Respiratory system clinical anatomy

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Respiratory System - Clinical Anatomy

Overview

The respiratory system is divided into the upper airway (nose, mouth, pharynx, larynx) and the lower airway (trachea, bronchial tree, lungs). Clinically, anatomy is best organized into four functional units: the thoracic cage & muscles, the tracheobronchial tree, the alveolar unit, and the pulmonary circulation.

1. Thoracic Cage & Muscles of Respiration

The rib cage houses both lungs, each surrounded by its own pleura. The apex is small - admitting only the trachea, esophagus, and great vessels. The base is formed by the diaphragm.

Diaphragm

The diaphragm is the principal muscle of respiration, responsible for 75% of the change in chest volume during quiet breathing. Its contraction causes the thoracic base to descend 1.5 to 7 cm, expanding lung volume.

Accessory Muscles

Muscle Group	Role
External intercostals	Assist inspiration (upward/outward rib movement)
Sternocleidomastoid	Elevates rib cage during forced inspiration
Scalene muscles	Prevent inward displacement of upper ribs
Pectoralis (with fixed arms)	Assists chest expansion
Abdominal muscles (rectus, obliques, transversus)	Active expiration
Internal intercostals	Assist downward rib movement (expiration)

Clinical point: Expiration is passive in the supine position but becomes active in the upright position and with increased effort.

Pharyngeal Muscles & Airway Patency

The genioglossus maintains tonic inspiratory activity to keep the tongue away from the posterior pharyngeal wall. The levator palati, tensor palati, palatopharyngeus, and palatoglossus prevent the soft palate from falling back, especially in the supine position. Loss of this tone is a major factor in obstructive sleep apnea.

2. Tracheobronchial Tree

Trachea

Begins at the lower border of the cricoid cartilage (the narrowest part of the adult airway: ~17 mm in men, ~13 mm in women)
Extends to the carina - average length 10-13 cm
Composed of C-shaped cartilaginous rings - anterior and lateral walls are cartilaginous; posterior wall is membranous
Bifurcates at the sternal angle (angle of Louis) into right and left mainstem bronchi

Right vs. Left Mainstem Bronchus

Feature	Right	Left
Angle with trachea	More linear (steeper takeoff ~25°)	More angular (~45°)
Length to first branch	~2.0 cm (men), ~1.5 cm (women) to RUL	5.0 cm (men), 4.5 cm (women)
Clinical implication	Foreign bodies more commonly enter right bronchus	Longer - more time for endobronchial tube malposition

Clinical point: Because the right mainstem bronchus is more in line with the trachea, aspirated foreign bodies, malpositioned endotracheal tubes, and inadvertent endobronchial intubation most commonly affect the right lung.

Dichotomous Branching - 23 Generations

Tracheobronchial tree: dichotomous branching (left) and segmental bronchi (right)

The airway divides through approximately 23 generations from trachea (generation 0) to alveolar sac (generation 23):

Zone	Generations	Structures	Function
Conducting zone	0-16	Trachea → bronchi → bronchioles → terminal bronchioles	Gas conduction only (anatomical dead space)
Transitional & respiratory zone	17-23	Respiratory bronchioles → alveolar ducts → alveolar sacs	Gas exchange begins

Each generation approximately doubles the number of airways
Cartilaginous support disappears at the bronchiolar level; smaller airways then depend on radial traction from surrounding parenchymal elastic recoil to stay patent
Mucosa transitions from ciliated columnar epithelium (upper airways) → cuboidal → flat type I alveolar epithelium (gas exchange zones)

Lobar & Segmental Anatomy

Right lung: 3 lobes (upper, middle, lower), 10 segments
- Upper lobe: Apical, Anterior, Posterior
- Middle lobe: Lateral, Medial
- Lower lobe: Superior, Anterior basal, Medial basal, Lateral basal, Posterior basal
Left lung: 2 lobes (upper, lower), 8-9 segments (apical and posterior are fused as apico-posterior; lingual replaces middle lobe)
- Upper lobe: Apico-posterior, Anterior, Superior lingular, Inferior lingular
- Lower lobe: Superior, Antero-medial basal, Lateral basal, Posterior basal

3. The Alveolar Unit

There are an estimated 300-500 million alveoli, providing a gas-exchange surface area of 50-100 m².

