Physio - Gas Exchange

Question 1

What are the typical values for the partial pressures of oxygen, and carbon dioxide in the atmosphere, alveoli, mixed venous blood, and arterial blood at sea level for a person with normal lung function?

Answer 1

Partial pressure in atmosphere:

• O2 = 760 mmHg
• CO2 = small

Partial pressure in mixed venous blood:

• O2 = 40 mm Hg
• CO2 = 46 mm Hg

Partial pressure in alveoli:

• O2 = 100 mm Hg
• CO2 = 40 mm Hg

Partial pressure in arterial blood:

• O2 = 100 mm Hg
• CO2 = 40 mm Hg

Question 2

What is the delivery and consumption of O2?

Answer 2

Basics:

• Normal resting conditons:
  
  • O2 used (tissues) = 250 ml/min
  • CO2 produced = 200 ml/min

Important Concept:

• O2 consumption & CO2 production = INDEPENDENT of the lung
  
  • relatively constant
  • respiratory quotient (RQ) = 0.8

Question 3

If all alveoli were the same, what is the role of Dalton’s Law?

Answer 3

Composition of inspired air

• Dalton’s Law:
  
  • Total pressure of mixed gas = SUM of (partial) pressures of component gases
• Alveolar air equilibrates with P_CO2 in venous blood which is 40
  mm Hg.
  
  • PO₂ in alveolar air is correspondingly reduced
• Water vapor plays a role in the lung
  
  • added partial pressure

Question 4

What is Fick’s Law of Diffusion?

Answer 4

Basics of Capillary Interface:

• Movement of O2 & CO2 from alv –> blood & vice versa = determined by DIFFUSION!

Fick’s Law of Diffusion:

• V_gas α [(P₁‐P₂)(A)(Sol)] / (T)(MW)
  
  • P1 - P2 = pressure gradient of gas
  • A = surface area for diffusion
  • Sol = solubility
  • T = thickeness of barrier
  • MW = molecular weight
• diffusion = PROPORTIONAL to solubility of gas in membrane
• diffusion = INVERSELY PROPORTIONAL to barrier thickness
  
  • ↓ diffusion rate = ↑ thickness

Question 5

What factors which may limit the diffusion of oxygen and carbon dioxide from
the alveolus to the blood or vice versa?

Answer 5

Normal Conditions:

• Normoxic air, resting CO, normal diffusion interface
  
  • gas exchange equilibrates by 1/3 of transit time
    
    • applies to O2 & CO2
  • safety factor for diffusion of gases

Exercise:

• Large ↑ in CO & Pulm blood flow
  
  • ↓ transit time
  • gas exchange = still reaches equilibrium at same rate

Thickening of Diffusion Barrier:

• May not reach equilibrium :(
• ↓PO2 or ↑blood flow

Question 6

How will dead space in the pulmonary system influences the gas composition in the alveoli and blood?

Answer 6

Anatomic Dead Space:

• V_T = V_A + V_DA
  
  • _​V_T = tidal volume
  • V_{A =} alv volume
  • V_{DA =} anatomic dead space vol
• ↑ Vt = ↓ contribution of V_DA
• ↓ Vt = ↑ contribution of V_DA

Physiologic Dead Space:

• Includes anatomic dead space + poorly ventilated alveoli
  
  • poorly ventilated alveoli = found in upper (apical) regions of lung
• V_DP = V_T (Pa_CO2‐PE_CO2)/(Pa_CO2)

Question 7

How can we predict the alveolar partial pressure of carbon dioxide based upon your knowledge of alveolar ventilation and carbon dioxide production by peripheral tissues?

Answer 7

Interdependence of Alv Ventilation & Alv CO2:

• sole source of CO₂ in alv = peripheral tissues
  
  • ⩒_CO2 (L/min)

Alv Ventilation Equation:

• (P_CO2)_{A =} ⩒_CO2/⩒_A

Relationship:

↑ CO₂ production –> ↑ PP of CO₂ –> ↓ PP of O₂

• Rate at which gas exchanges in alv & rate of CO2 production in tissues —> eventually effects PP of O₂

If we ↑ rate of ventilation & keep CO2 production rate constant….

• the PP of CO2 will eventually go ↓

Question 8

How can we predict the alveolar partial pressure of oxygen based upon the alveolar partial pressure of carbon dioxide, partial pressure of
oxygen in inspired air, and the respiratory quotient?

Answer 8

Interdependence of Alv CO2 & O2

• O₂ ,(P_O2)_I , in the inspired air would be diluted by water vapor (PH2O) and CO₂ from the peripheral tissues

Alveolar Gas Eq’n

• (P_O2)_A = (P_O2)_I – {(P_CO2)_A/R} + F
  
  • F = small & ignored.
  • I = inspired air
  • A = alveoli
  • R = respiratory quotient
    
    • = ⩒CO2/⩒O2
    • typically close to 0.8

Purpose:

• Allows us to estimate the PP of O₂ in the alv
  
  • we cannot directly measure this, but if we know the PP of CO2 & metabolism, then we can estimate

Note:

• Alveolar ventilation alters the Alv PP of O2 & CO2 (graph)
  
  • ↑ O2 = ↓ CO2
  • ↓ O2 = ↑ CO2

Question 9

List the factors which contribute to differing amounts of blood flow to regions of the lungs in an upright subject.

