Gas Movement

Question 1

What happens to the gas pressure gradient as it moves from mouth to the conducting airways? Why?

Answer 1

“Bulk Flow”
Difference btw airway and alveolar pressure progressively falls. Because:
1. Viscous and frictional forces decrease total gas or airway pressure
2. Diameter of airways decrease, but total cross-sectional area increases. This causes drop in total airway resistance, accomp. by decrease in airway pressure.

Question 2

Fick’s law for diffusion

Answer 2

Vgas = (A x D x (P1-P2)) / T
(= rate at which gas passes through gas-liquid interface)

Question 3

Diffusion Coefficient is directly proportional to…indirectly proportional to…

Answer 3

Directly: solubility of the gas in the tissues and fluids it must traverse
Inversely: square root of the gas’s molecular weight

Question 4

Diffusion Coefficient of CO2 vs O2

Answer 4

Dco2 20 times that of O2

Even though CO2 has higher molec. weight, it is 24 times more soluble.

Question 5

Partial pressure gradient of CO2 vs O2

Answer 5

Partial pressure gradient of CO2 is much less than that of O2

Question 6

Concentration of gas in the blood is dependent on what two factors?

Answer 6

Partial pressure and it solubility

Question 7

Rate at which the partial pressure of a gas equilibrates between the alveoli, blood, and tissues increases if the gas is more or less soluble?

Answer 7

Less soluble

Question 8

Relationship between anesthetic solubility and how quickly they act.

Answer 8

More insoluble anesthetics act quickly

More soluble drugs have a delayed effect

Question 9

Causes of arterial hypoxemia

Answer 9

1. Hypoventilation - an increase in PACO2 causes PAO2 to drop. No increase in PA-aO2
2. Reduced PB or FiO2 - decrease in total gas (barometric) pressure results in decreased PAO2. No increase in PA-aO2
3. Ventilation-perfusion mismatching - Low V/Q causes decrease in PaO2 and increase in normal PA-aO2
4. Shunt - Addition of mixed venous blood to the arterial circulation causes fall in PaO2 and increase in normal PA-aO2
5. Impaired diffusion - May result from thickening of the diffusion barrier or from a decrease in the SA available for gas exchange. If sufficiently severe, impaired diffusion leads to a decrease in PaO2 and increase in PA-aO2.

Question 10

Causes of inadequate alveolar ventilaiton

Answer 10

Abnormally low VE:

• Respiratory center depression
• Neuromuscular disease
• Severe chronic lung disease
• “restrictive” chest wall disorders

Inadequate increase in VE:
-PaCO2 will rise if VE is inadequate to compensate for an acute increase in either physiologic dead space of CO2 production.

Question 11

Equation for amount of oxygen transported as dissolved oxygen in blood.

Answer 11

Dissolved O2 (mL O2/100mL blood) = 0.003 x PaO2

Question 12

Most important factors which influence the relationship btw the partial pressure of oxygen in the arterial blood and the saturation of hemoglobin.

Answer 12

• pH
• Temp
• Concentration of inorganic phosphates, such as (2,3-DPG)

Question 13

What changes result in Right shift of oxyhemoglobin saturation curve?

Answer 13

Decreased pH
Increased Temp
Increased 2,3-DPG

Question 14

How is CO2 transported in the blood?

Answer 14

Most of it enters the RBC. The rest:

1. dissolved in plasma (relatively small amount)
2. Bound by hemoglobin to form carbamino compounds. Formation of compounds enhanced by presence of unsaturated hemoglobin (Haldane effect)
3. Hydrated (relatively slow since carbonic anhydrase is not present in the plasma)

Question 15

Bohr Effect

Answer 15

Describes the effect of PCO2 on the affinity of hemoglobin for O2
Tissues: Increased PCO2, lowers pH, decreases affinity of hemoglobin for O2 (greater O2 release to tissues)
Lungs: Decreased PCO2, raises pH, increases affinity of hemoglobin for O2 (allows for O2 loading)

Question 16

Haldane Effect

Answer 16

Describes the effect of PO2 on the affinity of hemoglobin for CO2.

• Deoxygenated hemoglobin has greater affinity for CO2 than does oxyhemoglobin.
• Results from enhanced ability of deoxygenated hemoglobin to form carbamino compounds and to accept the H+ released by the hydration of CO2.
• Enhances ability to load CO2 in the tissues and release it in the lungs