Diffusion and Gas Transport

Question 1

What two factors affect PO2 at the level of inspired gas?

Answer 1

barometric pressure and inspired oxygen fraction

Question 2

What are the two factors that affect PO2 at the level of alveolar gas?

Answer 2

oxygen consumption and alveolar ventilation

Question 3

What are the two factors that affect PO2 at the level of arterial blood?

Answer 3

venous admixture and ventilation perfusion ratio

Question 4

What are the two factors that affect PO2 at the level of the cell?

Answer 4

blood flow and Hg concentration

Question 5

The rate of transfer of a gas through a tissue sheet is proportional to what?

Answer 5

the area of the tissue sheet and the pressure gradient

Question 6

The rate of transfer of a gas through a tissue sheet is inversely proportional to what?

Answer 6

thickness

Question 7

What is Fick’s law of diffusion?

Answer 7

the rate of gas diffusion by volume per unit time

Vg: volume of gas diffusing

A: surface area available for diffusion

D= diffusion coefficient

t= thickness of diffusion barrier

deltaP: pressure gradient driving diffusion

Question 8

The diffusion coefficient (D) in Ficks law of diffusion is proportional to what variable? What is it inverse to?

Answer 8

solubility of gas in the medium

square root of the molecular weight

Question 9

Why is the increased size of air space in emphysema a problem for gas diffusion?

Answer 9

it increases the distance required to diffuse from center to capillary of alveolus, reducing diffusion across the membrane

Question 10

What is the most relevant rate limiting factor for alveolar gas transfer to blood?

Answer 10

Hb binding reaction rate

equilibration and diffusion between gas and blood takes a finite time that is generally less than the time the RBC spends in the pulmonary capillaries

Question 11

What does the rate of diffusion of a gas WITHIN a given solvent depend on?

Answer 11

concentration gradient

Question 12

Define diffusion limited gas transfer

Answer 12

In diffusion limited gas transfer, the diffusion pathway provides a barrier that limits gas transfer. Blood flow has NO effect on the exchange process

Question 13

What is the effective solubility (capacitance) for CO? and how does that affect diffusion?

Answer 13

binding between CO and Hb is very high,

Question 14

In non smokers what is the baseline level of CO?

Answer 14

none

Question 15

When comparing a more soluble v. less soluble gas, which one will have a slower rate of rise in blood tension?

Answer 15

less soluble gas

Question 16

Are low solubility gases more likely to be diffusion or perfusion limited?

Answer 16

Perfusion limited, because their partial pressure rises very quickly in blood

Question 17

Is oxygen generally perfusion or diffusion limited? why?

Answer 17

Perfusion limited

Question 18

What 2 processes limit diffusion?

Answer 18

blood-gas barrier movement

chemical reactions

Question 19

What is the diffusion Capacity (DL?)

Answer 19

the rate of gas transfer divided by the pressure gradient across the membrane

Question 20

Is diffusion resistance arrange in series or in parallel?

Answer 20

in series

Question 21

Why is carbon monoxide generally used to determine diffusing capacity? What equation does this use?

Answer 21

Its mean pulmonary capillary partial pressure is normally zero

Question 22

What two factors limit CO2 exchange, thus offsetting the fact that diffusion of CO2 is generally faster than O2 when looking just at solubility?

Answer 22

1. formation of bicarbonate
2. formation of carbamino compounds

Question 23

At any given time, what is the solubility of oxygen in plasma? If PO2 = 100 mmHg, what is the dissolved oxygen concentration?

Answer 23

.003 ml 02/100 ml blood/mmHg

.3 Vol%

Question 24

Define oxygen capacity in the Hb dissociation curve.

Answer 24

the maximum amount of O2 that can bind to Hb

Question 25

What is P50? what does it describe?

Answer 25

Partial pressure of O2 at which 50% is bound to Hb Expresses affinity of Hb for oxygen

Question 26

If Hb concentration varies, which changes: O2 capacity or P50?

Answer 26

O2 capacity only

Question 27

How do you calculate SO2 ?

Answer 27

Question 28

Is SO2 dependent or independent of oxygen capacity?

