7.2 Pulmonary Anatomy, Mechanics of Breathing & Gas Transport

7.2 Pulmonary Anatomy, Mechanics of Breathing & Gas Transport

Airway management, mechanical ventilation, and oxygen delivery are core clinical domains for the Certified Anesthesia Technologist (Cer.A.T.T.). Mastery of respiratory anatomy, neural innervation, respiratory mechanics, static lung capacities, and blood gas transport dynamics is vital when managing difficult airways, troubleshooting ventilator alarms, and interpreting blood gas values in the perioperative environment.

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Upper Airway Anatomy: Pharynx & Larynx

The respiratory tract begins at the nares and oral aperture, funneling inspired gases through the pharynx and larynx into the tracheobronchial tree.

The Pharynx

The pharynx is a fibromuscular tube extending from the base of the skull to the lower border of the cricoid cartilage at the level of the sixth cervical vertebra (C6), divided into three distinct anatomical regions:

1. Nasopharynx: Extends from the posterior nasal choanae to the soft palate. Houses the pharyngeal tonsils (adenoids) and the openings of the Eustachian (auditory) tubes. Non-collapsible; lined with pseudostratified ciliated columnar epithelium.
2. Oropharynx: Extends from the soft palate to the superior tip of the epiglottis. Communicates anteriorly with the oral cavity. Key structures include the palatine tonsils, base of the tongue, and the vallecula—the anatomical recess between the tongue base and the anterior surface of the epiglottis into which the curved Macintosh laryngoscope blade tip is positioned to indirectly elevate the epiglottis.
3. Laryngopharynx (Hypopharynx): Extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage (C6), where it becomes continuous with the esophagus. Contains the pyriform sinuses bilaterally, which flank the laryngeal inlet and are susceptible to perforation during blind instrumentation or aggressive esophageal stethoscope placement.

SAGITTAL UPPER AIRWAY DIVISION & LARYNGOSCOPIC LANDMARKS:

 [ Nasopharynx ]  --> Skull base to Soft Palate (Adenoids, Eustachian tubes)
        |
 [ Oropharynx ]   --> Soft Palate to Epiglottis Tip (Tongue Base, Vallecula)
        |             * Curved Mac Blade Tip Placed Here in Vallecula
 [ Hypopharynx ]  --> Epiglottis to Cricoid Lower Border (C6) (Pyriform Sinuses)
        |
 [ Larynx ]       --> Laryngeal Inlet & Vocal Cords (Guarded by Epiglottis)
                      * Straight Miller Blade Directly Elevates Epiglottis

The Larynx & The 9 Laryngeal Cartilages

The larynx serves as the primary sphincter guarding the lower respiratory tract against aspiration, while providing phonation. The laryngeal skeleton is formed by nine cartilages (three unpaired and three paired):

Airway Anatomy Pearl: The Cricothyroid Membrane is a fibrous band spanning the space between the inferior border of the thyroid cartilage and the superior border of the anterior cricoid ring. Measuring approximately 9 to 10 mm in height and 22 to 30 mm in width, it is relatively avascular and represents the standard anatomical target for emergency front-of-neck airway access (surgical or percutaneous needle cricothyrotomy) when a "cannot intubate, cannot oxygenate" (CICO) crisis occurs.

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Neural Innervation of the Larynx & Airway Reflexes

The larynx receives both sensory and motor innervation exclusively from two branches of the Vagus Nerve (Cranial Nerve X): the Superior Laryngeal Nerve (SLN) and the Recurrent Laryngeal Nerve (RLN).

                        VAGUS NERVE (CN X)
                         /              \
   [ Superior Laryngeal Nerve ]      [ Recurrent Laryngeal Nerve ]
         /             \                          |
  Internal Branch   External Branch               |
  (Sensory ABOVE    (Motor to ONLY          (Motor to ALL Intrinsic
   Vocal Cords)      Cricothyroid)           Muscles EXCEPT Cricothyroid;
                                             Sensory BELOW Vocal Cords)

1. Superior Laryngeal Nerve (SLN)

Arises from the inferior ganglion of the vagus nerve high in the neck and divides into two distinct branches:

