Gas Exchange Lecture 1: O2 and CO2 Transport/ Ventilation and perfusion

Question 1

Define Gas Content

Answer 1

• The measure of the number of molecules of a gas contained in a given volume of liquid
• Expressed at litres / litre (vol of gas molecules in a container at STp per litre of liquid)

Question 2

State Henry’s Law

Answer 2

A constant T, the amount of given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid

P = kC

Question 3

How are gas content and partial pressures related?

Answer 3

Gases dissolve in liquids by amounts that are in direct proportion to their partial pressures

Question 4

Partial pressure/gas content relationship for oxygen dissolving in blood

Answer 4

For each mmHg P_O2, 0.003 mL O₂/100 mL blood is dissolved

Question 5

3 key characteristics of hemoglobin

Answer 5

• High affinity for O₂
• 12 - 15 gm of Hb per 100 mL of blood
• Max carrying capacity for O₂ (saturation)

Question 6

Equations for total blood oxygen content

Answer 6

Total blood O₂ content = HbO₂ + dissolved O₂

Question 7

Equation for HbO₂

Answer 7

[Hgb] x O₂ saturation x binding capacity

Question 8

Equation for dissolved oxygen

Answer 8

P_O2 x solubiltiy

Question 9

What is chloride shift?

Answer 9

The exchange of bicarbonate and chloride ions across the RBC membrane to allow bicarbonate to enter the plasma

Question 10

3 methods of CO2 transport in the blood

Answer 10

• 70 - 80% as bicarbonate
• 5 - 10% dissolved in plasma
• 5 - 10% bound to Hb as carbamino compounds

Question 11

CO2 content in blood relationship to partial pressure

Answer 11

Roughly linear disscoaition curve (relative to O2 curve)

Question 12

How can changes in CO₂ shift the HbO₂ dissociation curve to the right?

Answer 12

Bohr effect: increased CO₂, increased temp, increased 2,3 DPG, decrease pH

Question 13

How can changes in CO2 lead to a left shift of the HbO2 dissociation curve?

Answer 13

• Decreased CO2
• Decreased temp
• Decreased 2,3 DPG
• Increased pH

Question 14

What is the influence of O2 on the CO2 curve?

Answer 14

Haldane effect = increased loading of CO2 on deoxygenated Hb (increased CO2 carryign capacity of deoxygenated blood)

Question 15

Ideal alveolar gas equation fo CO₂

Answer 15

Question 16

Ideal alveolar gas equation for O₂

Answer 16

Question 17

What does the rate of O2 delivery to the alveoli depend on?

Answer 17

Ventilation and inspired PO₂

Question 18

What does the rate of O₂ uptake into blood depend on

Answer 18

The O₂ needs of the tissues (VO₂)

Question 19

Why is P_AO₂ lower than P_iO2?

Answer 19

Because P_AO₂ reflects a balance between the rate of O₂ delivery to the alveoli and the rate of O₂ uptake by alveolar capillary blood

Question 20

What determines the P_O2 and P_CO2 levels in the alveolus?

Answer 20

Ventilation-perfusion ratio

Question 21

Graphic effect of alveolar ventilation on P_AO₂ and P_ACO₂

Answer 21

Inverse relationship

Question 22

What exactly does the ideal alveolar gas equation for O2 represent?

Answer 22

The greatest arterial PO2 that such a lung could produce. So if P_aO₂ = P_AO₂, then the lung is operating at its highest efficiency

Question 23

How do you calculate (A-a) DO2?

Answer 23

Ideal alveolar PO2 - measured arterial PO2

Question 24

What exactly does A-a DO2 measure and why is it useful?

Answer 24

A measure of gas exchange efficiency useful in:

• Determining the efficiency of gas exchange in a given lung
• Helping to detemine the cause of hypoxemia

Question 25

Define hypoxemia

Answer 25

An abnormally low PO₂ in arterial blood for the subject's age

Question 26

5 causes of hypoxemia

Answer 26

1. Decreased PiO2 2. Hypoventilation 3. V/Q mismatch 4. Shunt 5. Diffusion limitation

Question 27

Pressure values when perfusion is 0

Answer 27

P_AO₂ = PiO₂ P_ACO₂ = PiCO₂ = 0

Question 28

Pressure values when ventilation is 0

Answer 28

PaO₂ = mixed venous PvO₂ PaCO₂ = PvCO₂

Question 29

What is wasted perfusion?

Answer 29

Blood flowing to alveoli with no ventilation = shunt (or shunt-like effect)

Question 30

What is wasted ventilation?

Answer 30

Air entering alveoli with no perfusion = dead space

Question 31

2 extremes of V/Q mismatch

Answer 31

Dead space Shunt

Question 32

4 examples of pulmonary shunt

Answer 32

1. Pulmonary edema 2. Consolidtion 3. Atelectasis 4. Airway obstruction (i.e. tumor, mucus, foreign body)

Question 33

Define pulmonary shunt

Answer 33

A region of the lung receives blood flow but is not ventilated

Question 34

Define extrapulmonary shunting

Answer 34

Blood bypasses the pulmonary circulation altogether

Question 35

3 examples of extrapulmonary shunting

Answer 35

1. Ductus arteriosus / heart septal defects 2. Thebesian veins (drain a small portion of coronary venous blood directly into the left atrium) 3. Bronchial circulation (a portion of bronchial veins drains into the pulmonary veins)

Question 36

When does hypoxemia occur from V/Q mismatch

Answer 36

When V/Q is LOW

Question 37

Why is dead space not a cause of hypoxemia?

Answer 37

V is infinitely higher than Q so V/Q = infinity and hypoxemia only occurs if V/Q is LOW

Question 38

How to calculate dead space

Answer 38

Question 39

Where and why can diffusion limitation be observed?

Answer 39

Elite athletes due to huge cardiac output and short transit time

Question 40

Effects of O2 therapy for V/Q inequality and shunt

Answer 40

Effective in V/Q mismatch, but highly ineffective in shunt

Question 41

Upon discovering a patient with hypoxemia, how do you determine the cause?

Answer 41