Complete Anatomy of the Respiratory System

Overview

1. Ventilation-perfusion matching - delivering O2 to the body and removing CO2
2. Host defense - protecting against airborne pathogens, chemicals, and particulates

1. Upper Airway

Nose

Pharynx

• Nasopharynx - posterior to the nasal cavity; contains the pharyngeal tonsil (adenoids) and the opening of the Eustachian tube
• Oropharynx - from the soft palate to the epiglottis; contains the palatine tonsils
• Laryngopharynx (hypopharynx) - from the epiglottis to the cricoid cartilage

Larynx

• Thyroid cartilage - the largest; forms the "Adam's apple"
• Cricoid cartilage - the only complete ring in the airway; the narrowest part of the adult trachea (avg. 17 mm in men, 13 mm in women)
• Arytenoid cartilages - control vocal cord movement
• Epiglottis - leaf-shaped; prevents aspiration during swallowing

2. Tracheobronchial Tree

Trachea

Mainstem Bronchi

• Right mainstem bronchus: shorter, wider, and lies in a more linear arrangement with the trachea - this is why aspirated foreign bodies and misplaced endotracheal tubes preferentially enter the right side. The right upper lobe bronchus takes off ~2.0 cm (men) / ~1.5 cm (women) from the carina.
• Left mainstem bronchus: longer (~5.0 cm in men, ~4.5 cm in women) and lies at a more angular orientation from the trachea. It divides into the left upper lobe bronchus and left lower lobe bronchus.

Dichotomous Division - 23 Generations

Dichotomous division of airways (left) and segmental bronchi (right). - Morgan & Mikhail's Clinical Anesthesiology

Bronchi vs. Bronchioles

• Bronchi have cartilaginous support in their walls and submucosal glands
• Bronchioles lack cartilage; patency depends on radial traction from surrounding elastic tissue - airway diameter becomes dependent on lung volume
• As the airway progresses distally, mucosa transitions from ciliated columnar epithelium → cuboidal → flat alveolar epithelium

3. Lungs

Gross Structure

• Right lung - 10 bronchopulmonary segments:
  • Upper lobe: apical, anterior, posterior
  • Middle lobe: medial, lateral
  • Lower lobe: superior, anterior basal, medial basal, lateral basal, posterior basal
• Left lung - 8-10 bronchopulmonary segments:
  • Upper lobe: apico-posterior, anterior, superior lingular, inferior lingular
  • Lower lobe: superior, antero-medial basal, lateral basal, posterior basal

Pleura

• Visceral pleura - directly covers the lung surface; has no pain fibers
• Parietal pleura - lines the inner chest wall, diaphragm, and mediastinum; richly innervated (pain-sensitive)
• Pleural space - a potential space between the two layers containing a thin film of fluid (~10-20 mL); the resting intrapleural pressure is approximately -5 cm H2O (subatmospheric), which keeps the lungs expanded against the chest wall

4. Alveoli and the Blood-Air Barrier

Alveolar Wall Structure

Cross-section of alveolus showing the thin (gas-exchange) side and interstitial space. Epithelium, basement membrane, and endothelium layers are visible. - Morgan & Mikhail's Clinical Anesthesiology

• Thin side (<0.4 μm thick): alveolar epithelium + fused basement membranes + capillary endothelium - this is where O2 and CO2 diffuse
• Thick side (1-2 μm): contains a true pulmonary interstitial space (elastin, collagen, nerve fibers) - where fluid and solute exchange occurs; provides structural support

Alveolar Cell Types

Surfactant

5. Pulmonary Circulation and Lymphatics

Dual Blood Supply

• The pulmonary artery from the right ventricle carries deoxygenated blood, divides into right and left branches supplying each lung
• Blood passes through the pulmonary capillaries (O2 taken up, CO2 eliminated)
• Four pulmonary veins (2 from each lung) return oxygenated blood to the left atrium
• Pulmonary arteries and veins have thinner walls with less smooth muscle than systemic vessels
• Normal mean pulmonary arterial pressure: ~15 mmHg (much lower than systemic ~100 mmHg)