Answer 9

Upright posture:

• ↓ blood flow as you move from base –> apex of lung
  
  • due to effect of GRAVITY on BP in arteries & veins above & below the level of the atria
• Pul vessels = easily distended
  
  • gravity makes vessels have a greater diameter
    
    • ↑ blood flow at base of lung
    • ↓ blood flow at apex of lung
  • vessels at top = higher resistance (smaller diameter)
    
    • tendency to collapse

Regional Perfusion of the Lung

• Zone 1 = apex of lung
  
  • LOW blood flow due to top of lung (small pressure gradient) –>
    
    • collapsed & no blood flow
• Zone 2 = middle of lung
  
  • blood vessels = slightly larger diamter due to gravity
• Zone 3 = base of lung
  
  • HIGHEST blood flow (largest pressure differnce) –>
    
    • most exchange occuring

Question 10

What are the effect of inequalities of ventilation & perfusion of the lung?

Answer 10

Apex of Lung:

• Ventilation:
  
  • Pip = more (-)
  • ↑ transmural pressure gradient & ↑ alveoli size
  • ↓ compliance & ↓ ventilation
• Perfusion:
  
  • ↓ intravascular pressure & ↓ recruitment
  • distention
  • ↑ resistance
  • ↓ blood floow

Base of Lung:

• Ventilation:
  
  • Pip = less (-)
  • ↓ tranmural pressure gradient & ↓ alveoli ↓
  • ↑ compliance & ↑ ventilation
• Perfusion:
  
  • ↑ vascular pressure & ↑ recruitment
  • ↓ resistance
  • ↑ blood flow

When upright…

• apex = excess ventilation, ↓ perfusion
  
  • high V/Q ratio (>1)
  • high O2 tension & low CO2 in alv
• base = excess perfusion, ↓ ventilation
  
  • low V/Q ratio (<1)
  • low O2 & high CO2

Question 11

What can cause arterial hypoxemia?

Answer 11

• Inequalities in ventilation –> perfusion across the lung
  
  • can cause ARTERIAL HYPOXEMIA
• V-Q mismatch!
  
  • poorly ventilated path + well ventilated path come together
    
    • average will favor value closer to poorly ventilated (greater wt)

How to treat this?

• Suppliment w/ O2

Question 12

What are mechanisms to compensate for change in pressure gradient of O2 in lungs?

Answer 12

Poor Ventilation:

• Regions w/ inadequate ventilation (V/Q ratio < 1)
  
  • leads to constriction in blood vessels immediately above that alveoli
    
    • moves blood away from poorly ventilated –> toward better ventilated alveoli

Poor Perfusion:

• Regions w/ inadequate blood flow; (V/Q ratio > 1)
  
  • leads to constriction of alveoli to bring the ration closer to 1
    
    • decrease use of poorly perfused alveoli

Question 13

How is O2 transported in the blood?

Answer 13

Basics:

• Total amount of oxygen in arterial blood is the sum of O2 physically dissolved in plasma + O2 bound to Hb

Dissolved O2 = (solubility)x(P_O2)

• Dissolved O2 = 0.3 ml/100 ml

O2 bound to Hb = [Hb]x(ml O2/g Hb)x(% Hb Saturation)

• Amount of O2 bound to Hb = 19.6 ml O2/100 ml
• Saturation = 97%
• CARRIES WAY MORE BLOOD than dissolved

Total amount of O2/100 ml blood =

• 19.6 ml + 0.3 ml = 19.9 ml

Question 14

O2‐Hb DISSOCIATION CURVE

Answer 14

Arteries:

• HIGHER pp of O2

Veins

• LOWER pp of O2

Question 15

What is the effect of H+, Temp, and 2,3BPG on O2 binding to Hb?

Answer 15

• ALL REDUCE AFFINITY for O₂
  
  • _​Right shift on dissociation curve
    
    • aka BOHR EFFECT

Question 16

What are the 3 ways CO₂ can be transported in the blood?

Answer 16

1. Physically dissolved in blood = 5‐10%.
   
   • Solubility of CO2 in plasma is much greater that O2
   • Solubility constant = 0.06 ml CO2/100 ml/mm Hg
2. Bound to Hb = 5‐10%
   
   • Haldane Effect
     
     • ↑ O2 tension = ↓ binding for CO2
3. Converted to HCO₃ via carbonic anhydrase = 80‐90%.
   
   • MAJORITY transported this way!