• Internal Branch (Internal Laryngeal Nerve):
  • Function: Entirely sensory.
  • Territory: Provides sensation to the base of the tongue, epiglottis, vallecula, aryepiglottic folds, and the laryngeal vestibule down to the superior surface of the true vocal cords.
  • Clinical Application: Anesthetized during awake fiberoptic intubation by applying local anesthetic topically to the piriform fossae or by performing a bilateral transcutaneous block at the greater cornu of the hyoid bone.
• External Branch (External Laryngeal Nerve):
  • Function: Strictly motor.
  • Territory: Innervates only one muscle: the cricothyroid muscle.
  • Muscle Action: The cricothyroid muscle tilts the thyroid cartilage forward, which tenses and elongates the vocal cords, raising vocal pitch.
  • Injury Manifestation: Unilateral injury causes voice fatigue and loss of high-register phonation; frequently injured during thyroid surgery due to proximity to the superior thyroid artery.

2. Recurrent Laryngeal Nerve (RLN)

• Sensory Territory: Sensation to the laryngeal mucosa below the level of the true vocal cords down through the subglottis and upper trachea.
• Motor Territory: Supplies all intrinsic muscles of the larynx EXCEPT the cricothyroid muscle.
• Intrinsic Muscle Actions:
  • Posterior Cricoarytenoids (PCA): The sole abductors of the vocal cords (mnemonic: PCA = Pull Cords Apart).
  • Lateral Cricoarytenoids (LCA): Primary adductors of the vocal cords (mnemonic: LCA = Leaves Cords Aside/Together).
  • Thyroarytenoids & Vocalis: Shorten and relax the vocal cords.

Anatomical Asymmetry of the Recurrent Laryngeal Nerves

• Left RLN: Leaves the vagus nerve in the mediastinum, loops under the aortic arch (immediately posterior to the ligamentum arteriosum), and ascends through the tracheoesophageal groove back to the larynx. Its long intrathoracic course makes it vulnerable to compression by mediastinal masses, aortic aneurysms, an enlarged left atrium (Ortner's syndrome), or surgical traction during thoracic and cardiac operations.
• Right RLN: Leaves the vagus nerve in the root of the neck, loops under the right subclavian artery, and ascends through the right tracheoesophageal groove.

Clinical Manifestations of Nerve Injuries

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Tracheobronchial Tree & Segmental Architecture

The lower respiratory tract begins at the subglottic exit of the cricoid cartilage and descends into the thoracic cavity:

                          TRACHEA (C6 to T4-T5)
                          Length: 10-15 cm; 16-20 C-rings
                                    |
                              CARINA (T4-T5)
                               /          \
                              /            \
    RIGHT MAIN BRONCHUS      /              \     LEFT MAIN BRONCHUS
    - Angle: 25° (Vertical)                  - Angle: 45° (Horizontal)
    - Length: ~2.5 cm (Short)                - Length: ~5.0 cm (Long)
    - Caliber: ~1.5 cm (Wide)                - Caliber: ~1.0 cm (Narrower)
    - 3 Lobes (Sup, Mid, Inf)                - 2 Lobes + Lingula

1. The Trachea

• Extends from the cricoid cartilage (level of C6) to the bifurcation at the carina (level of T4–T5, corresponding surface landmark: the sternal angle of Louis).
• Measures 10 to 15 cm in length and 2.0 to 2.5 cm in external diameter in adults.
• Composed of 16 to 20 anterior C-shaped hyaline cartilages that prevent airway collapse under negative intrathoracic pressures. The posterior wall is flat, lacking cartilage, composed instead of the smooth trachealis muscle and fibroelastic tissue abutting the anterior wall of the esophagus.

2. Mainstem Bronchi: Right vs. Left Comparison

Endotracheal Depth Pearl: Because the adult right upper lobe bronchus branches off the right mainstem only 2.0 to 2.5 cm distal to the carina, advancing an endotracheal tube just 2 to 3 cm too deep will intubate the right mainstem and frequently occlude the right upper lobe orifice, resulting in rapid right upper lobe and left lung atelectasis.

3. Lobes and Bronchopulmonary Segments

• Right Lung: Composed of 3 lobes (Superior, Middle, Inferior) separated by the horizontal and oblique fissures; divided into 10 bronchopulmonary segments.
• Left Lung: Composed of 2 lobes (Superior and Inferior) separated by a single oblique fissure; divided into 8 to 9 segments. The anteromedial aspect of the left superior lobe contains the cardiac notch and the lingula (the anatomical homologue of the right middle lobe).