• Arises from the thoracic aorta and intercostal arteries
• Supplies <4% of cardiac output
• Sustains metabolic needs of the tracheobronchial tree down to the terminal bronchioles
• Anastomoses with pulmonary arterial circulation along the airways

Alveolar vs. Extra-alveolar Vessels

• Extra-alveolar vessels lie in loose peribronchovascular connective tissue; expand as lung volume increases
• Alveolar vessels (capillaries) lie within the alveolar walls and are subject to alveolar pressure

Pulmonary Lymphatics

• Superficial (pleural) network - along visceral pleura
• Deep (peribronchial) network - along airways and blood vessels

6. Rib Cage and Muscles of Respiration

Rib Cage

• Contains both lungs, each surrounded by its own pleura
• The apex allows entry of the trachea, esophagus, and great vessels
• The base is formed by the diaphragm
• Each rib (except the last two) articulates posteriorly with a vertebra and is angulated downward as it attaches anteriorly to the sternum; upward and outward rib movement expands the chest

Primary Muscles of Inspiration

Accessory Muscles of Inspiration (recruited during increased demand)

• Sternocleidomastoid - elevates the rib cage
• Scalene muscles - prevent inward displacement of upper ribs
• Pectoralis muscles - assist chest expansion when arms are on a fixed support

Muscles of Expiration

• Expiration is passive during normal breathing (elastic recoil of the lung and chest wall)
• Becomes active in the upright position and with increased effort:
  • Abdominal muscles (rectus abdominis, external and internal oblique, transversus) - compress the abdomen and assist downward movement of the ribs
  • Internal intercostals - aid downward rib movement

7. Neural Control of Respiration (Ventilatory Pump)

Central Nervous System Controllers

• Cerebral cortex (parietal cortex): controls voluntary breathing; projects to motor neurons via corticospinal tracts
• Brainstem: controls automatic breathing
  • Pontine center (pneumotaxic center): fine-tunes rhythm
  • Dorsal respiratory group (DRG) in medulla: drives inspiration
  • Ventral respiratory group (VRG) in medulla: active in both inspiration and expiration

Spinal Cord Pathways

• Corticospinal tract - carries voluntary breathing signals
• Reticulospinal tract - carries automatic breathing signals
• These are separate pathways; damage to one can preserve the other (explains "Ondine's curse" - loss of automatic breathing while voluntary is preserved)

Motor Innervation of Respiratory Muscles

Feedback Mechanisms

• Central chemoreceptors (medullary ventral surface): respond primarily to changes in cerebrospinal fluid pH/PCO2
• Peripheral chemoreceptors (carotid and aortic bodies): respond to hypoxemia (PaO2 <60 mmHg), hypercapnia, and acidosis
• Pulmonary stretch receptors: in airway smooth muscle; Hering-Breuer reflex (inhibit inspiration when lung is over-inflated)
• J-receptors (juxtacapillary receptors): in alveolar walls; respond to pulmonary edema and embolism, causing rapid shallow breathing and dyspnea

8. Cellular Anatomy of the Airway Wall

Conducting Airway Epithelium (Pseudostratified columnar)

Bronchial Wall Layers (from lumen outward)

1. Mucosa - epithelium + basement membrane
2. Lamina propria - contains lymphoid tissue, mast cells, and immune cells
3. Smooth muscle - arranged in spiral bands; regulates airway diameter
4. Submucosa - contains seromucous glands (between smooth muscle bands and cartilaginous plates)
5. Cartilaginous plates (in bronchi only)
6. Adventitia

9. Connective Tissue Framework (Elastic Continuum)

• Collagen fibers: provide tensile strength, resist overdistension
• Elastin fibers: provide elastic recoil, allow the lung to return to its resting volume after inspiration
• Glycosaminoglycans (proteoglycans): form a gel-like matrix in the interstitium; also regulate epithelial cell phenotype, cytokine signaling, and cell behavior

• Axial fibers: from the hilum outward along airways and blood vessels
• Peripheral fibers: from the visceral pleura inward
• Parenchymal fibers: interconnect the alveolar walls

Summary: Functional Zones

• Murray & Nadel's Textbook of Respiratory Medicine, 2-Volume - Chapters 1 (Anatomy) and 130 (Neuromuscular)
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter 23 (Functional Respiratory Anatomy)