Alveolar Cell Types

Cell	Description	Function
Type I pneumocyte	Flat, form tight 1-nm junctions	Primary gas-exchange surface; barrier prevents albumin leak
Type II pneumocyte	Round, contain lamellar bodies; more numerous	Produce surfactant; can divide and regenerate type I cells
Pulmonary alveolar macrophages	Mobile	Innate immune defense
APUD cells, mast cells, lymphocytes	Present in lower airways	Immune/neuroendocrine functions

Clinical point: Type II pneumocytes are the progenitor cells of the alveolar epithelium. Surfactant deficiency (as in premature neonates) leads to neonatal respiratory distress syndrome. Loss of type I cells in injury (e.g., ARDS) must be repaired via type II cell differentiation.

Alveolar-Capillary Membrane

The blood-gas barrier consists of (thin to thick sides):

Flat type I pneumocyte + fused basement membranes + capillary endothelium (thin "gas-exchanging" side)
A thick side containing the interstitial space (fluid accumulates here first in pulmonary edema before flooding alveoli)

4. Pulmonary Circulation & Lymphatics

The lungs receive a dual blood supply:

Pulmonary Circulation (Gas Exchange)

Right ventricle → pulmonary artery → right and left branches → pulmonary capillaries → four pulmonary veins → left atrium
Pulmonary capillaries average ~10 μm in diameter - barely enough for a single red cell to pass
Pulmonary vessels have thinner walls and less smooth muscle than systemic vessels - hence lower resistance and pressures (normal mPAP ~15 mmHg vs. systemic ~93 mmHg)

Bronchial Circulation (Nutrition)

Arises from the thoracic aorta and intercostal arteries
Supplies the walls of the tracheobronchial tree as far as the terminal bronchioles
Accounts for <4% of cardiac output
Anastomoses with the pulmonary circulation - contributes to normal venous admixture ("physiological shunt")

Lymphatics

The lungs have an extensive lymphatic network draining to hilar and mediastinal nodes
Clinically relevant in pulmonary edema (lymphatics can compensate up to a point before fluid floods the interstitium and alveoli), and in spread of lung carcinoma

5. Respiratory Control - Neuroanatomy

The ventilatory pump is regulated by a distributed neural network:

Component	Location	Function
Voluntary control	Parietal cortex → corticospinal tracts	Volitional breathing (speech, breath-holding)
Automatic control	Pons (pneumotaxic & apneustic centers) + medulla (pre-Bötzinger complex)	Rhythm and drive to breathe
Spinal cord	Reticulospinal tracts → anterior horn cells	Conducts impulses to respiratory muscles
Phrenic nerve	C3, C4, C5 ("C3, 4, 5 keeps the diaphragm alive")	Diaphragm innervation
Feedback receptors	Peripheral chemoreceptors (carotid/aortic bodies), central chemoreceptors (medulla), lung stretch receptors	Fine-tuning based on PaO₂, PaCO₂, pH, and lung volume

Clinical point: Cervical spinal cord injuries at or above C3 will paralyze the diaphragm and require mechanical ventilation. Injuries between C3-C5 may allow partial diaphragm function.

6. Key Clinical Correlates

Anatomical Feature	Clinical Significance
Right mainstem bronchus more vertical	Right-sided endobronchial intubation; right-sided aspiration pneumonia
Cricoid - narrowest adult airway	Cricoid pressure (Sellick maneuver) to prevent aspiration; narrowest in children is subglottic
Diaphragm - phrenic nerve (C3-C5)	High cervical cord injury → respiratory failure
Type II pneumocytes produce surfactant	RDS in prematurity; loss in ARDS
Pulmonary capillaries ~10 μm	Fat emboli, thromboemboli can obstruct
Pleural space is potential space	Pneumothorax, hemothorax, pleural effusion all occur here
23 airway generations	Generations 0-16 = anatomical dead space (~150 mL in adults)
Right lung 3 lobes, Left lung 2 lobes	Surgical resections: pneumonectomy, lobectomy, segmentectomy

Sources:

Morgan and Mikhail's Clinical Anesthesiology, 7e - Functional Respiratory Anatomy, pp. 910-915
Murray & Nadel's Textbook of Respiratory Medicine - Functional Anatomy of the Respiratory System, Ch. 130

Clinical physiology of respiratory system

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Clinical Physiology of the Respiratory System

Overview

Respiratory physiology encompasses the mechanics of breathing, gas exchange, lung volumes, ventilation-perfusion relationships, oxygen transport, control of breathing, and the response to pathological states. Each topic has direct clinical implications.