4. Respiratory Mechanics & Diaphragmatic Innervation

• The Diaphragm: The primary muscle of inspiration, responsible for 75% of resting tidal airflow. Diaphragmatic contraction pulls the central tendon downward, expanding thoracic volume and creating negative intrathoracic pressure (-5 to -8 cmH2O) that draws air into the lungs.
• Motor Innervation: The diaphragm is innervated solely by the bilateral Phrenic Nerves, which originate from the C3, C4, and C5 cervical ventral rami (clinical mnemonic: "C3, 4, 5 keeps the diaphragm alive"). Accidental blockade of the phrenic nerve commonly occurs during an interscalene brachial plexus block (occurring in nearly 100% of cases), causing transient ipsilateral diaphragmatic paralysis and a 25% reduction in vital capacity.

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Lung Volumes, Capacities & Functional Residual Capacity (FRC)

Pulmonary ventilation is assessed using standard static lung volumes and dynamic capacities derived from multiple volumes:

LUNG VOLUMES & CAPACITIES RELATIONSHIP:

+-------------------------------------------------------------------+
| TOTAL LUNG CAPACITY (TLC ~ 5800 - 6000 mL)                        |
| +-----------------------------------------------+ +-------------+ |
| | VITAL CAPACITY (VC ~ 4500 - 5000 mL)          | | RESIDUAL    | |
| | +---------------+ +---------------+ +---------+ | | VOLUME      | |
| | | INSPIRATORY   | | TIDAL VOLUME  | | EXP.    | | | (RV ~ 1200) | |
| | | RESERVE (IRV) | | (VT ~ 500 mL) | | RESERVE | | |             | |
| | | ~ 3000 mL     | | (6-8 mL/kg)   | | (ERV)   | | |             | |
| | |               | |               | | ~ 1100  | | |             | |
| | +---------------+ +---------------+ +---------+ | +-------------+ |
| |                                     | FUNCTIONAL RESIDUAL       | |
| |                                     | CAPACITY (FRC ~ 2400 mL)  | |
| |                                     | [ERV + RV]                | |
| +-------------------------------------+---------------------------+ |
+-------------------------------------------------------------------+

Individual Static Volumes

1. Tidal Volume (VT): The volume of air inspired or expired during each normal, quiet respiratory cycle. Normal value is 6 to 8 mL/kg of Ideal Body Weight (IBW) (~400 to 500 mL in a 70 kg adult).
2. Inspiratory Reserve Volume (IRV): The maximal volume of gas that can be forcefully inhaled beyond a normal tidal inspiration (~3000 mL).
3. Expiratory Reserve Volume (ERV): The maximal volume of gas that can be forcefully exhaled at the end of a normal tidal exhalation (~1100 mL).
4. Residual Volume (RV): The volume of gas remaining in the pulmonary parenchymal architecture following maximal exhalation (~1200 mL). RV cannot be measured by direct spirometry; it requires gas dilution (helium wash-in) or body plethysmography.

Combined Lung Capacities

1. Vital Capacity (VC = VT + IRV + ERV): The maximum volume of air that can be exhaled following maximal inspiration (~4500 to 5000 mL, or 60 to 70 mL/kg).
2. Inspiratory Capacity (IC = VT + IRV): The maximal volume that can be inhaled from resting end-expiratory position (~3500 mL).
3. Total Lung Capacity (TLC = VC + RV): The total volume of gas contained within the lungs following maximal inspiration (~5800 to 6000 mL).
4. Functional Residual Capacity (FRC = ERV + RV): The volume of gas remaining in the lungs at the end of a normal, unforced tidal expiration (~30 mL/kg, or ~2400 mL in an adult male).

The Critical Role of FRC in Perioperative Anesthesia

The FRC represents the balance point between two opposing mechanical forces: the inward elastic recoil of the lungs versus the outward elastic recoil of the chest wall. In clinical anesthesia, FRC is the single most vital lung parameter:

• Oxygen Reservoir During Apnea: Gas exchange does not cease between breaths; alveolar oxygen continues to diffuse across pulmonary capillaries during exhalation. FRC functions as the body's internal oxygen reservoir.
• Denitrogenation (Preoxygenation): Breathing 100% O2 prior to induction washes nitrogen out of the FRC, replacing a 2400 mL volume of 21% O2 (approx. 450 mL of O2) with pure oxygen (~2200 mL of O2). This extends safe apnea time without desaturation from 1 to 2 minutes up to 6 to 8 minutes in a healthy adult.