Classification of Pneumonia

I. Classification by Epidemiological Setting (Most Clinically Used)

1. Community-Acquired Pneumonia (CAP)

• Most common serious infectious cause of hospitalization and death in the United States
• Hospitalization rates increase exponentially with age (1-2/1000 in young adults vs. ~40/1000 in adults >85 years)
• Seasonal peaks November-March, coinciding with influenza and respiratory viral activity

2. Hospital-Acquired Pneumonia (HAP)

• Incidence: 5-10 cases per 1000 hospital admissions
• Second most common cause of hospital-acquired infection
• Caused predominantly by multidrug-resistant (MDR) organisms

• S. aureus (10-20%; 50% of which are MRSA)
• Pseudomonas aeruginosa (10-20%; 28-35% resistant to cefepime)
• Enteric Gram-negative bacilli (20-40%; 19-29% resistant to piperacillin-tazobactam)
• Acinetobacter baumannii (5-10%; 56-61% resistant to carbapenems)

3. Ventilator-Associated Pneumonia (VAP)

• Occurs in 16% of patients ventilated for more than 1 day
• Overall mortality ~13%
• The CDC/NHSN defines a surveillance algorithm with escalating categories:

4. Healthcare-Associated Pneumonia (HCAP) - Historical

• Hospitalization ≥2 days in the past 90 days
• Broad-spectrum antibiotics within past 3 months
• Immune suppression / corticosteroids
• Prior respiratory isolation of MRSA or P. aeruginosa
• Residence in a nursing home / long-term care facility

II. Classification by Causative Agent (Etiological)

A. Bacterial Pneumonia

B. Viral Pneumonia

• Influenza A and B (most common in adults)
• SARS-CoV-2 (COVID-19 pneumonia)
• Respiratory syncytial virus (RSV) - especially in elderly and immunocompromised
• Parainfluenza, adenovirus, human metapneumovirus
• Cytomegalovirus (CMV) - in immunocompromised

C. Fungal Pneumonia

• Endemic fungi: Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis
• Opportunistic fungi: Pneumocystis jirovecii (PCP - in HIV/AIDS, CD4 <200), Aspergillus, Candida, Cryptococcus neoformans
• Pneumocystis jirovecii: ubiquitous organism; colonizes >50% of adults; causes diffuse, bilateral, subacute but severe pneumonia in immunocompromised patients

D. Parasitic Pneumonia

• Toxoplasma gondii, Strongyloides stercoralis, Paragonimus spp.
• Generally seen in immunocompromised or tropical settings

III. Classification by Anatomical / Radiological Pattern

1. Lobar Pneumonia

• Consolidation involving an entire lobe or major segment
• Classic example: Streptococcus pneumoniae
• Pathological stages: congestion → red hepatization → grey hepatization → resolution
• Radiograph: homogeneous lobar or segmental consolidation, often with air bronchograms

2. Bronchopneumonia (Lobular Pneumonia)

• Patchy, multifocal consolidation centered on bronchioles, spreading to alveoli
• Bilateral, often basal predominance
• Common with S. aureus, Gram-negative bacilli, H. influenzae
• Often seen in debilitated patients, the elderly, post-surgical patients, and aspiration

3. Interstitial Pneumonia (Atypical Pneumonia Pattern)

• Inflammation primarily in the interstitium (walls of alveoli and bronchioles)
• Bilateral, diffuse, reticulonodular or ground-glass pattern on chest X-ray
• Classic "atypical" organisms: Mycoplasma pneumoniae, Chlamydophila, Legionella, viruses
• May also be seen with Pneumocystis jirovecii (bilateral perihilar ground-glass)

IV. Classification by Typical vs. Atypical Presentation

Note: The typical/atypical distinction is a clinical guide, not absolute. Legionella is "atypical" by organism but can present quite severely with lobar consolidation. Co-infections are also common.

