.
• Five embryologic stages of lung maturation, each with a distinct structural hallmark:
  • Embryonic (wk 4–7): lung buds branch into main/lobar/segmental bronchi.
  • Pseudoglandular (wk 7–17): airway branching complete down to terminal bronchioles; gland-like appearance on histology; lung is non-viable if born in this stage.
  • Canalicular (wk 17–26): respiratory bronchioles form; vascularization increases; type II pneumocytes appear near the end of this stage.
  • Saccular (wk 26–birth): terminal saccules form, thinning of interstitium; surfactant production ramps up.
  • Alveolar (wk 36–~8 yrs postnatal): true alveoli proliferate; most alveolar development is postnatal.

Tracheoesophageal Fistula (TEF) / Esophageal Atresia

• Caused by abnormal partitioning of the tracheoesophageal septum → incomplete separation of trachea from esophagus.
• Most common configuration: proximal esophagus ends blindly (atresia) while the distal esophageal segment communicates with the trachea.
• This is the single most common congenital anomaly of the lower respiratory tract.
• Clinical clues:
  • Copious oral secretions / drooling, choking with first feeds.
  • Inability to pass an orogastric tube — tube coils in the blind pouch.
  • Air visible in the stomach on imaging (because the distal fistula lets air into the GI tract).
  • Recurrent aspiration pneumonia.
• Polyhydramnios results because the fetus cannot swallow amniotic fluid effectively.
• Associated with the VACTERL association (Vertebral, Anal, Cardiac, TE fistula, Renal, Limb anomalies) — always screen for the others if one is found.

Diaphragm Development

• Four embryologic contributors fuse to form the diaphragm:
  1. Septum transversum — becomes the central tendon.
  2. Pleuroperitoneal membranes — close the pericardioperitoneal canals.
  3. Dorsal mesentery of the esophagus — forms the crura.
  4. Body wall musculature — contributes the peripheral rim.
• Innervated by the phrenic nerve (C3–C5) — reflects the diaphragm's cervical origin and explains referred shoulder-tip pain with diaphragmatic irritation.

Conducting Zone vs Respiratory Zone

• Cartilage and mucus-producing goblet cells extend as far as the end of the bronchi; smooth muscle extends slightly further, to the end of the terminal bronchioles (and is sparse beyond that point).
• Cilia are present through the respiratory bronchioles and disappear at the alveolar duct — the mucociliary escalator physically cannot clear debris that reaches the alveoli, which is why alveolar macrophages take over that job distally.

Alveolar Cell Types

• Club (Clara) cells: nonciliated secretory cells of the bronchioles; detoxify inhaled substances via cytochrome P-450, secrete a surfactant-like component, and serve as a bronchiolar stem cell population.
• Alveolar macrophages ("dust cells"): phagocytose inhaled particulates and pathogens; hemosiderin-laden forms ("heart failure cells") accumulate after alveolar hemorrhage or pulmonary edema.
• Pores of Kohn: small openings connecting adjacent alveoli, allowing collateral air flow — but also a route by which infection (e.g., pneumonia) can spread between alveoli.

Gross Lobar Anatomy & the Right/Left Asymmetry

• Right lung: 3 lobes (upper, middle, lower). Left lung: 2 lobes (upper, lower) plus the lingula, a tongue-like projection that is the developmental counterpart of the right middle lobe — the left lung is smaller to make room for the heart.
• At each hilum, the relationship of the pulmonary artery to the bronchus differs by side: the artery sits anterior to the bronchus on the right and superior to it on the left — a fact worth memorizing as a fixed right-anterior/left-superior pairing.
• The right main bronchus is wider, more vertical, and shorter than the left, so aspirated material (and inhaled foreign bodies generally) preferentially enters the right lung — most often the right lower lobe in an upright or supine patient (see Lung Defenses for the aspiration-position specifics).

Diaphragm: Structures at Each Level

• A simple way to keep the order straight: the number of structures at each level roughly tracks the vertebral level itself — 2 at T8 (IVC, phrenic), a couple at T10 (esophagus, vagus — 2 trunks), and 3 at T12 (aorta, thoracic duct, azygos).
• The diaphragm is innervated by the phrenic nerve (C3–C5, per the embryology above); irritation refers pain to the shoulder (C5) and trapezius ridge (C3–C4).
• Phrenic nerve injury (e.g., from a mediastinal tumor) causes elevation of the ipsilateral hemidiaphragm on imaging.

