The Mechanics of Breathing

• What is airway resistance?

  The opposition to airflow within the respiratory tract, primarily influenced by the diameter of the airways and the flow of air.
• What factors influence airway resistance?

  • Airway diameter: Smaller diameters (e.g., during bronchoconstriction) increase resistance; larger diameters (e.g., during bronchodilation) decrease resistance.
  • Airway length: Longer airways increase resistance.
  • Air viscosity: Higher viscosity of air (e.g., due to humidity or temperature) increases resistance.
  • Elasticity of the airway walls: Loss of elasticity increases resistance.
• How is airway resistance mathematically defined and what is its relationship to airflow?

  Airway resistance (R) is defined by the equation:

  $R\ =$$\ \frac{\Delta P}{Q}$

  Where:

  • $\Delta P$ is the pressure difference between the atmosphere and the alveoli.
  • $Q$ is the airflow rate

  The relationship implies that as resistance increases, the airflow (Q) decreases for a given pressure difference.
• How does airway resistance change with the size of the airways?

  • Airway resistance is inversely proportional to the radius of the airway raised to the fourth power. This is known as Poiseuille’s Law.
  • A small decrease in airway diameter results in a significant increase in resistance, whereas an increase in diameter leads to a substantial reduction in resistance.
• How does airway resistance affect the mechanics of breathing?

  Increased airway resistance requires greater effort to move air in and out of the lungs. This leads to:

  • Increased work of breathing: The muscles must exert more force to overcome the higher resistance.
  • Decreased airflow: Reduced airflow limits the amount of oxygen reaching the alveoli and increases the CO2 retention.
• How do respiratory diseases impact airway resistance?

  Respiratory diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and bronchitis, increase airway resistance through mechanisms like:

  • Bronchoconstriction
  • Inflammation
  • Mucus production
  • Loss of lung elasticity
• How does Chronic Obstructive Pulmonary Disease (COPD) affect airway resistance?

  COPD involves both chronic inflammation and airway remodeling, leading to:

  • Thickened airway walls due to inflammation.
  • Mucus hypersecretion, which obstructs airflow.
  • Loss of lung elasticity (e.g., emphysema), which reduces the ability of the airways to stay open during exhalation, increasing airway resistance.
• Why is airway resistance higher during an asthma attack?

  During an asthma attack, airway resistance increases due to:

  • Bronchoconstriction: The smooth muscle surrounding the airways contracts, narrowing the airways.
  • Inflammation: Swelling of the airway linings reduces the effective diameter of the airways.
  • Mucus production: Excess mucus further obstructs airflow, contributing to higher resistance.
• What happens to airway resistance in a patient with emphysema?

  In emphysema, a form of COPD, the alveolar walls break down, causing a loss of lung elasticity. This leads to:

  • Airway collapse: During exhalation, airways may collapse because of the loss of supporting elastic fibers, increasing airway resistance.
  • Reduced airflow: The inability to properly expel air from the lungs increases airway resistance and reduces effective ventilation.
• How can bronchodilators affect airway resistance?

  • Bronchodilators relax the smooth muscles in the airways, leading to bronchodilation, which increases airway diameter and reduces resistance.
  • This makes breathing easier by improving airflow and reducing the work of breathing, particularly in conditions like asthma and COPD.
• What is the relationship between airway resistance and the work of breathing?

  • As airway resistance increases (e.g., due to narrowing of the airways in asthma or COPD), the work of breathing increases.
  • The body has to exert more muscular effort to overcome the higher resistance, making breathing more difficult and tiring, and potentially leading to respiratory fatigue.
• How does airway resistance change during normal breathing versus forced breathing?

  • Normal breathing: Airway resistance is relatively low because the airways are wide open and airflow is smooth.
  • Forced breathing: Resistance increases, especially during forced expiration, because the airways may collapse or narrow, leading to more effort being required to exhale air. This is particularly evident in conditions like asthma or COPD, where forced breathing exacerbates resistance.
• What is lung compliance?

  The ability of the lungs to stretch and expand in response to changes in pressure. It is a measure of the lung's ability to expand when a given amount of pressure is applied.
• What does high lung compliance indicate?

  The lungs are easily stretched, while low lung compliance means that the lungs are stiff and resistant to expansion.
• How is lung compliance calculated?

  $Compliance\ =$$\ \frac{\Delta V}{\Delta P}$

  Where:

  $\Delta V$ = change in lung volume (mL/L)

  $\Delta P$ = change in transpulmonary pressure (the difference between the pressure inside the lungs (intra-alveolar pressure) and the pressure outside the lungs (pleural pressure)). (cm $H_2O$)
• What factors affect lung compliance?

  • Elasticity of lung tissue: The intrinsic ability of lung tissue to return to its original shape after being stretched (due to elastin and collagen fibers).
  • Surface tension of the alveoli: The tension at the air-liquid interface in the alveoli, which is reduced by surfactant, improving compliance.
  • Chest wall compliance: The ability of the chest wall to expand with lung expansion.
  • Age: As a person ages, lung tissue may become stiffer, leading to reduced compliance.
• How does lung compliance change in restrictive lung diseases (e.g., pulmonary fibrosis, ARDS, and interstitial lung disease)?


  • $\downarrow$Lung compliance
  • Increased stiffness of the lung tissue, which reduces the lung's ability to expand.
  • The alveolar walls and the interstitial spaces become thickened or scarred, making it harder for the lungs to stretch and inflate.
• How does lung compliance change in obstructive lung diseases (e.g., asthma, COPD)?

