Arterial Blood Gasses

Question 1

Oxygen “off-loading” characteristics

Answer 1

• most oxygen is bound to hemoglobin
• tissues can only use freely-dissolved O₂ ==> O₂ must unbind from hemoglovin
• ODC reflects equilibrium between O₂ binding and unbinding
  
  • decreased pH or increased P_aCO2 ==> rightward shift of ODC curve
  • rightward shift ==> increased O₂ unloading @ tissues (beneficial during exercise)

Question 2

Quantity of O₂ delivery/minute =

Answer 2

• = CO x concentration of arterial O₂
• D_O2 = Q x C_aO2
• C_aO2 = (S_aO2x [Hb] x 1.39) + (0.003 x P_aCO2)
  
  • S_aO2 = saturation of hemoglobin
  • 1.39 (ml/gram) = max volume of O₂ that can combine w/1 g of hemoglobn
  • ([Hb] x 1.39) = “oxygen carrying capacity”

Question 3

Hypoxemia definition

Answer 3

• reduced arterial free oxygen + reduced percent saturation of hemoglobin (S_aO2) ==>
• decreased O₂ delivery

Question 4

Oxygen consumption equation

Answer 4

• =difference between O₂ content of arterial and venous blood
• V_O2 = (Q x C_aO2) - (Q x C_vO2) = Q x (C_aO2- C_vO2)
• Ignoring small dissolved O₂ component ==>
  
  • V_O2 = Q x (S_aO2 - S_vO2) x [Hb] x 1.39

Question 5

Relationship between oxygen tension and transport towards/into tissues [diagram]

Answer 5

Question 6

Measures that asses arterial oxygenation

Answer 6

1. arterial oxygen tension (P_aO2)
2. oxy-hemoglobin saturation (S_aO2)
3. alveolar-arterial pressure gradient for oxygen
4. blood oxygen content (C_aO2)

Question 7

Desturation vs. hypoxemia

Answer 7

• saturation = % hemoglobin occupied (S_aO2)
• hypoxemia = low arterial oxyen tension (P_aO2)
• hypoxemia w/out desaturated: P_aO2 < 65 mmHG & S_aO2 > 90%
• desaturation w/out hypoxemia: PaO2 > 65 mmHG & SaO2​ < 90%

Question 8

Main causes of hypoxemia (+ major physiologic problem)

Answer 8

• P_aO2 < 80 Torr (<65 Torr @ Denver)

1. low ambient P_O2 in inspired air (altitude)
   
   • normal A-a gradient
   • problem w/O₂ ==> alveoli
2. hypoventilation: increased alveolar P_ACO2 ==> decreased P_AO2​
   
   • normal A-a gradient
   • problem w/O₂​ ==> alveoli
3. diffusion limitation between alveoli and pulmonary capillaries
   
   • widened A-a gradient
4. V/Q mismatch
   
   • widened A-a gradient
   • responds readily to 100% oxygen
5. Shunt
   
   • widened A-a gradient
     ​
   • does not respond to 100% oxygen

Question 9

Main methods of CO₂ transport w/in blood

Answer 9

• Dissolved CO₂
• Bicarbonate ion (HCO₃^-): Carbonic acid (H₂CO₃) is converted to water and carbon dioxide via carbonic anyhydrase
  
  • H₂O + CO₂ <==> H₂CO₃ <==> H⁺ + HCO₃^-
• Carbamino acids: bound to proteins (e.g. hemoglobin)