1. Mechanics of Breathing

Inspiration (Active Process)

Muscular contraction causes the diaphragm to descend and ribs to pivot outward, expanding the thoracic cavity. This generates negative intrapleural pressure (drops from -5 to -8 or -9 cm H₂O during inspiration), which lowers alveolar pressure below atmospheric, establishing a gradient that drives airflow inward.

Transpulmonary pressure (P_TP):

P_TP = P_alveolar - P_intrapleural

At end-expiration: P_TP = 0 - (-5) = +5 cm H₂O

Expiration (Passive at Rest)

Elastic recoil of the previously expanded lung parenchyma drives gas out. No muscular work is required during quiet breathing. Active expiration recruits abdominal muscles and internal intercostals.

Clinical point: Normal spontaneous breathing spends more time in exhalation (Te) than inspiration (Ti) because a compliant lung fills quickly. An I:E ratio of 1:2 is the normal baseline.

Minute Ventilation (V̇E)

V̇E = Respiratory Rate × Tidal Volume

Normal = 6-8 L/min at rest.

2. Elastic Properties - Compliance & Surface Tension

Compliance

Compliance (C) = ΔVolume / ΔPressure - the ease of lung distension.

Component	Normal Value	Notes
Lung compliance (CL)	150-200 mL/cm H₂O	Reduced by fibrosis, edema, ARDS
Chest wall compliance (CW)	200 mL/cm H₂O	Reduced by obesity, ascites, supine position
Total compliance (lung + chest wall)	~100 mL/cm H₂O	1/C_total = 1/CL + 1/CW

Clinical point: A stiff lung (low compliance) requires greater effort or pressure to achieve the same tidal volume. In ARDS, lung compliance drops dramatically - ventilating these patients with low tidal volumes (6 mL/kg IBW) protects against barotrauma.

Surface Tension & Surfactant

The air-fluid interface in alveoli behaves like a bubble, obeying Laplace's Law:

Pressure = 2 × Surface tension / Radius

Without surfactant, smaller alveoli (higher pressure due to smaller radius) would empty into larger ones - causing alveolar collapse. Surfactant (produced by type II pneumocytes) reduces surface tension in proportion to its concentration:

As alveoli shrink → surfactant becomes more concentrated → surface tension falls → prevents further collapse
As alveoli enlarge → surfactant becomes more dilute → surface tension rises → prevents overdistension

This mechanism stabilizes alveolar size and prevents atelectasis.

Clinical point: Surfactant deficiency in premature neonates causes Neonatal Respiratory Distress Syndrome (NRDS). In ARDS, damaged type II cells cannot maintain surfactant, worsening alveolar collapse.

3. Lung Volumes & Capacities

Parameter	Abbreviation	Normal (Adult)	Definition
Tidal Volume	VT	~500 mL	Volume per normal breath
Inspiratory Reserve Volume	IRV	~3000 mL	Extra volume above VT
Expiratory Reserve Volume	ERV	~1200 mL	Extra volume exhaled beyond VT
Residual Volume	RV	~1200 mL	Volume remaining after maximal exhalation (cannot be measured by spirometry)
Inspiratory Capacity	IC	VT + IRV	Max inspired from FRC
Functional Residual Capacity	FRC	ERV + RV (~2400 mL)	Volume at end-expiration at rest
Vital Capacity	VC	IRV + VT + ERV (~4600 mL)	Max volume from full expiration to full inspiration
Total Lung Capacity	TLC	~6000 mL	All four volumes combined
Forced Expiratory Volume (1 sec)	FEV₁	>80% of FVC	Key spirometric parameter
FEV₁/FVC ratio	-	>0.7 (>70%)	<70% = obstructive defect

Clinical point: FRC is the lung volume at rest where elastic inward recoil of the lung balances elastic outward recoil of the chest wall. FRC falls in supine position, obesity, pregnancy, and with general anesthesia (by ~15-20%). A reduced FRC below closing capacity (CC) causes small airway closure during tidal breathing, leading to intrapulmonary shunting and hypoxemia.