V. Classification by Host Immune Status

Immunocompetent Host

Immunocompromised Host

• HIV/AIDS (CD4 <200): Pneumocystis jirovecii, Cryptococcus, CMV, M. tuberculosis, MAC
• Neutropenic patients (chemotherapy, hematologic malignancy): Aspergillus spp., Candida, Gram-negative bacilli, Mucor
• Solid organ transplant: CMV, Aspergillus, Pneumocystis jirovecii, Nocardia
• Corticosteroid therapy: Aspergillus, P. jirovecii, atypical mycobacteria

VI. Classification by Severity (Site-of-Care Decision)

CURB-65 Score (simpler, commonly used)

Pneumonia Severity Index (PSI / PORT Score)

• Stratifies by 20 variables (age, comorbidities, vital signs, labs, imaging)
• Five classes: Class I-II (outpatient), Class III (borderline - observation), Class IV-V (hospital/ICU)
• Mortality: Class I = 0.1%; Class II = 0.6%; Class III = 2.8%; Class IV = 8.2%; Class V = 29.2%
• More validated than CURB-65 but more complex to calculate

IDSA/ATS Severe CAP Criteria

• Hypotension requiring vasopressors (septic shock)
• Respiratory failure requiring mechanical ventilation

• Respiratory rate ≥30 breaths/min
• PaO2/FiO2 ≤250
• Multilobar infiltrates
• Confusion/disorientation
• BUN ≥20 mg/dL (uremia)
• WBC <4000 cells/µL (leukopenia)
• Platelet count <100,000/µL
• Core temperature <36°C (hypothermia)
• Hypotension requiring IV fluid resuscitation

VII. Special Clinical Syndromes

Aspiration Pneumonia

• Caused by macroaspiration of oropharyngeal contents (often less virulent organisms requiring a large inoculum)
• Distinguished from aspiration pneumonitis (chemical inflammation from gastric acid without infection - usually rapid recovery)
• Risk factors: stroke, esophageal disorders, impaired consciousness, alcoholism, poor dentition, acid-suppressive medications
• Community setting: normal oral flora - anaerobes, Streptococcus species
• Hospital setting: nosocomial organisms added - Gram-negative bacilli, S. aureus
• Typically affects dependent segments: posterior segment of right upper lobe (supine) and right lower lobe (upright)
• Complications: necrotizing pneumonia, lung abscess (cavity >1 cm), empyema - especially with anaerobes, S. aureus, Klebsiella, Pseudomonas

Necrotizing Pneumonia / Lung Abscess

• Multiple cavities ≤1 cm = necrotizing pneumonia
• One or more cavities >1 cm communicating with a bronchus = lung abscess
• Organisms: anaerobes (indolent), S. aureus, Klebsiella pneumoniae, P. aeruginosa (acute)
• CA-MRSA can cause a severe, necrotizing post-influenza pneumonia via exotoxin (Panton-Valentine leukocidin)

Summary Diagram

PNEUMONIA
│
├── By SETTING
│   ├── Community-Acquired (CAP)
│   ├── Hospital-Acquired (HAP) - ≥48h after admission
│   └── Ventilator-Associated (VAP) - ≥72h after intubation
│
├── By ETIOLOGY
│   ├── Bacterial (typical & atypical)
│   ├── Viral
│   ├── Fungal
│   └── Parasitic
│
├── By ANATOMY / RADIOLOGY
│   ├── Lobar (consolidation)
│   ├── Bronchopneumonia (patchy/lobular)
│   └── Interstitial (diffuse bilateral)
│
├── By HOST
│   ├── Immunocompetent
│   └── Immunocompromised (HIV, neutropenic, transplant)
│
├── By SEVERITY
│   ├── Non-severe (outpatient/ward)
│   └── Severe (ICU) — CURB-65 ≥3 or IDSA/ATS criteria
│
└── Special Syndromes
    ├── Aspiration pneumonia / pneumonitis
    ├── Necrotizing pneumonia / lung abscess
    └── Opportunistic pneumonia (PCP, Aspergillus, CMV)

• Fishman's Pulmonary Diseases and Disorders, 2-Volume Set - Ch. 122, 125
• Goldman-Cecil Medicine, International Edition - Ch. 84
• Current Surgical Therapy, 14e - Hospital-Acquired and Ventilator-Associated Pneumonia
• Harrison's Principles of Internal Medicine, 22e - Community-Acquired Pneumonia
• Frameworks for Internal Medicine - Aspiration Pneumonia