Pulmonary Ventilation

• What is pulmonary ventilation?

  Pulmonary ventilation (or minute ventilation) refers to the total volume of air moved into and out of the lungs per minute. It is calculated as:

  Minute Ventilation = Tidal Volume x Respiratory rate

  Where:

  • Tidal volume (VT) = volume of air per breath (typically ~500mL)
  • Respiratory rate (RR) = number of breaths per minute (typically ~12-20bpm)
• What is alveolar ventilation?

  Alveolar ventilation (VA) is the volume of air that reaches the alveoli per minute, participating in gas exchange.
• Why is alveolar ventilation important?

  Clinically relevant than minute ventilation because it excludes that anatomical dead space.

  Alveolar ventilation is critical for maintaining oxygenation and carbon dioxide removal
• How do you calculate alveolar ventilation?

  VA = (VT - VD) x f

  Where:

  • VT = Tidal volume (mL)
  • VD = Dead space volume (mL) (typically 150mL in an average adult)
  • f = Respiratory rate (breaths per minute)
• A patient has a tidal volume of 500mL, a respiratory rate of 12 breaths per minute, and an anatomical dead space of 150mL. What is their alveolar ventilation?

  VA = (500 - 150) x 12

  VA = 350 x 12 = 4200 mL/min
• What happens to alveolar ventilation if tidal volume increases while respiratory rate remains constant?

  
  

  Alveolar ventilation increases because more fresh air reaches the alveoli per breath. Since dead space volume remains constant, a greater proportion of each breath contributes to gas exchange.
• A patient doubles their respiratory rate from 12 to 24 breaths per minute, keeping a tidal volume of 500 mL. How does this affect alveolar ventilation?

  VA = (500-150) x 24

  VA = 350 x 24 = 8400mL/min

  • This results in a doubling of alveolar ventilation, increasing oxygen delivery and CO2 removal
• How does shallow, rapid breathing affect alveolar ventilation?

  Shallow, rapid breathing decreases alveolar ventilation because a larger proportion of each breath is wasted in the dead space. For example:

  • If VT = 200 mL and RR = 30 breaths/min, dead space (150 mL) takes up most of each breath, leading to minimal alveolar ventilation.
• Why is alveolar ventilation more clinically relevant than minute ventilation?

  •  Minute ventilation includes dead space ventilation, which does not contribute to gas exchange.
  • Alveolar ventilation directly determines oxygen delivery and CO₂ removal, making it a more accurate measure of respiratory efficiency.
• What muscles are involved in inspiration, and how do they contribute?

  • Diaphragm contracts and moves downward, increasing thoracic cavity volume.
  • External intercostal muscles contract, elevating the ribs and expanding the chest wall.
• How does Boyle’s Law explain inspiration?

  Boyle’s Law states that pressure and volume are inversely proportional.

  • When the thoracic cavity expands, lung volume increases, causing intrapulmonary pressure to decrease below atmospheric pressure.
  • This creates a pressure gradient that draws air into the lungs from the atmosphere.
• How does passive expiration occur?

  Passive expiration is the relaxation of inspiratory muscles:

  • The diaphragm relaxes and moves upward.
  • The external intercostals relax, reducing thoracic volume.
  • Elastic recoil of the lungs decreases lung volume, increasing intrapulmonary pressure above atmospheric pressure, forcing air out.
• How does forced expiration differ from passive expiration?

  Forced expiration requires active muscle contraction:

  • Internal intercostal muscles contract, pulling the ribs downward.
  • Abdominal muscles contract, pushing the diaphragm upward.
  • These actions increase intrapulmonary pressure more rapidly, expelling air forcefully.
• How does surfactant facilitate pulmonary ventilation?

  Surfactant, produced by type II alveolar cells, reduces surface tension in the alveoli, preventing alveolar collapse (atelectasis) and ensuring easier lung expansion during inspiration.
• How does airway resistance affect ventilation?

  Higher airway resistance (e.g., due to asthma, bronchoconstriction) makes breathing more difficult.

  • Bronchodilation (via sympathetic activation) reduces resistance.
  • Bronchoconstriction (via parasympathetic activation) increases resistance.
• What brain structures control ventilation?

  • Medulla oblongata (contains respiratory centers that regulate rhythm).
  • Pons (modifies breathing patterns).
  • The phrenic nerve controls diaphragm contraction.
• What are the pressure changes during inspiration and expiration involving intrapulmonary pressure, interpleural pressure and air movement?

  Inspiration:

  • Intrapulmonary pressure: decreases (below atm)
  • Intrapleural pressure: more -ve
  • Air movement: air enters lungs

  Expiration:

  • Intrapulmonary pressure: increases (above atm)
  • Intrapleural pressure: more +ve
  • Air movement: air leaves lungs

  
• What are the approximate alveolar partial pressures of oxygen and carbon dioxide?

  • PAO₂ ≈ 14 kPa
  • PACO₂ ≈ 5 kPa

  These values ensure diffusion of oxygen into the blood and removal carbon dioxide
• What are the typical arterial partial pressures of oxygen and carbon dioxide?

  • PaO₂ = 13 kPa
  • PaCO₂ = 5 kPa

  These pressures indicate efficient gas exchange and normal respiratory function.
• What are the typical venous partial pressures of oxygen and carbon dioxide?

