RESPIRATORY FOUNDATIONS

Respiratory Foundations

The respiratory system maintains oxygen delivery and carbon dioxide elimination through the coordinated processes of ventilation, perfusion, and diffusion. Respiratory disease develops when airflow, lung expansion, pulmonary blood flow, alveolar-capillary exchange, or ventilatory control becomes impaired (Norris, 2020; Sisson et al., 2019).

Core purpose: Review the physiologic processes needed to interpret obstructive, restrictive, alveolar, and pulmonary vascular disorders.

Key clinical framework: Respiratory dysfunction can usually be traced to impaired ventilation, perfusion, diffusion, ventilation-perfusion matching, or oxygen delivery (Norris, 2020; Sisson et al., 2019).

Clinical priority: Respiratory rate, work of breathing, mental status, oxygenation, ventilation, and hemodynamic stability must be interpreted together. A reassuring oxygen saturation does not exclude ventilatory failure, inadequate oxygen delivery, or impending respiratory fatigue.

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Functional Anatomy

  • The conducting airways move, warm, humidify, and filter inspired air but do not participate directly in gas exchange (Norris, 2020; Sisson et al., 2019).
  • The respiratory zone includes the respiratory bronchioles, alveolar ducts, and alveoli, where oxygen and carbon dioxide diffuse between alveolar gas and pulmonary capillary blood (Norris, 2020; Sisson et al., 2019).
  • Type I alveolar cells form most of the gas-exchange surface. Type II alveolar cells produce surfactant and contribute to alveolar epithelial repair (Sisson et al., 2019).
  • Surfactant reduces surface tension, improves compliance, and helps prevent alveolar collapse during expiration (Norris, 2020; Sisson et al., 2019).

Ventilation

Ventilation is the movement of air between the atmosphere and the alveoli. Effective alveolar ventilation depends on respiratory rate, tidal volume, airway patency, respiratory-muscle function, and the proportion of each breath that reaches gas-exchanging alveoli.

Minute ventilation is calculated as:

Minute ventilation = respiratory rate × tidal volume

Some inhaled air remains within the conducting airways and does not participate in gas exchange. This volume is called anatomic dead space. Alveolar ventilation therefore depends on the volume of air reaching functioning alveoli rather than minute ventilation alone (Norris, 2020; Sisson et al., 2019).

A patient can be tachypneic and still hypoventilate when breaths are too shallow to provide adequate alveolar ventilation. This distinction helps explain why respiratory rate alone cannot establish ventilatory adequacy.

Airway Resistance

Airway resistance increases as airway radius decreases. Bronchoconstriction, mucosal edema, mucus, inflammation, secretions, and structural narrowing can therefore substantially restrict airflow.

Resistance is especially important during expiration. Intrathoracic pressure rises as air moves out of the lungs, increasing compression of the intrathoracic airways. Obstructive disorders prolong expiration and may prevent complete emptying before the next breath begins (Norris, 2020; Sisson et al., 2019).

Loss of elastic recoil reduces the pressure that normally helps hold small airways open during exhalation. The resulting expiratory airway collapse limits airflow and promotes air trapping, particularly in emphysema (Norris, 2020; Sisson et al., 2019).

Compliance and Elastic Recoil

Compliance describes the change in lung volume produced by a change in pressure.

  • High compliance: The lungs inflate easily but may empty poorly because elastic recoil is reduced. This pattern is associated with emphysema.
  • Low compliance: Greater pressure is required to expand the lungs. Pulmonary fibrosis, pulmonary edema, and ARDS reduce compliance.

Patients with reduced compliance commonly adopt rapid, shallow breathing because large tidal volumes require greater elastic work. Patients with obstructive disease may use slower, deeper breaths and prolonged expiration to reduce airflow resistance and allow more complete emptying (Sisson et al., 2019).

Surfactant

Type II alveolar cells produce surfactant, which reduces surface tension at the alveolar air-fluid interface. Surfactant helps stabilize alveoli, limits collapse during expiration, and reduces the pressure required for inflation.

Loss or inactivation of surfactant increases surface tension, promotes atelectasis, reduces compliance, and increases the work of breathing. Surfactant dysfunction contributes to the alveolar instability found in ARDS (Norris, 2020; Sisson et al., 2019).

