Alveolar Gas Exchange

Question 1

What is alveolar ventilation?

Answer 1

• Volume of air reaching the alveoli
• VA(Dot)= V_A x F
• 4 L/min is average
• To find V_A you take Vt- Dead space

Question 2

What is perfusion (Q)?

Answer 2

• Blood from the right ventricle
• 5 L/ min
• not asked to calculate, but it is CO=HR x SV

Question 3

What factors determine how much CO2 and O₂ can cross the alveoli barrier?

Answer 3

• J= (SA)xDx (P₁-P₂)/ Distance
• solved for each gas individually

Question 4

Under normal resting conditions how much CO2 and O2 are exchanged/min?

Answer 4

• CO2 is 200 mL
• O2 is 250 mL

Question 5

What does surface area correspond to?

Answer 5

• # of alveoli in the lung

Question 6

What happens to total gas exchanged as surface area increases?

Answer 6

• J increases
• They are directly proporitonal

Question 7

What makes up the distance between alveoli and blood?

Answer 7

• fluid layer
• alveolar epithelium
• interstitial space
• blood vessel wall

Question 8

As distance increases J will __.

Answer 8

As distance increases J will decrease

Question 9

What conditions will increase the distance for gas to travel?

Answer 9

• ILD
• CHF
• Sarcoidosis

Question 10

What conditions will decrease the SA of alveoli?

Answer 10

• Pneumonia
• ARDS
• COPD
• Neoplasm

Question 11

What doe sthe diffusion coefficient for each gas depend on? How do CO2 and O2 relate to these?

Answer 11

• Solubiltiy of gas in water and molecular weight
• O2 is less soluble in water than CO2
• Molecular weight of CO2 is greater than O2

Question 12

What is the average DL_O2 ?

Answer 12

21 ml O₂ / min/mm Hg

Question 13

How do you measure the DL_O2?

Answer 13

• Using CO because it will bind to Hb and there will be no dissolved CO
• Patient inhales a single breath of air with small amount of CO

Question 14

Describe the “reflex path” for a patient with decreased CO2 normal PaO2 due to pneumonia.

Answer 14

• Inc. diffusion distance
• Dec. J of 02
• Dec. Pa O2
• Increases firing of Peripheral chemoreceptors
• Inc Vt and frequency
• Inc min alveolar ventilation
  
  • Inc. Pa O2
  • also causes hyperventilation which dec PaCO2
    
    • Increases pH which sets of chemoreceptors to slightly decrease hyperventilation

Question 15

When will a patient with lung disease first notice limitations?

Answer 15

• During exercise because their need for oxygen will exceed the ability of lungs to allow for diffusion

Question 16

Will a change in PaO2 or PaCO2 occur first with lung disease?

Answer 16

PaO2 due to the limitations of oxygen diffusion

Question 17

How does surfactant impact small alveoli?

Answer 17

• It reduces Tension to a great amount so that the alveoli will no longer collapse into the larger ones
• It acts on large ones but not as significantly as small alveoli