Factors That Severely Depress FRC

Closing Capacity (CC) Relationship: Closing Capacity is the lung volume at which small, non-cartilaginous airways in dependent lung zones begin to collapse during expiration (CC = Closing Volume + RV). In healthy young adults, FRC is significantly larger than CC. However, when FRC falls below CC (due to supine positioning, anesthesia, or advanced age >65), airway closure occurs during normal tidal ventilation, resulting in intrapulmonary shunt, atelectasis, and rapid hypoxemia.

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The Thoracic Rib Cage

The ASATT outline names the sternum, ribs, and xiphoid process as rib cage landmarks.

• Sternum: The manubrium, body, and xiphoid process (the small inferior tip). The sternal angle (angle of Louis), where the manubrium meets the body, marks the second rib and roughly the level of the carina.
• Ribs: Twelve pairs. Ribs 1 to 7 are true ribs attached directly to the sternum by costal cartilage, ribs 8 to 10 are false ribs attached indirectly, and ribs 11 and 12 are floating ribs with no anterior attachment.
• Intercostal spaces: The intercostal vein, artery, and nerve run along the lower border of each rib, so needles and chest tubes pass just over the upper border of the rib below.
• Clinical landmarks: Chest compressions go on the lower half of the sternum while avoiding the xiphoid process, which can injure the liver. The subxiphoid approach is used for pericardiocentesis, and chest tubes are commonly placed in the fourth or fifth intercostal space near the mid-axillary line.

Gas Transport & Alveolar Diffusion Mechanics

Gas movement from the ambient atmosphere down to cellular mitochondria relies on physical gas laws governing diffusion and solubility:

1. Fick's Law of Alveolar-Capillary Diffusion

Fick's Law states that the rate of diffusion (V(gas)) of a gas across the alveolar-capillary membrane is directly proportional to the surface area of the membrane (A), the diffusion constant (D), and the partial pressure gradient across the membrane (Δ P), and inversely proportional to the membrane thickness (T):

$\text{Rate of Diffusion} \propto \frac{A \times D \times (P_1 - P_2)}{T}$

• Clinical Impact: Alveolar diffusion is severely impaired when membrane thickness increases (pulmonary edema, interstitial pulmonary fibrosis, acute respiratory distress syndrome) or when functional alveolar surface area is lost (emphysema, pulmonary embolism, atelectasis).

2. Henry's Law of Gas Solubility

Henry's Law dictates that at a constant temperature, the amount of a given gas dissolved in a liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid:

$\text{Dissolved Gas Concentration} = \alpha \times P$

• Dissolved Oxygen in Blood: Oxygen solubility coefficient (α) in plasma is 0.003 mL O₂ / 100 mL blood / mmHg PaO₂.
  • At a normal arterial PaO₂ of 100 mmHg, dissolved O₂ equals: 100 × 0.003 = 0.30 mL O₂ / dL blood. This dissolved fraction is insufficient to sustain human life without hemoglobin.
• Dissolved Carbon Dioxide in Blood: Carbon dioxide is 20 to 24 times more soluble in aqueous blood than oxygen (α = 0.067 mL CO₂ / 100 mL blood / mmHg PaCO₂). Consequently, carbon dioxide diffuses across the alveolar-capillary membrane far more rapidly than oxygen, explaining why pulmonary diffusion defects cause hypoxemia long before hypercapnia occurs.

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The Oxyhemoglobin Dissociation Curve

Hemoglobin is a tetrameric protein containing four heme moieties, each capable of binding one molecule of oxygen (O₂). The binding of the first oxygen molecule induces a conformational shift in the globin subunits, sequentially increasing the affinity of the remaining heme sites for subsequent oxygen molecules. This phenomenon—positive cooperativity—produces the characteristic sigmoidal (S-shaped) oxyhemoglobin dissociation curve.