  • $\uparrow$ lung compliance (initially)
  • Air trapping and hyperinflation of the lungs occur, causing lungs to be more distended
  • Over time, the loss of the elastic recoil in the lungs die to damage of the alveolar walls (in COPD) can also decrease lung compliance.
  • Despite the $\uparrow$ in lung volume, ability of lungs to expel air (due to reduced recoil) is impaired.
• Why is lung compliance important in the mechanics of breathing?

  Lung compliance is critical for efficient ventilation. It directly impacts how easily the lungs can expand during breathing.
•  What happens when lung compliance is high?

   When lung compliance is high, the lungs can expand easily with minimal pressure, allowing for efficient airflow into and out of the lungs.
•  What happens when lung compliance is low?

  When lung compliance is low, more pressure is required to achieve the same volume of air, making it harder for the lungs to expand and resulting in difficulties with breathing.
• How does altered lung compliance affect breathing?

   Altered lung compliance can make breathing less efficient. Low compliance may result in shallow, laboured breaths, while high compliance may lead to air trapping and difficulty exhaling.
• How does lung compliance affect mechanical ventilation?

   In patients with respiratory disease, altered lung compliance can affect the effectiveness of mechanical ventilation. For instance, low compliance requires higher pressure settings, while high compliance may lead to over-expansion and difficulty in managing ventilation
• Why is reduced lung compliance a concern in diseases with insufficient surfactant?

  In diseases like Neonatal Respiratory Distress Syndrome (NRDS), insufficient surfactant leads to increased surface tension in the alveoli. This makes it harder for the lungs to expand, resulting in reduced compliance and difficulty in breathing.
• What is elastic recoil in the context of lung function?

  Elastic recoil refers to the ability of the lungs to return to their resting volume after being stretched. It is a property of the lung tissue that helps with exhalation.
• How is elastic recoil related to lung compliance?

  Elastic recoil and lung compliance are inversely related. Elastic recoil helps the lungs return to their resting position after stretching, while lung compliance measures how easily the lungs expand with pressure. They balance each other in lung function.
• How does high lung compliance affect elastic recoil?

  High lung compliance means the lungs stretch easily but do not snap back effectively. This is usually associated with low elastic recoil, making the lungs less able to return to their resting volume after expansion.
• How does low lung compliance affect elastic recoil?

  Low lung compliance means the lungs are stiffer and harder to expand, which is typically associated with high elastic recoil. The lungs are resistant to expansion but return to their resting volume more easily after being stretched.
• Why is the balance between lung compliance and elastic recoil important?

  The balance between lung compliance and elastic recoil is crucial for effective breathing. High compliance allows for easy lung expansion, while adequate elastic recoil ensures that the lungs return to their resting volume for efficient exhalation.
• Give an example of a disease that increases lung compliance and explain why.

  • Emphysema (a type of COPD) increases lung compliance.
  • In emphysema, the destruction of alveolar walls leads to a loss of elastic fibres, which reduces the lung’s ability to recoil.
  • This causes the lungs to expand more easily (increased compliance) but decreases their ability to expel air, leading to air trapping and difficulty exhaling.
•  Give an example of a disease that decreases lung compliance and explain why.

  • Pulmonary fibrosis decreases lung compliance.
  • In pulmonary fibrosis, the formation of scar tissue (fibrosis) in the lungs increases stiffness and reduces the ability of the lungs to expand.
  • This leads to decreased compliance and difficulty with inspiration, requiring more effort to expand the lungs.
• What is pulmonary surfactant?

  Pulmonary surfactant is a complex mixture of lipids and proteins secreted by type II alveolar cells (pneumocytes) in the lungs.
• What is the main function of pulmonary surfactant in respiration?

  The main function of pulmonary surfactant is to reduce surface tension at the air-liquid interface within the alveoli.
• How does pulmonary surfactant help prevent alveolar collapse?

  By reducing surface tension, pulmonary surfactant prevents the collapse of alveoli during exhalation. It stabilises the alveoli and helps maintain their open structure.
• What effect does pulmonary surfactant have on lung compliance?

  Pulmonary surfactant increases lung compliance, making it easier to expand the lungs during breathing.
• How does pulmonary surfactant facilitate easier breathing?

  By reducing surface tension and increasing lung compliance, pulmonary surfactant helps maintain proper lung volume and makes it easier to breathe.
• What are the key components of pulmonary surfactant?

  • Phospholipids: Primarily dipalmitoylphosphatidylcholine (DPPC), which lowers surface tension.
  • Proteins: Such as surfactant proteins A, B, C, and D, which help in surfactant synthesis, spreading, and host defence.
  • Cholesterol: Helps in stabilising the surfactant structure.
• What happens if insufficient pulmonary surfactant is produced in neonates?

  Insufficient pulmonary surfactant production in neonates leads to neonatal respiratory distress syndrome (NRDS). This condition is most common in premature infants, as surfactant production increases late in gestation. Without enough surfactant, surface tension within the alveoli increases, causing alveolar collapse, impaired gas exchange, and respiratory failure.
• What are the signs and symptoms of neonatal respiratory distress syndrome (NRDS)?

  • Rapid, labored breathing (tachypnea)
  • Grunting during exhalation
  • Nasal flaring
  • Cyanosis (bluish color of the skin due to low oxygen levels)
  • Decreased breath sounds These symptoms typically appear shortly after birth, especially in premature infants.
• How does insufficient pulmonary surfactant lead to respiratory distress in adults?

  In adults, insufficient pulmonary surfactant can occur in conditions such as acute respiratory distress syndrome (ARDS), which is often caused by trauma, pneumonia, or sepsis. When surfactant is deficient, surface tension in the alveoli increases, leading to alveolar collapse (atelectasis), reduced lung compliance, impaired gas exchange, and hypoxemia. This can lead to acute respiratory failure if untreated.