Diffusion & Perfusion

Question 1

Solubility coeffecient definition + a_O2 & a_CO2

Answer 1

• tendency of a molecule to dissolve in a liquid
• a_O2 = 0.0013 mM/Torr ==> oxygen is not very soluble in blood
• a_CO2 = 0.03 mM/Torr ==> CO₂ ~ 20x more soluble than O₂

Question 2

Equation for arterial content of O₂ (total blood concentration)

Answer 2

C_aO2 = hemoglobin-bound O₂ + freely-dissolved O₂

Question 3

ODC (oxy-hemoglobin dissociation curve) characteristics

Answer 3

• relates oxygen saturation of hemoglobin (S_O2) to the P_O2 in blood

Question 4

Factors that contribute to easy gas diffusion at alveoli

Answer 4

• Large surface area of alveolar membrane
• Thin alveolar membrane
• large O₂ pressure gradient between alveoli and cappilaries
  
  • low O₂ solubility in blood +
  • O₂ quickly binds to hemoglobin ==>
  • P_capO2 remains low & gradient high

Question 5

Diseases that impact O₂ diffusion

Answer 5

• interstitial disease ==> thickening of alveolar wall ==> slower rate of diffusion
• emphysema ==> breakdown of lung tissue ==> decreased surface area ==> slower diffusion
• abnormalities in hemobglobin (e.g. anemia) ==> smaller pressure gradient ==> decreased diffusion

Question 6

Perfusion definition/general characteristics

Answer 6

• Perfusion = Q
• amount of blood flow of pulmonary circulation available for gas exchange per min
• Q = CO = 5L/min @ rest

Question 7

Main factors that regulate perfusion (4)

Answer 7

1. O₂ tension
2. Chemical agents
3. Capillary recruitment
4. Gravity

Question 8

O₂ tension regulation of perfusion

Answer 8

• “O₂ tension” = P_AO2
• Low P_AO2 ==> hypoxic pumonary vasoconstriction ==> decreased local blood flow and shift to other regions

Question 9

Chemical agents regulation of perfusion

Answer 9

• Products of arachidonic acid metabolism:
  
  • thromboxane A₂ ==> vasoconstriction
  • Prostacyclin ==> vasodilation

Question 10

Capillary recruitment regulation of perfusion

Answer 10

• moderate exercise ==> increase CO ==>
• increased blood flow accommodated via recruitment of new capillaries

Question 11

Gravity regulation/impact on perfusion

Answer 11

• pulmonary BP @ apex << BP @ base
• @ base ==> more capillaries open + higher blood flow

Question 12

Major types of perfusion problems (3)

Answer 12

• alveolar dead-space
• shunts
• ventilation/perfusion (V/Q) mismatch

Question 13

Alveolar-dead space characteristics

Answer 13

• alveoli that are well-ventilated but lack perfusion
• no blood flow ==> no gas exchange ==> wasted ventilation

Question 14

Shunt characteristics

Answer 14

• blood perfused regions w/out ventilation
• small amount of shunt is normal
• doesn’t generally lead to increased arterial P_CO2 (compensated by increased ventilation via central chemoreceptors)

Question 15

V/Q mismatch characteristics

Answer 15

• V/Q = ratio of ventilation versus perfusion
• varying V/Q ratios throughout the lung ==> decreased arterial oxygenation (even when total V & Q are normal)
• since hemoglobin is generally near saturation ==> high V/Q regions will not be able to compensate for low V/Q regions

Question 16

Causes of V/Q mismatch

Answer 16

• Gravity
• COPD