THE OXYHEMOGLOBIN DISSOCIATION CURVE & LANDMARKS:

  100 |                                       * (PaO2 100, SaO2 98-100%)
      |                                  .-' 
   80 |                             .-'      * (PaO2 60, SaO2 90% - "Knee")
S     |                         .-'
O  60 |                     .-'
2     |                  .-'                 * (PaO2 40, SaO2 75% - Mixed Venous)
(%)40 |              .-'
      |          * (PaO2 26.7, SaO2 50% - P50)
   20 |      .-'
      |  .-' 
    0 +------------------------------------------------------------
      0       20       40       60       80       100      120
                            PaO2 (mmHg)

     <--- LEFT SHIFT                          RIGHT SHIFT --->
     - High pH (Alkalosis)                   - Low pH (Acidosis)
     - Low PaCO2                             - High PaCO2
     - Hypothermia                           - Hyperthermia
     - Low 2,3-DPG                           - High 2,3-DPG
     (Holds O2 tightly)                      (Releases O2 to tissues)

Critical Curve Landmarks Tested on Certification Exams

1. P₅₀ Reference Point: The partial pressure of oxygen at which hemoglobin is exactly 50% saturated. Normal baseline value in a healthy adult (at pH 7.40, PaCO₂ 40 mmHg, and temperature 37°C) is 26.7 mmHg (commonly rounded to 27 mmHg).
2. The Mixed Venous Point: At a PaO₂ of 40 mmHg, hemoglobin saturation (SvO₂) is normally 75%.
3. The "Knee" of the Curve: At a PaO₂ of 60 mmHg, hemoglobin saturation (SaO₂) is approximately 90%.
   • Clinical Warning: Above 60 mmHg, the curve is relatively flat; large changes in PaO₂ produce minimal changes in saturation. Below 60 mmHg, the curve enters its steep descent; minor drops in oxygen tension cause catastrophic precipitously rapid arterial desaturation.
4. Normal Arterial Point: At a PaO₂ of 100 mmHg, hemoglobin saturation is 98% to 100%.

Curve Shifts: Physiology & Mnemonic

A shift in the curve alters hemoglobin's binding affinity for oxygen, changing the P₅₀ value:

1. Right Shift (Decreased Affinity → Promotes Tissue Oxygen Unloading)

• The curve shifts rightward: for any given PaO₂, hemoglobin saturation is lower. The P₅₀ increases to > 26.7 mmHg.
• Physiological Benefit: Hemoglobin binds oxygen more loosely, releasing oxygen into metabolically active capillary beds that need it most.
• Causes (Mnemonic: CADET, face RIGHT!):
  • C — Elevated CO₂ (Hypercapnia)
  • A — Acidosis (Low arterial pH / High [H⁺])
  • D — Elevated 2,3-Diphosphoglycerate (2,3-DPG)
  • E — Exercise
  • T — Elevated Temperature (Hyperthermia, Febrile states)

2. Left Shift (Increased Affinity → Inhibits Tissue Oxygen Unloading)

• The curve shifts leftward: hemoglobin binds oxygen tenaciously, refusing to release it to peripheral tissues at physiological capillary partial pressures. The P₅₀ decreases to < 26.7 mmHg.
• Causes:
  • Alkalosis (Elevated arterial pH / Low [H⁺])
  • Hypocapnia (Low PaCO₂)
  • Hypothermia (decreased metabolic temperature)
  • Decreased erythrocyte 2,3-DPG (frequently seen following massive transfusion of banked packed red blood cells, which become depleted of 2,3-DPG during cold storage)
  • Carboxyhemoglobin (COHb): Carbon monoxide binds to hemoglobin with 200 times the affinity of O₂, locking the remaining subunits in a high-affinity relaxed state and shifting the curve dramatically leftward.
  • Methemoglobin (MetHb): Oxidized iron (Fe³⁺) cannot bind O₂ and shifts remaining ferrous heme leftward.
  • Fetal Hemoglobin (HbF): Lacks beta chains (composed of two alpha and two gamma subunits), preventing 2,3-DPG binding. This generates a baseline leftward shift (P₅₀ ≈ 19 mmHg), enabling the fetus to extract oxygen across the placental interface from maternal adult hemoglobin (HbA).

The Bohr Effect vs. The Haldane Effect

These two fundamental respiratory phenomena describe reciprocal gas transport interactions: