Pulmonary Ventilation

Detailed Notes

14.1 Pulmonary Ventilation — Definition and Overview

Pulmonary ventilation, commonly called breathing, is the movement of air between the atmosphere and the alveoli of the lungs. It consists of two phases: inspiration (inhalation) , during which air flows into the lungs, and expiration (exhalation) , during which air flows out. Ventilation is entirely a mechanical process driven by pressure gradients — air always moves from regions of higher pressure to regions of lower pressure. The respiratory muscles create these pressure gradients by changing the volume of the thoracic cavity, and Boyle's law dictates the resulting pressure changes.

14.2 Pressure Relationships in the Thorax

Understanding pulmonary ventilation requires familiarity with four key pressures, all measured relative to atmospheric (barometric) pressure:

• Atmospheric pressure (Patm) : The pressure exerted by the column of air surrounding the body. At sea level, Patm = 760 mmHg. This is the reference value — all other respiratory pressures are expressed relative to it. Atmospheric pressure is treated as 0 mmHg in respiratory physiology (i.e., other pressures are reported as positive or negative relative to it).

• Intrapulmonary (alveolar) pressure (Palv) : The pressure within the alveoli. During a breath-hold with the glottis open, Palv equals Patm (0 mmHg). During inspiration, Palv drops below Patm (to approximately −1 mmHg), creating the pressure gradient that drives air inward. During expiration, Palv rises above Patm (to approximately +1 mmHg), forcing air outward.

• Intrapleural pressure (Pip) : The pressure within the pleural cavity — the thin, fluid-filled space between the visceral pleura (covering the lungs) and the parietal pleura (lining the thoracic wall). Pip is always negative relative to Patm under normal conditions, typically about −4 mmHg (756 mmHg absolute) at rest. This negative pressure is created by the opposing elastic recoil forces of the lungs (pulling inward) and the chest wall (pulling outward), which together generate a slight suction in the pleural cavity. Intrapleural pressure becomes even more negative during inspiration (approximately −6 to −8 mmHg) as the chest wall expands further.

• Transpulmonary pressure (Ptp) : The difference between intrapulmonary and intrapleural pressure: Ptp = Palv − Pip. Transpulmonary pressure represents the distending force that keeps the lungs inflated against their natural tendency to collapse (elastic recoil). At rest: Ptp = 0 − (−4) = +4 mmHg. A larger transpulmonary pressure means greater lung expansion; its loss (as in pneumothorax) causes lung collapse.

Table 14.1: Respiratory Pressures

14.3 Boyle's Law — The Physics of Breathing

Boyle's law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume:

P ∝ 1 / V — as volume increases, pressure decreases; as volume decreases, pressure increases.

This simple gas law is the physical principle that makes pulmonary ventilation possible:

1. During inspiration: The respiratory muscles contract to increase the volume of the thoracic cavity. According to Boyle's law, this increase in volume causes the intrapulmonary pressure (Palv) to decrease below atmospheric pressure. Air then flows down its pressure gradient — from the higher-pressure atmosphere into the lower-pressure alveoli.

1. During expiration: The respiratory muscles relax (or contract, in forced expiration) to decrease the volume of the thoracic cavity. Boyle's law dictates that this volume decrease causes Palv to increase above atmospheric pressure. Air then flows down its pressure gradient — from the higher-pressure alveoli out into the atmosphere.

The pressure changes during quiet breathing are remarkably small (approximately ±1 mmHg), yet they drive roughly 500 mL of air (tidal volume) into and out of the lungs with each breath.

14.4 Inspiration — Quiet and Forced

Quiet Inspiration (Eupnea)

Quiet inspiration at rest is an active process driven by the contraction of two primary muscle groups:

1. Diaphragm: The most important inspiratory muscle. When it contracts, its dome-shaped central tendon flattens and descends into the abdominal cavity, increasing the vertical dimension of the thoracic cavity. The diaphragm is innervated by the phrenic nerves (C3–C5 spinal roots). It accounts for approximately 75% of the volume change during quiet breathing.