Answer 28

independent

Question 29

If P50 drops, what direction does the SO2 v. PO2 relationship go?

Answer 29

Right shift

Question 30

If PO2 is normal, do you still have to worry about CO toxicity?

Answer 30

Yes. ## Footnote Oxygen is not the only gas that has an affinity for Hb. CO binds very strongly with Hb. Its affinity is about 200 times greater than O2 and desaturates only at very low PCO. This combination of factors makes CO potentially lethal even if PO2 is normal, since CaO2 is drastically reduced for a significant duration even after a transient exposure to CO.

Question 31

Does increased CO2 in the blood cause an increase or a drop in pH?

Answer 31

drop in pH - produces acid

Question 32

Define the Bohr effect

Answer 32

tissues are more acidic than lungs, facilitating O2 release to tissues that are metabolically active Tissues are more acidic due to production of CO2

Question 33

What are three conditions that raise 2,3 DPG? What does this mean?

Answer 33

Chronic hypoxia due to: altitude lung disease severe anemia Increases P50 and shifts curve to the right

Question 34

HOw do you calculate total oxygen delivery to tissues?

Answer 34

O2 delivery = arterial O2 concentration \* blood flow

Question 35

Define hypoxic hypoxia

Answer 35

hypoxic hypoxia (a low PO2 in the arterial blood);

Question 36

Define anemic hypoxia

Answer 36

(when oxygen carrying capacity is lowered by a low hemoglobin concentration in the blood);

Question 37

Define circulatory hypoxia

Answer 37

e.g., shock or hypoperfusion of the tissues

Question 38

Define histotoxic hypoxia

Answer 38

such as cyanide poisoning; this is the only form of tissue hypoxia where available oxygen cannot be utilized and the O2 extraction is lowered

Question 39

What enzyme is responsible for the production of bicarbonate?

Answer 39

carbonic anhydrase

Question 40

Define chloride shift in bicarbonate metabolism

Answer 40

When chloride bicarbonate exchanger moves bicarb out of the cell in exchange for chloride to preserve electric neutrality

Question 41

Does reduced or oxyhemoglobin bind more CO2?

Answer 41

Reduced Hb

Question 42

Define the Haldane effect

Answer 42

When O2 displaces HHb infavor of HbO2 in the lungs; this results in release of CO2 from bicarbonate and additional carbamino Hb breakdown

Question 43

What are the two main drivers of the Bohr-Haldane effect?

Answer 43

1. deprotonated Hb binds O2 more strongly than protonated Hb 2. deoxyHb binds protons more strongly than oxyhemoglobin

Question 44

Across a small range of PCO2, what does the shape of the CO2 dissociation curve mean ?

Answer 44

larger changes in CO2 without saturation effects

Question 45

What factors determine the rate of diffusion across the alveolar-capillary barrier at a given instant? If the diffusing capacity is constant can the rate of diffusion vary?

Answer 45

Question 46

How does rate of diffusion affect oxygen consumption?

Answer 46

The rate of diffusion of O2 should not affect the O2 consumption unless the diffusion falls below the minimum required to maintain V O2 .

Question 47

Calculate diffusing capacity using CO. What's a normal value? how much does it increase during exercise?

Answer 47

DL = ventilation rate CO/PACO normal: 25 increases by 2-3x

Question 48

With falling FiO2, will diffusion limitation value increase or decrease?

Answer 48

increase

Question 49

In health, what effect does exercise have on diffusion capacity?

Answer 49

increases it

Question 50

How do you calculate the O2 content of blood?

Answer 50

fractional HbO2 saturation x [Hb] x amount o2 bound in Hb i. e. 100% O2 sat, 15 gm Hb/100mL blood, 1.34 mL O2/gm Hb 1. 0 x 15 x 1.34 = 20.1 mL o2/100 ml blood = 20.1 vol% O2

Question 51

How do you calculate O2 uptake or consumption per 100 mL of blood?

Answer 51

arterial O2 content - venous o2 content = O2 removed from blood by metabolism

Question 52

How do you calculate the amount of O2 consumed per minute?

Answer 52

(arterial O2 content - venous O2 content) X cardiac output