Lesson 1

What is ventilation and what are its two critical physiologic functions?

• Ventilation is the exchange of O₂ and CO₂ between the atmosphere and the lungs.

• It delivers O₂ to hemoglobin for aerobic metabolism and removes CO₂, the primary end-product of aerobic metabolism.

Why is coordination between the brain and respiratory muscles necessary for ventilation?

• Ventilation requires cyclic pressure changes produced by coordinated respiratory muscle activity.

• These changes create pressure gradients that move air down its pressure gradient into and out of the lungs.

During quiet inspiration, which muscles contract and how do they change thoracic dimensions?

• The diaphragm and external intercostals contract.

• The diaphragm increases the superior-inferior dimension, and external intercostals increase the anteroposterior diameter of the chest.

What law explains how changes in thoracic volume cause airflow?

• Boyle’s Law.

• Increasing thoracic volume decreases intrathoracic pressure, creating a gradient that allows airflow into the lungs.

Why is expiration normally passive during tidal breathing?

• Expiration occurs due to elastic recoil of the lungs and chest wall.

• No significant muscle activity is required during normal tidal breathing.

When does expiration become an active process?

• During increased minute ventilation (exercise) or lung disease such as COPD.

• Active expiration uses abdominal muscles and internal intercostals.

Which muscles are accessory muscles of inspiration and when are they recruited clinically?

• Sternocleidomastoid and scalene muscles.

• They are recruited during increased work of breathing or respiratory distress.

Which muscles produce active expiration?

• Rectus abdominis

• Transversus abdominis

• Internal obliques

• External obliques

Mnemonic: “Let the air out (exhale) of my TIREs”

T = Transversus abdominis

I = Internal oblique

R = Rectus abdominis

E = External oblique

What clinical respiratory function requires a vital capacity of at least 15 mL/kg?

• An effective cough requires a vital capacity ≥15 mL/kg.

• This is important in anesthesia for assessing postoperative airway clearance ability.

Why is airflow said to follow the path of least resistance?

• Air moves according to pressure gradients and airway resistance.

• Therefore airflow preferentially moves toward regions with lower resistance and higher compliance

What are the three functional divisions of the airway and their roles?

1. Conducting zone
   

   • Air conduction only

   • No gas exchange (anatomic dead space)

2. 
   

3. Transitional zone
   

   • Air conduction and some gas exchange

4. 
   

5. Respiratory zone
   

   • Primary site of gas exchange

6. 
   

Which airway structures belong to the conducting zone?

• Nares

• Mouth

• Trachea

• Bronchi

• Bronchioles

• Ends at the terminal bronchioles.

Why is the conducting zone considered anatomical dead space?

• It does not participate in gas exchange.

• Its function is gas movement, warming, humidification, and filtration

Which structure marks the transition between conducting and respiratory zones?

• Respiratory bronchioles mark the beginning of the transitional zone

Which structures form the respiratory zone?

• Alveolar ducts

• Alveolar sacs

• Alveoli

These structures are responsible for gas exchange by diffusion

Across which cells does gas exchange primarily occur?

• Gas exchange occurs across Type I pneumocytes.

• These are thin squamous epithelial cells optimized for diffusion

Why are bronchioles and alveolar ducts prone to collapse?

• They lack cartilage, making them susceptible to external compression

Define transpulmonary pressure (TPP)

• TPP = Alveolar pressure – Pleural pressure.

• It represents the distending pressure that keeps airways open

What determines airway patency during breathing?

• Airway patency depends on positive transpulmonary pressure.

• If TPP is negative, airway collapse occurs

Define alveolar pressure and intrapleural pressure.

• Alveolar pressure (P_alv) = pressure inside the airway/alveoli.

• Pleural pressure (P_ip) = pressure outside the airway within the pleural space

During normal tidal breathing, what are the relationships between pressures?

• Transpulmonary pressure = always positive

• Intrapleural pressure = always negative

• Alveolar pressure fluctuates slightly positive or negative

When does alveolar pressure become negative and positive?

• Negative during inspiration → air flows in

• Positive during expiration → air flows out

Why is there no airflow at FRC or end-inspiration?

• Airflow requires a pressure gradient.

• At FRC and end inspiration, alveolar pressure equals atmospheric pressure, so flow stops.

When can intrapleural pressure become positive?

• Normally never during quiet breathing.

• It becomes positive during forced expiration or pathologic states like pneumothorax

What are the pressure relationships at FRC?

• Alveolar pressure = 0

• Intrapleural pressure = −5

• TPP = +5

• No airflow

What occurs during inspiration regarding pressures?

• Alveolar pressure becomes slightly negative (~ −1).

• Intrapleural pressure becomes more negative (~ −8).

• TPP increases → air flows into the lungs

What are the pressures at end-inspiration?

• Alveolar pressure returns to 0.

• Intrapleural pressure remains more negative (~ −8).

• TPP ≈ +8.

Airflow stops again

What happens during quiet expiration?

• Alveolar pressure becomes +1.

• Intrapleural pressure returns toward −5.

• TPP ≈ +6.

• Air flows out of the lungs

Why does airway collapse occur during forced expiration?

• During forced expiration:
  
  

  • Pleural pressure becomes positive

  • TPP may become negative

• 
  

• This can cause dynamic airway compression

Why is understanding transpulmonary pressure clinically important in anesthesia?

• Positive pressure ventilation increases alveolar pressure, altering TPP.

• Excessive pressures can cause:
  
  

  • Barotrauma

  • Alveolar overdistension

  • Reduced venous return