1. External intercostal muscles: These muscles run obliquely between the ribs (fibers directed downward and forward). When they contract, they elevate the ribs and swing the sternum outward, increasing the anteroposterior and lateral dimensions of the thoracic cavity (the "bucket handle" and "pump handle" movements, respectively).

The combined effect of diaphragm descent and rib elevation increases thoracic volume → decreases Palv (Boyle's law) → air flows into the lungs.

Forced (Deep) Inspiration

During exercise or respiratory distress, additional accessory inspiratory muscles are recruited to produce a larger, more rapid volume increase:

• Sternocleidomastoid (SCM) : Elevates the sternum.
• Scalene muscles (anterior, middle, posterior): Elevate and fix the first two ribs, providing a stable anchor for the external intercostals.
• Pectoralis minor: Elevates ribs 3–5 when the scapula is fixed (e.g., when leaning on a table or arms — the "tripod position" adopted by patients in respiratory distress).

These accessory muscles are not used during quiet breathing in a healthy individual. Their activation signals increased work of breathing and is a clinical sign of respiratory distress.

14.5 Expiration — Quiet and Forced

Quiet Expiration

Quiet expiration at rest is largely a passive process — it does not require active muscle contraction. Instead, three passive forces drive air out of the lungs:

1. Elastic recoil of the lungs: The lungs are rich in elastin fibers, which were stretched during inspiration like a stretched rubber band. When the inspiratory muscles relax, this stored elastic energy recoils the lungs inward, decreasing alveolar volume.

1. Elastic recoil of the chest wall: The ribs and costal cartilages, deformed during inspiration, spring back to their resting position.

1. Relaxation of inspiratory muscles: The diaphragm relaxes and domes back upward; the external intercostals relax, allowing the ribs to descend.

These forces decrease thoracic volume → increase Palv (Boyle's law) → air flows out.

Importantly, the internal intercostal muscles are relaxed during quiet expiration — they do not participate.

Forced Expiration

Forced expiration (as in coughing, sneezing, or blowing out candles) is an active process that recruits additional muscles:

• Internal intercostal muscles: Their fibers run perpendicular to the external intercostals (directed downward and backward). When they contract, they forcefully depress the ribs, further decreasing thoracic volume.

• Abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis): These muscles contract to compress the abdominal contents, which pushes the relaxed diaphragm upward more forcefully and rapidly than passive recoil alone. The abdominal muscles are the primary drivers of forced expiration.

14.6 Lung Compliance

Lung compliance is a measure of the lung's "stretchability" — how easily the lungs expand in response to a given change in transpulmonary pressure. It is defined mathematically as:

Compliance (C) = ΔV / ΔP

Where ΔV is the change in lung volume and ΔP is the change in transpulmonary pressure. High compliance means the lungs expand easily (like a fresh, thin-walled balloon). Low compliance means the lungs are stiff and require more pressure to expand (like a thick, new balloon that is hard to blow up).

Determinants of Lung Compliance

Two main factors determine lung compliance:

1. Elastic tissue content: The elastin and collagen fibers within the lung parenchyma provide the structural framework. Elastin allows stretch; collagen resists overstretch. In fibrosis or aging, changes in the elastin-to-collagen ratio can reduce compliance. In emphysema, destruction of alveolar walls and elastic tissue increases compliance — the lungs become "floppy" and easy to inflate but difficult to deflate (loss of elastic recoil).

1. Surface tension at the air–liquid interface: The inner surface of each alveolus is lined with a thin film of fluid. At this air–water interface, water molecules are much more attracted to each other (cohesion) than to air molecules, creating surface tension — a force that tends to collapse the alveolus. Surface tension accounts for approximately two-thirds of the lung's elastic recoil force. This is the major factor opposing lung expansion and is the target of surfactant.

Table 14.2: Determinants of Lung Compliance

14.7 Surface Tension and Surfactant

Surface Tension and the Law of Laplace

The thin film of alveolar fluid lining each alveolus generates surface tension, which creates a collapsing pressure. The relationship between surface tension, alveolar radius, and collapsing pressure is described by the Law of Laplace:

P = 2T / r

Where P is the collapsing pressure (the pressure tending to collapse the alveolus), T is the surface tension, and r is the radius of the alveolus.

This equation reveals a critical problem for the lungs: smaller alveoli (smaller r) would generate a higher collapsing pressure than larger alveoli (larger r) , assuming equal surface tension. According to the Law of Laplace, small alveoli would therefore tend to collapse and empty their air into larger, adjacent alveoli — a phenomenon called alveolar instability. In a lung with thousands of alveoli of different sizes, this would cause widespread alveolar collapse (atelectasis).

Surfactant — The Solution

The lungs solve this problem with pulmonary surfactant, a complex mixture of phospholipids (primarily dipalmitoylphosphatidylcholine / DPPC) , proteins, and ions secreted by type II alveolar pneumocytes (type II pneumocytes) . Surfactant reduces surface tension in two crucial ways:

1. Overall reduction: By interposing itself between water molecules at the air–liquid interface, surfactant dramatically lowers the total surface tension, reducing the work of breathing.

1. Surface tension varies with alveolar size: As an alveolus shrinks during expiration, surfactant molecules become more concentrated at the surface, lowering surface tension even further. This means that smaller alveoli have lower surface tension than larger ones — the opposite of what would happen without surfactant. This equalizes the collapsing pressure (P) between alveoli of different sizes, preventing small alveoli from collapsing into larger ones. This property is essential for maintaining alveolar stability.

Clinical Correlate: Infant Respiratory Distress Syndrome (IRDS)

Type II pneumocytes do not begin producing adequate surfactant until approximately 26–28 weeks of gestation, and surfactant production does not reach sufficient levels until about 35 weeks. Premature infants born before this time lack sufficient surfactant and develop infant respiratory distress syndrome (IRDS) , also called hyaline membrane disease. Without surfactant, their alveoli collapse with every exhalation (atelectasis), and tremendous effort is required to re-inflate them — leading to respiratory failure. Treatment includes exogenous surfactant administration into the airways and, when possible, maternal corticosteroids before premature delivery (betamethasone/dexamethasone) to accelerate fetal surfactant production.

14.8 Airway Resistance

Airway resistance is the opposition to airflow through the conducting airways, determined by airway diameter, airflow velocity, and whether flow is laminar or turbulent. The relationship is analogous to blood flow resistance, governed by Poiseuille's law:

Resistance (R) ∝ 1 / r⁴

Where r is the airway radius. Even small changes in airway diameter produce large changes in resistance.

Site of Highest Resistance

A common misconception is that the smallest airways (bronchioles) offer the greatest resistance. In fact, the highest total airway resistance occurs in the medium-sized bronchi (approximately generations 4–8 of the bronchial tree). This is because:

• The trachea and main bronchi have large diameters → low individual resistance.
• The bronchioles are small in diameter but extremely numerous, and their total cross-sectional area is enormous (comparable to the parallel arrangement of capillaries in the systemic circulation) → low total resistance.
• The medium bronchi have a moderate individual diameter with a moderate total cross-sectional area → the "sweet spot" for highest resistance.

Neural and Chemical Regulation of Airway Diameter

Airway smooth muscle tone is under autonomic nervous system control:

• Sympathetic stimulation (β₂-adrenergic receptors) : Activation of β₂ receptors on bronchial smooth muscle causes bronchodilation (relaxation of smooth muscle → increased airway diameter → decreased resistance). Circulating epinephrine from the adrenal medulla also stimulates β₂ receptors. This is the basis for β₂-agonists (e.g., albuterol/salbutamol) in asthma treatment.

• Parasympathetic stimulation (muscarinic receptors) : Acetylcholine released from vagus nerve (CN X) endings causes bronchoconstriction (contraction of smooth muscle → decreased airway diameter → increased resistance). This is the basis for anticholinergic bronchodilators (e.g., ipratropium).

• Chemical mediators: Histamine released from mast cells during allergic reactions is a potent bronchoconstrictor — central to the pathophysiology of allergic asthma. Other bronchoconstrictors include leukotrienes (also from mast cells) and prostaglandin D₂. Carbon dioxide has a local effect: high CO₂ levels cause bronchodilation (matching ventilation to perfusion), while low CO₂ causes bronchoconstriction.

Table 14.3: Regulation of Airway Smooth Muscle Tone

14.9 Lung Volumes and Capacities

Pulmonary function testing divides the total air in the respiratory system into four primary volumes (non-overlapping, directly measured or calculated) and four capacities (sums of two or more volumes).

Lung Volumes

Table 14.4: The Four Primary Lung Volumes

Residual volume is a critical concept — it is the air that cannot be voluntarily exhaled. It exists because the lungs are held open by the negative intrapleural pressure; even at maximal expiratory effort, the airways collapse before all air is expelled, trapping the residual volume. This trapped air keeps the alveoli partially inflated and prevents complete lung collapse.

Lung Capacities

Capacities are combinations of two or more lung volumes:

Table 14.5: Lung Capacities — Formulas and Values

Functional residual capacity (FRC) is physiologically important because it is the resting position of the respiratory system — the point at which the inward elastic recoil of the lungs is exactly balanced by the outward recoil of the chest wall. At FRC, the respiratory muscles are relaxed, and this is the volume from which a normal tidal breath begins.

Visual Summary of Volumes and Capacities

TLC  ┌─────────────────────────────┐
     │          IRV                │  Inspiratory Reserve Volume
     ├─────────────────────────────┤  ─── IC (Inspiratory Capacity)
     │          TV                 │  Tidal Volume
FRC →├─────────────────────────────┤  ─── FRC (Functional Residual Capacity)
     │          ERV                │  Expiratory Reserve Volume
     ├─────────────────────────────┤
RV   │          RV                 │  Residual Volume (cannot be exhaled)
     └─────────────────────────────┘
      VC = IRV + TV + ERV
      TLC = VC + RV

14.10 Spirometry — Measuring Lung Function

Spirometry is the most common pulmonary function test. The patient inhales and exhales through a mouthpiece connected to a spirometer, which records volume changes over time. The resulting tracing — a spirogram — provides direct measurement or calculation of several lung volumes and capacities.

What Spirometry CAN Measure

• Tidal volume (TV) — directly from the resting breathing trace.
• Inspiratory reserve volume (IRV) — from a maximal inspiration following a normal tidal inspiration.
• Expiratory reserve volume (ERV) — from a maximal expiration following a normal tidal expiration.
• Vital capacity (VC) — from a maximal expiration following a maximal inspiration (or vice versa). Can be measured as slow vital capacity (SVC) or forced vital capacity (FVC) .

From these, inspiratory capacity (IC = TV + IRV) can also be calculated.

What Spirometry CANNOT Measure

Spirometry cannot measure residual volume (RV) because, by definition, RV is the air that cannot be exhaled. Since the spirometer only records air that actually passes through the mouthpiece, RV is invisible to it. Consequently, any capacity that includes RV is also not directly measurable by spirometry:

• Functional residual capacity (FRC = ERV + RV) — cannot be measured.
• Total lung capacity (TLC = VC + RV) — cannot be measured.

To measure RV (and therefore FRC and TLC), additional techniques are required:

• Helium dilution method: The patient breathes from a spirometer containing a known concentration of helium (an inert, non-absorbable gas). Helium equilibrates between the spirometer and the patient's lungs; from the dilution, the unknown lung volume (FRC) is calculated.
• Body plethysmography: The patient sits in an airtight booth and breathes against a closed shutter. Pressure changes at the mouth and in the box are used to calculate thoracic gas volume (FRC) by Boyle's law.