Obstructive vs. Restrictive Patterns

Feature	Obstructive (e.g., COPD, Asthma)	Restrictive (e.g., fibrosis, obesity)
FVC	Normal or ↓	↓↓
FEV₁	↓↓	↓
FEV₁/FVC	<0.7	Normal or ↑
TLC	↑ (air trapping)	↓
RV	↑	↓

4. Airway Resistance

Normal total airway resistance: 0.5-2 cm H₂O/L/s

The largest contribution comes from medium-sized bronchi (generation 4-7) - not from large or small airways.

Flow Types

Laminar flow: Concentric cylinders; governed by Hagen-Poiseuille law (resistance ∝ 1/r⁴); occurs in small airways (<1 mm)
Turbulent flow: Random movement; resistance ∝ gas density / r⁵; occurs in large airways at high flow rates

Reynolds number predicts flow type:

Re = (velocity × diameter × density) / viscosity

Re < 1000 → laminar
Re > 1500 → turbulent

Clinical point: Helium has a lower density-to-viscosity ratio than oxygen. A Heliox (He-O₂) mixture reduces turbulent flow and airway resistance in upper airway obstruction (e.g., croup, subglottic stenosis) - a useful temporizing bridge.

Factors Increasing Airway Resistance

Bronchospasm (asthma)
Mucosal edema (anaphylaxis, infection)
Secretions
Low lung volume (loss of radial traction on small airways) - corrected by PEEP

5. Ventilation-Perfusion (V̇/Q̇) Relationships

The cornerstone of clinical respiratory physiology.

Normal V̇/Q̇ = 0.8

(Total alveolar ventilation ~4 L/min; total perfusion ~5 L/min)

Gravity-Dependent Zonal Differences (West's Zones)

V/Q distribution in the lung by zone - apex to base

In the upright lung:

Zone	Location	V̇/Q̇	PaO₂	PaCO₂	Mechanism
Zone 1 (apex)	Upper	3.0 (highest)	130 mmHg	28 mmHg	Minimal perfusion; relative over-ventilation
Zone 2 (mid)	Middle	~1.0	-	-	Balanced ventilation and perfusion
Zone 3 (base)	Lower	0.6 (lowest)	89 mmHg	42 mmHg	Highest perfusion; relative under-ventilation

Clinical relevance: Tuberculosis preferentially affects the lung apex (Zone 1 - high V̇/Q̇, high O₂ tension) because M. tuberculosis is an obligate aerobe that thrives in oxygen-rich environments.

V̇/Q̇ Extremes - Spectrum of Defects

V̇/Q̇	Condition	Cause	Gas exchange effect
V̇/Q̇ = ∞	Dead space	No perfusion (PE, shock)	PAO₂ = 150 mmHg; PACO₂ = 0; wasted ventilation
V̇/Q̇ high	High V̇/Q̇	Reduced perfusion	High PO₂, low PCO₂ in capillary blood
V̇/Q̇ = 0.8	Normal	Matched V/Q	Normal gas exchange
V̇/Q̇ low	Low V̇/Q̇	Reduced ventilation (mucus plugging, partial obstruction)	Low PO₂, high PCO₂ in capillary blood
V̇/Q̇ = 0	Shunt	No ventilation (consolidation, atelectasis, R→L cardiac shunt)	PaO₂ = 40 mmHg; PaCO₂ = 46 mmHg (same as mixed venous)

Key Clinical Distinction: Dead Space vs. Shunt

Dead space (high V̇/Q̇): hypercapnia dominates; CO₂ rises because it cannot be eliminated. Example: pulmonary embolism
Shunt (V̇/Q̇ = 0): hypoxemia that is refractory to supplemental O₂ (blood bypasses alveoli entirely). Example: lobar pneumonia, ARDS, ASD with Eisenmenger

Key point: Low V̇/Q̇ (not absolute shunt) can be at least partially corrected by increasing FiO₂. True shunt cannot - this is the clinical basis for the 100% O₂ test.