Perfusion

Perfusion is the movement of blood through the pulmonary circulation. The pulmonary vascular system normally operates under lower pressure and resistance than the systemic circulation.

Pulmonary perfusion may be impaired by:

  • Pulmonary embolism
  • Reduced cardiac output
  • Pulmonary vascular remodeling
  • Excessive alveolar pressure
  • Destruction of pulmonary capillaries
  • Hypoxic pulmonary vasoconstriction

The pulmonary circulation must deliver blood to ventilated alveoli for gas exchange to occur.

Diffusion

Diffusion is the movement of oxygen and carbon dioxide across the alveolar-capillary membrane. Gas transfer is influenced by:

  • Available surface area
  • Membrane thickness
  • The partial-pressure gradient
  • The diffusion properties of the gas
  • Capillary transit time

Pulmonary fibrosis impairs diffusion by thickening and remodeling the interstitium. Emphysema reduces the surface area available for exchange by destroying alveolar walls and pulmonary capillaries (Norris, 2020; Sisson et al., 2019).

Adequate gas exchange requires ventilation of the alveoli, perfusion of the pulmonary capillary bed, and diffusion across the alveolar-capillary membrane. Disease may impair one or more of these processes, producing hypoxemia, hypercapnia, or both (Norris, 2020; Sisson et al., 2019).

Ventilation-Perfusion Relationships

Optimal gas exchange requires ventilation and perfusion to reach the same lung units.

Low Ventilation-Perfusion Ratio

A low ventilation-perfusion ratio occurs when perfusion exceeds ventilation. Causes include:

  • Asthma
  • COPD
  • Pneumonia
  • Pulmonary edema
  • Atelectasis
  • Partial airway obstruction

Blood leaving these regions is incompletely oxygenated.

Shunt

A shunt represents the extreme form of low ventilation relative to perfusion. Blood passes through unventilated alveoli and returns to the systemic circulation without adequate oxygenation.

Shunt-related hypoxemia may respond poorly to supplemental oxygen because oxygen cannot reach alveoli that are collapsed or filled with fluid. Significant shunting contributes to hypoxemia in pulmonary edema and ARDS (Norris, 2020; Sisson et al., 2019).

High Ventilation-Perfusion Ratio

A high ventilation-perfusion ratio occurs when ventilation exceeds perfusion. The extreme form is alveolar dead space, in which an alveolus is ventilated but receives little or no blood flow.

Pulmonary embolism is the major example. The affected alveoli continue to receive air, but vascular obstruction prevents effective gas exchange.

Oxygen Transport

Most oxygen is carried bound to hemoglobin, while a small proportion is dissolved in plasma. PaO₂ reflects dissolved oxygen tension. Oxygen saturation reflects the percentage of available hemoglobin-binding sites occupied by oxygen.

The oxygen-hemoglobin dissociation curve shifts according to physiologic conditions:

  • Increased temperature, increased PaCO₂, increased hydrogen-ion concentration, and increased 2,3-bisphosphoglycerate shift the curve to the right and promote tissue unloading.
  • Decreased temperature, decreased PaCO₂, alkalosis, and reduced 2,3-bisphosphoglycerate shift the curve to the left and increase hemoglobin’s affinity for oxygen.

A normal oxygen saturation does not guarantee adequate tissue oxygen delivery. Hemoglobin concentration, cardiac output, perfusion, and cellular oxygen use also matter (Norris, 2020).

Carbon Dioxide Elimination

Carbon dioxide elimination depends primarily on alveolar ventilation. Hypoventilation raises PaCO₂ and may produce respiratory acidosis. Hyperventilation lowers PaCO₂ and may produce respiratory alkalosis.

Early hypoxemic lung disease commonly stimulates tachypnea, producing a normal or decreased PaCO₂. A normalizing or rising PaCO₂ in a patient who remains tachypneic can indicate worsening airflow obstruction, respiratory-muscle fatigue, or declining alveolar ventilation.

Pulmonary Function Testing

Spirometry helps distinguish obstructive from restrictive ventilatory patterns.