Forced Expiratory Maneuvers and the FEV₁/FVC Ratio

The most clinically useful spirometric maneuver is the forced vital capacity (FVC) test: the patient takes a maximal inspiration and then exhales as hard and as fast as possible until no more air can be expelled. From this, two key parameters are derived:

• Forced vital capacity (FVC) : The total volume of air exhaled during the maneuver.
• Forced expiratory volume in 1 second (FEV₁) : The volume of air exhaled during the first second of the FVC maneuver.

The ratio of these two values — FEV₁ / FVC — is the single most important parameter for distinguishing between obstructive and restrictive lung disease:

Table 14.6: Obstructive vs. Restrictive Patterns on Spirometry

In obstructive disease, the airways are narrowed, so although total lung capacity may be normal (or even elevated due to air trapping), getting the air out takes longer — hence FEV₁ drops disproportionately relative to FVC, lowering the FEV₁/FVC ratio. In restrictive disease, all lung volumes are reduced symmetrically, so both FEV₁ and FVC decrease proportionally, and the ratio stays normal or may even be elevated.

14.11 Clinical Correlations

Asthma

Asthma is an obstructive disorder characterized by three interrelated features: (1) airway inflammation (chronic eosinophilic inflammation of the bronchial walls), (2) bronchial hyperresponsiveness (exaggerated bronchoconstriction in response to triggers), and (3) reversible airflow obstruction. During an acute asthma attack, triggers such as allergens, cold air, or exercise provoke mast cell degranulation with the release of histamine, leukotrienes, and other mediators → bronchoconstriction, mucosal edema, and mucus hypersecretion → increased airway resistance → difficulty exhaling (wheezing, prolonged expiration). Spirometry shows a reduced FEV₁/FVC ratio, and the obstruction is at least partially reversible with bronchodilators (a key diagnostic criterion).

COPD and Emphysema

Chronic obstructive pulmonary disease (COPD) encompasses emphysema and chronic bronchitis, both characterized by persistent, largely irreversible airflow obstruction. In emphysema, destruction of alveolar walls and elastic tissue causes:

• Loss of elastic recoil → airways collapse during expiration (dynamic airway compression) → air trapping → hyperinflation.
• Increased compliance (floppy lungs — easy to inflate but hard to deflate).
• Increased residual volume (RV) and functional residual capacity (FRC) , with a barrel chest on physical examination.
• Spirometry: Reduced FEV₁/FVC ratio (obstructive pattern), decreased FEV₁, normal or increased TLC (via plethysmography).

Infant Respiratory Distress Syndrome (IRDS)

As discussed in Section 14.7, premature infants with insufficient surfactant develop diffuse atelectasis (alveolar collapse). Clinically: tachypnea, nasal flaring, grunting, retractions, and hypoxemia within hours of birth. Chest X-ray shows a "ground-glass" appearance with air bronchograms. Administration of exogenous surfactant through the endotracheal tube is lifesaving.

Pneumothorax

A pneumothorax occurs when air enters the pleural cavity, abolishing the negative intrapleural pressure. Without the negative Pip to oppose elastic recoil, the transpulmonary pressure drops to zero or near-zero, and the lung collapses away from the chest wall. Causes include trauma (penetrating chest wound), spontaneous rupture of a subpleural bleb (common in tall, thin young males), or iatrogenic causes (central line insertion, mechanical ventilation). A tension pneumothorax occurs when air enters the pleural space but cannot exit (one-way valve effect), progressively shifting the mediastinum, compressing the contralateral lung, and obstructing venous return — a life-threatening emergency requiring immediate needle decompression.

Atelectasis

Atelectasis is the collapse of lung tissue — either partial or complete. It can result from:

• Surfactant deficiency (IRDS).
• Airway obstruction (mucus plug, foreign body) → absorption atelectasis: trapped distal air is absorbed into the blood, and the alveoli collapse.
• Pleural effusion or pneumothorax → compression atelectasis.
• Postoperative atelectasis from shallow breathing after anesthesia (encouraging deep breathing and incentive spirometry helps prevent this).