Thoracentesis

• Needle enters just above the rib at the chosen intercostal space, to avoid the neurovascular bundle that runs just beneath each rib.
• Avoid inserting below the 9th rib to reduce the risk of injuring abdominal organs (liver on the right, spleen on the left).

Laplace's Law and the Need for Surfactant

• Small alveoli generate higher collapsing pressure than large ones for the same surface tension — without a stabilizing mechanism, small alveoli would empty into large ones.
• Surfactant lowers surface tension proportionally more in small alveoli (its molecules pack more densely as the alveolus shrinks) → equalizes pressures across alveoli of different sizes → prevents atelectasis.

Surfactant Biology

• Produced by type II pneumocytes — cuboidal, surfactant-storing cells with characteristic lamellar bodies.
• Type II pneumocytes are also the reserve/progenitor cell that repopulates type I pneumocytes (the thin, gas-exchanging cell type) after lung injury.
• Main phospholipid component: dipalmitoylphosphatidylcholine.
• Fetal lung maturity can be estimated from amniotic fluid: a lecithin:sphingomyelin ratio ≥ 2 suggests adequate surfactant production (a ratio < 1.5 predicts a substantial NRDS risk); the foam stability index and surfactant-albumin ratio are additional screening tests used for the same purpose.

Neonatal Respiratory Distress Syndrome (NRDS)

• Risk increases sharply with decreasing gestational age; also increased by maternal diabetes (fetal hyperinsulinemia delays surfactant maturation) and cesarean delivery without labor.
• Antenatal maternal corticosteroids accelerate fetal surfactant production when preterm delivery is anticipated.
• Postnatal management: exogenous surfactant administration, supplemental oxygen, and positive pressure support.
• Complications of prematurity/NRDS management: retinopathy of prematurity (high FiO2), bronchopulmonary dysplasia (barotrauma/oxygen toxicity), intraventricular hemorrhage.
• Widespread alveolar collapse (atelectasis) is itself a direct downstream complication of surfactant deficiency in NRDS.

• Viral rhinitis (common cold): overwhelmingly rhinovirus and coronavirus, also caused by adenovirus and parainfluenza virus; self-limited. In adults, parainfluenza virus itself tends to produce an illness resembling the ordinary common cold, spread by respiratory droplets.
• Bacterial rhinitis: usually a secondary superinfection following an initial viral rhinitis; commonly caused by Streptococcus, Staphylococcus, and H. influenzae.
• Acute sinusitis: usually a viral prodrome; bacterial superinfection (S. pneumoniae, H. influenzae, Moraxella) suspected if symptoms persist beyond ~10 days or worsen after initial improvement ("double worsening").
• Otitis externa: infection of the external auditory canal, classically linked to moisture exposure ("swimmer's ear"); the leading cause is Pseudomonas aeruginosa.
• Otitis media: same bacterial culprits as sinusitis; most common in young children due to a shorter, more horizontal eustachian tube.
• Allergic rhinitis: type I (IgE-mediated) hypersensitivity; nasal eosinophilia on smear.
• Laryngitis: hoarseness from vocal cord inflammation — infectious (often viral) or from vocal overuse.

Nose & Paranasal Sinuses

• Rhinosinusitis: obstruction of normal sinus drainage → inflammation and pain over the affected sinus. The maxillary sinus is most often involved, in part because its drainage opening sits high on its medial wall, so it must drain against gravity. Usually viral at onset (e.g., rhinovirus), with occasional bacterial superinfection.
• Epistaxis: most nosebleeds originate anteriorly, from a rich vascular anastomosis on the septum (the Kiesselbach plexus). Severe, harder-to-control bleeds instead arise more posteriorly, from a branch of the maxillary artery. Common triggers include trauma, dryness/foreign body, and allergic rhinitis.
• Choanal atresia: congenital blockage of the posterior nasal opening; suspect it when a newborn cannot have a catheter passed through the nose into the pharynx. Bilateral cases are a neonatal emergency (infants are obligate nose-breathers) and often occur as part of a broader malformation pattern that also includes eye, cardiac, growth, genitourinary, and ear anomalies.

Croup vs Epiglottitis