Obstructive Pattern

Obstructive disease limits expiratory airflow.

Expected findings include:

  • Reduced FEV₁
  • Reduced FEV₁/FVC ratio
  • Prolonged expiration
  • Air trapping with increased residual volume
  • Possible hyperinflation with increased total lung capacity

Asthma may demonstrate reversible obstruction. COPD produces persistent, incompletely reversible obstruction.

Restrictive Pattern

Restrictive disease limits lung expansion.

Expected findings include:

  • Reduced FVC
  • Reduced total lung capacity
  • Normal or increased FEV₁/FVC ratio
  • Reduced compliance in parenchymal restrictive disease

Spirometry can suggest restriction, but total lung capacity is required to confirm it (Norris, 2020; Sisson et al., 2019).

Diffusing Capacity

The diffusing capacity for carbon monoxide, or DLCO, estimates gas transfer across the alveolar-capillary interface.

DLCO may be reduced in:

  • Emphysema
  • Pulmonary fibrosis
  • Pulmonary vascular disease
  • Anemia

DLCO may be normal or increased in asthma. Interpretation must account for hemoglobin concentration and the broader pulmonary-function pattern.

Arterial Blood Gas Interpretation

A systematic review includes:

  1. Determine whether the pH indicates acidemia or alkalemia.
  2. Evaluate PaCO₂ for a respiratory contribution.
  3. Evaluate bicarbonate for a metabolic contribution.
  4. Determine whether compensation is appropriate.
  5. Assess oxygenation.
  6. Consider whether more than one acid-base disorder is present.

Common respiratory patterns include:

  • Acute respiratory acidosis: Elevated PaCO₂ with decreased pH and minimal bicarbonate compensation
  • Chronic respiratory acidosis: Elevated PaCO₂ with increased bicarbonate from renal compensation
  • Acute respiratory alkalosis: Decreased PaCO₂ with increased pH
  • Hypoxemic respiratory failure: Inadequate oxygenation caused by ventilation-perfusion mismatch, shunt, diffusion impairment, or reduced inspired oxygen
  • Ventilatory failure: Inadequate carbon dioxide elimination caused by insufficient alveolar ventilation

Evaluating Dyspnea

Dyspnea should be evaluated according to onset, progression, provoking factors, associated symptoms, and risk factors.

High-yield questions include:

  • Was the onset sudden or gradual?
  • Does it occur at rest or with exertion?
  • Is it episodic or persistent?
  • Is there orthopnea or paroxysmal nocturnal dyspnea?
  • Are cough, wheezing, sputum, fever, pleuritic pain, hemoptysis, or edema present?
  • Are there thromboembolic, occupational, medication, smoking, or environmental risks?
  • Has exercise tolerance changed?
  • Can the patient speak in complete sentences?
  • Is there altered mental status, cyanosis, or exhaustion?

Sudden dyspnea raises concern for pulmonary embolism, pneumothorax, acute pulmonary edema, severe asthma, anaphylaxis, or an acute cardiac event. Progressive dyspnea suggests obstructive lung disease, interstitial lung disease, heart failure, anemia, malignancy, or deconditioning (Ma & Lucey, 2012).

Red Flags

Urgent evaluation is required for:

  • Severe respiratory distress
  • Inability to speak in complete sentences
  • Cyanosis
  • Altered mental status
  • Exhaustion
  • Silent or markedly diminished breath sounds
  • Stridor
  • Hemodynamic instability
  • Rapidly increasing oxygen requirement
  • New hemoptysis
  • Sudden pleuritic chest pain with thromboembolic risk
  • Rising PaCO₂ with acidemia

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References

Ma, I., & Lucey, C. R. (2012). Dyspnea. In M. C. Henderson, L. M. Tierney Jr., & G. W. Smetana (Eds.), The patient history: An evidence-based approach to differential diagnosis (2nd ed., pp. 286–297). McGraw-Hill Medical.

Norris, T. L. (2020). Porth’s essentials of pathophysiology (5th ed.). Wolters Kluwer.

Sisson, T. H., Claar, D., Chesnutt, M. S., & Prendergast, T. J. (2019). Pulmonary disease. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed.). McGraw-Hill Education.