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Block B: Respiration > Gas Exchange Lecture 1: O2 and CO2 Transport/ Ventilation and perfusion > Flashcards

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

1
Q

Define Gas Content

A
  • 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)
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2
Q

State Henry’s Law

A

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

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3
Q

How are gas content and partial pressures related?

A

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

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4
Q

Partial pressure/gas content relationship for oxygen dissolving in blood

A

For each mmHg PO2, 0.003 mL O2/100 mL blood is dissolved

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5
Q

3 key characteristics of hemoglobin

A
  • High affinity for O2
  • 12 - 15 gm of Hb per 100 mL of blood
  • Max carrying capacity for O2 (saturation)
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6
Q

Equations for total blood oxygen content

A

Total blood O2 content = HbO2 + dissolved O2

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7
Q

Equation for HbO2

A

[Hgb] x O2 saturation x binding capacity

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8
Q

Equation for dissolved oxygen

A

PO2 x solubiltiy

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9
Q

What is chloride shift?

A

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

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10
Q

3 methods of CO2 transport in the blood

A
  • 70 - 80% as bicarbonate
  • 5 - 10% dissolved in plasma
  • 5 - 10% bound to Hb as carbamino compounds
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11
Q

CO2 content in blood relationship to partial pressure

A

Roughly linear disscoaition curve (relative to O2 curve)

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12
Q

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

A

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

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13
Q

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

A
  • Decreased CO2
  • Decreased temp
  • Decreased 2,3 DPG
  • Increased pH
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14
Q

What is the influence of O2 on the CO2 curve?

A

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

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15
Q

Ideal alveolar gas equation fo CO2

A
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16
Q

Ideal alveolar gas equation for O2

A
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17
Q

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

A

Ventilation and inspired PO2

18
Q

What does the rate of O2 uptake into blood depend on

A

The O2 needs of the tissues (VO2)

19
Q

Why is PAO2 lower than PiO2?

A

Because PAO2 reflects a balance between the rate of O2 delivery to the alveoli and the rate of O2 uptake by alveolar capillary blood

20
Q

What determines the PO2 and PCO2 levels in the alveolus?

A

Ventilation-perfusion ratio

21
Q

Graphic effect of alveolar ventilation on PAO2 and PACO2

A

Inverse relationship

22
Q

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

A

The greatest arterial PO2 that such a lung could produce. So if PaO2 = PAO2, then the lung is operating at its highest efficiency

23
Q

How do you calculate (A-a) DO2?

A

Ideal alveolar PO2 - measured arterial PO2

24
Q

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

A

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
25
Q

Define hypoxemia

A

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

26
Q

5 causes of hypoxemia

A
  1. Decreased PiO2
  2. Hypoventilation
  3. V/Q mismatch
  4. Shunt
  5. Diffusion limitation
27
Q

Pressure values when perfusion is 0

A

PAO2 = PiO2

PACO2 = PiCO2 = 0

28
Q

Pressure values when ventilation is 0

A

PaO2 = mixed venous PvO2

PaCO2 = PvCO2

29
Q

What is wasted perfusion?

A

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

30
Q

What is wasted ventilation?

A

Air entering alveoli with no perfusion = dead space

31
Q

2 extremes of V/Q mismatch

A

Dead space

Shunt

32
Q

4 examples of pulmonary shunt

A
  1. Pulmonary edema
  2. Consolidtion
  3. Atelectasis
  4. Airway obstruction (i.e. tumor, mucus, foreign body)
33
Q

Define pulmonary shunt

A

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

34
Q

Define extrapulmonary shunting

A

Blood bypasses the pulmonary circulation altogether

35
Q

3 examples of extrapulmonary shunting

A
  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)
36
Q

When does hypoxemia occur from V/Q mismatch

A

When V/Q is LOW

37
Q

Why is dead space not a cause of hypoxemia?

A

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

38
Q

How to calculate dead space

A
39
Q

Where and why can diffusion limitation be observed?

A

Elite athletes due to huge cardiac output and short transit time

40
Q

Effects of O2 therapy for V/Q inequality and shunt

A

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

41
Q

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

A

Block B: Respiration (4 decks)

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