  • PvO₂ = 5 kPa
  • PvCO₂ = 6 kPa

  Venous blood is oxygen-poor and carbon dioxide-rich before reaching the lungs for gas exchange.
•  What happens to alveolar oxygen partial pressure (PAO₂) when ventilation increases?

  • ↑ Ventilation = ↑ PAO₂
  • More oxygen enters the alveoli, increasing PAO₂ and enhancing oxygen diffusion into the blood.
•  How does increased ventilation affect alveolar carbon dioxide partial pressure (PACO₂)?

  • ↑ Ventilation = ↓ PACO₂
  • More CO₂ is expelled, lowering alveolar CO₂ levels and increasing the gradient for CO₂ diffusion from blood to alveoli.
•  How does ventilation affect the partial pressure gradient between alveoli and blood?

  • ↑ Ventilation = ↑ Partial Pressure Gradient = ↑ Gas Exchange Efficiency
  • This ensures proper oxygen delivery to tissues and CO₂ removal.
• What are lung volumes and lung capacities?

  • Lung volumes refer to individual amounts of air in the lungs during different phases of respiration.
  • Lung capacities are combinations of two or more lung volumes that reflect overall lung function.
• What is tidal volume (VT)?

  Tidal volume is the amount of air inhaled or exhaled in a normal breath at rest (~500 mL in adults).
• What is inspiratory reserve volume (IRV)?

  IRV is the additional air that can be forcibly inhaled after a normal inspiration (~2,500-3,000 mL).
•  What is expiratory reserve volume (ERV)?

   ERV is the extra amount of air that can be forcibly exhaled after a normal expiration (~1,000-1,200 mL).
• What is residual volume (RV), and why is it important?

  Residual volume is the air remaining in the lungs after maximal exhalation (~1,200 mL). It prevents lung collapse by maintaining alveolar inflation.
• What is vital capacity (VC), and how is it calculated?

  Vital capacity is the maximum amount of air that can be exhaled after a maximal inhalation.

  VC = IRV + VT + ERV
• What is functional residual capacity (FRC), and how is it calculated?

  FRC is the amount of air remaining in the lungs after normal expiration.

  Formula:

  FRC = ERV + RV
•  What is total lung capacity (TLC), and how is it calculated?

  TLC is the maximum volume of air the lungs can hold.

  Formula:

  TLC=IRV+VT+ERV+RV
• What factors influence lung volumes and capacities?

  • Age (lung function decreases with age)
  • Sex (males generally have larger lung volumes than females)
  • Height (taller individuals have larger lung volumes)
  • Lung compliance (elasticity of lung tissue)
  • Diseases (e.g., COPD reduces expiratory capacity, fibrosis restricts lung expansion)
• What is the pleural cavity, and where is it located?

  The pleural cavity is the thin, fluid-filled space between the two pleural membranes:

  • Parietal pleura (lines the thoracic cavity)
  • Visceral pleura (covers the lungs)
  • It is located within the thoracic cavity and surrounds each lung separately.
• What is the function of the pleural cavity in respiration?

  1. Reducing friction – The small amount of pleural fluid (~10–20 mL) lubricates the pleural surfaces, preventing friction during lung expansion and contraction.
  2. Creating surface tension – The fluid generates surface tension that helps keep the lungs adhered to the thoracic wall, allowing them to expand and contract efficiently.
  3. Facilitating negative pressure – The pleural cavity maintains a sub-atmospheric (negative) pressure, which prevents lung collapse and aids in normal breathing mechanics.
• Why is intrapleural pressure normally negative, and what is its typical value?

  • Intrapleural pressure is normally negative (~ -4 to -6 mmHg at rest) due to:

  1. The outward pull of the chest wall
  2. The inward elastic recoil of the lungs

  • This negative pressure prevents lung collapse and allows lung expansion during inspiration.
•  What is pneumothorax?

  Pneumothorax is the presence of air in the pleural cavity, which disrupts the negative pressure, leading to lung collapse.
• What are the main causes of pneumothorax?

  • Spontaneous pneumothorax – Occurs without trauma, often due to a ruptured lung bleb (common in tall, thin individuals and smokers).
  • Traumatic pneumothorax – Caused by external injury (e.g., stab wound, rib fracture).
  • Tension pneumothorax – A severe type where air enters but cannot escape, leading to increasing pressure that compresses the lungs and heart.
  • Iatrogenic pneumothorax – Occurs due to medical procedures (e.g., lung biopsy, mechanical ventilation).
•  What happens to lung function in pneumothorax?

  • Loss of negative intrapleural pressure leads to lung collapse on the affected side.
  • Reduced lung expansion impairs gas exchange, causing hypoxia.
  • In severe cases (tension pneumothorax), it can cause mediastinal shift, reducing venous return and cardiac output.
• What are the common symptoms of pneumothorax?

  • Sudden chest pain (sharp and pleuritic)
  • Shortness of breath (dyspnea)
  • Decreased or absent breath sounds on the affected side
  • Hyperresonance on percussion
  • In severe cases (tension pneumothorax):
  • Hypotension
  • Tachycardia
  • Tracheal deviation (away from affected side)
• How is pneumothorax treated?

  1. Small, stable pneumothorax – May resolve spontaneously with oxygen therapy.
  2. Larger pneumothorax – Requires needle aspiration or chest tube (thoracostomy) to remove air and re-expand the lung.
  3. Tension pneumothorax (emergency) – Requires immediate needle decompression followed by chest tube placement.