Venous Admixture (Physiologic Shunt Equation)

$$\frac{\dot{Q}_S}{\dot{Q}_T} = \frac{Cc'O_2 - CaO_2}{Cc'O_2 - C\bar{v}O_2}$$

Where Cc'O₂ = ideal pulmonary end-capillary O₂ content; CaO₂ = arterial O₂ content; CvO₂ = mixed venous O₂ content

Normal shunt fraction = <5%

6. Gas Exchange & the Alveolar Gas Equation

Alveolar Gas Equation

$$P_AO_2 = F_iO_2 \times (P_{atm} - P_{H_2O}) - \frac{P_aCO_2}{R}$$

Where R = respiratory quotient (~0.8); PH₂O = 47 mmHg at 37°C

At sea level breathing room air (FiO₂ = 0.21):

PAO₂ = 0.21 × (760 - 47) - 40/0.8 = ~100 mmHg

Alveolar-Arterial (A-a) Gradient

A-a gradient = PAO₂ - PaO₂

Normal A-a gradient: 5-15 mmHg in young adults (increases with age and FiO₂)
Estimated normal: ~(Age/4) + 4 mmHg

Elevated A-a gradient causes of hypoxemia:

V̇/Q̇ mismatch
Right-to-left shunt
Diffusion impairment (rare at rest; occurs with exercise or thickened membrane)

Normal A-a gradient causes of hypoxemia:

Hypoventilation (PaCO₂ rises, O₂ is displaced from alveoli)
Low inspired FiO₂ (altitude)

Diffusion

Gas exchange follows Fick's Law of Diffusion:

Volume of gas transferred ∝ (Area × ΔP × Diffusion coefficient) / Thickness

CO₂ diffuses 20× faster than O₂ despite similar partial pressure gradients
Therefore, diffusion impairment causes hypoxemia before hypercapnia
DLCO (diffusing capacity for CO) is the clinical test - reduced in emphysema, ILD, pulmonary hypertension

7. Oxygen Transport

The Oxygen Cascade

PO₂ falls stepwise: Atmosphere (160 mmHg) → Trachea (149 mmHg) → Alveolus (~100 mmHg) → Arterial blood (~95 mmHg) → Tissue (~40 mmHg) → Mitochondria (~5 mmHg)

Oxygen Content of Blood (CaO₂)

CaO₂ = (Hb × 1.34 × SaO₂) + (0.003 × PaO₂)

Hb-bound O₂ dominates (1.34 mL O₂/g Hb)
Dissolved O₂ (0.003 × PaO₂) is minor under normal conditions but critical with hyperbaric O₂ therapy

Oxygen-Hemoglobin Dissociation Curve

The sigmoidal shape has major clinical implications:

Segment	Clinical significance
Flat upper portion (PaO₂ >60 mmHg)	SaO₂ stays >90% even with significant drop in PaO₂ - protects against mild hypoxemia
Steep lower portion (PaO₂ <60 mmHg)	Small drop in PaO₂ causes large drop in SaO₂ - the "cliff" below which O₂ delivery crashes

Bohr Effect - Rightward shift (↑ P50, ↓ O₂ affinity, ↑ O₂ unloading at tissues):

↑ Temperature, ↑ PCO₂, ↑ [H⁺] (acidosis), ↑ 2,3-DPG
Physiologically appropriate during exercise (tissues get more O₂)

Leftward shift (↓ P50, ↑ O₂ affinity, ↓ tissue unloading):

↓ Temperature, ↓ PCO₂, alkalosis, ↓ 2,3-DPG, fetal Hb (HbF), CO poisoning, methemoglobinemia

Clinical point: Stored blood has low 2,3-DPG, shifting the curve left - massive transfusion patients may have impaired O₂ delivery to tissues despite adequate hemoglobin.

Causes of Hypoxemia - Summary Table

Mechanism	PaO₂	PaCO₂	A-a gradient	Response to O₂
Hypoventilation	↓	↑	Normal	Good
Low FiO₂ (altitude)	↓	↓	Normal	Good
V̇/Q̇ mismatch (low V̇/Q̇)	↓	Normal/↑	↑	Partial
Shunt (V̇/Q̇ = 0)	↓↓	Normal	↑	Poor
Diffusion impairment	↓ (exercise)	Normal	↑	Good

8. Control of Breathing

Central Controllers

Center	Location	Function
Pre-Bötzinger complex	Medulla	Primary rhythm generator (automatic breathing)
Dorsal respiratory group	Medulla	Inspiration timing
Ventral respiratory group	Medulla	Expiration and forced breathing
Pneumotaxic center	Upper pons	Limits inspiration, promotes rate
Apneustic center	Lower pons	Prolongs inspiration
Cortex	Parietal lobe	Voluntary breathing (speech, breath-holding)

Chemoreceptors

Central chemoreceptors (medulla):

Respond to CO₂/pH in CSF (CO₂ crosses BBB and forms H⁺ + HCO₃⁻)
Primary driver of ventilatory drive under normal conditions
Not directly sensitive to O₂

Peripheral chemoreceptors (carotid & aortic bodies):

Respond to ↓ PaO₂ (primary), ↑ PaCO₂, ↑ [H⁺]
Carotid bodies are the more important clinical ones (glossopharyngeal nerve, CN IX)
Drive the hypoxic ventilatory response - activated when PaO₂ falls below ~60 mmHg

Critical clinical point: In COPD patients with chronic hypercapnia, central chemoreceptors become desensitized to CO₂. These patients rely on the hypoxic drive (peripheral chemoreceptors) to maintain ventilation. Giving high-flow O₂ without monitoring can blunt this drive and cause CO₂ narcosis. Target SpO₂ 88-92% in known hypercapnic COPD.

Mechanical Receptors

Pulmonary stretch receptors (Hering-Breuer reflex): Activated by lung inflation; inhibit further inspiration - prevent over-inflation
Irritant receptors: Respond to dust, smoke, noxious gases → cough, bronchoconstriction
J-receptors (juxtacapillary): Stimulated by pulmonary congestion, edema → rapid shallow breathing and dyspnea in heart failure

9. Hypoxic Pulmonary Vasoconstriction (HPV)

A unique property of pulmonary vessels (opposite to systemic circulation):

Low alveolar PO₂ → local vasoconstriction → diverts blood away from poorly ventilated areas → maintains V̇/Q̇ matching
Mediators: inhibition of K⁺ channels, membrane depolarization, Ca²⁺ influx in pulmonary arterial smooth muscle
HPV is inhibited by volatile anesthetic agents (clinically relevant during one-lung ventilation)

Clinical point: In chronic hypoxia (e.g., high altitude, COPD), generalized HPV → global pulmonary hypertension → right ventricular hypertrophy → cor pulmonale.

10. CO₂ Transport

CO₂ is transported in three forms:

Form	% of Total	Notes
Bicarbonate (HCO₃⁻)	~70%	CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (carbonic anhydrase in RBCs); HCO₃⁻ exits RBC in exchange for Cl⁻ (chloride shift)
Dissolved CO₂	~10%	PaCO₂ = 40 mmHg
Carbamino compounds	~20%	CO₂ bound to Hb and plasma proteins

Haldane Effect: Deoxygenated Hb binds CO₂ more efficiently than oxygenated Hb - explains why venous blood carries more CO₂ than arterial blood at the same PCO₂.

Clinical Summary: Putting It Together

Physiological Concept	Clinical Condition	Key Mechanism
Low compliance	ARDS, fibrosis	High ventilating pressures needed; risk of barotrauma
Surfactant loss	NRDS, ARDS	Alveolar collapse, worsened by Laplace
Dead space ↑	Pulmonary embolism	Wasted ventilation, ↑ PaCO₂, ↑ A-a gradient
Shunt	Consolidation, atelectasis	Hypoxemia refractory to O₂
Low V̇/Q̇	COPD, asthma	Partial O₂ response; chronic hypercapnia
HPV suppression	Volatile anesthetics	V̇/Q̇ mismatch during one-lung ventilation
Loss of hypoxic drive	High-flow O₂ in COPD	CO₂ retention, respiratory acidosis
FRC < closing capacity	Supine, obese, elderly	Dynamic airway collapse, shunt, hypoxemia

Sources:

Morgan & Mikhail's Clinical Anesthesiology, 7e - Ch. 23: Respiratory Physiology & Anesthesia, pp. 910-960
Fischer's Mastery of Surgery, 8e - Normal Pulmonary Physiology, p. 264
Costanzo Physiology, 7th Edition - V/Q Defects and Control of Breathing, pp. 234-240

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