Ventilation/Diffusion/Gas Transport

Question 1

What is the conventional way fractional gas concentrations are reported?

Answer 1

As though there were no water present Fg = Pg/(Pb-Ph2o)

Question 2

What does the pressure of water vapor in the airways depend on?

Answer 2

Temperature

Question 3

What is convention for expressing volumes of ventilated gas (ex: TV)?

Answer 3

BTPS (body temp, ambient pressure, saturated with water vapor)

Question 4

What is the convention for expressing metabolic rates (O2 consumption, CO2 production)?

Answer 4

STPD (because volumes at STPD direct relate to moles)

Question 5

What is Henry’s Law?

Answer 5

C(gas) = K*P(gas)

Question 6

What does K indicate in Henry’s Law?

Answer 6

Solubility (depends on specific gas, solvent, and temperature)

Question 7

How do the solubilities of O2 and CO2 compare?

Answer 7

CO2 is MUCH more soluble in blood than O2

Question 8

How does diffusion occur within a medium vs between different media?

Answer 8

In a medium - down concentration gradient Between media - down partial pressure gradient

Question 9

Why does convection occur?

Answer 9

Breathing makes the air (medium) move in a circulatory way, contributing to gas exchange and diffusion.

Question 10

What is minute ventilation (V dot)?

Answer 10

Amount of air inspired/minute (V dot I) or expired/minute (V dot E)

Question 11

How do you calculate minute ventilation?

Answer 11

VT x breathing frequency Normal: 500 mL x 15/min = 7.5 L/min

Question 12

Where does the last air inspired in each tidal breath go?

Answer 12

It stays in the anatomic dead space (conducting airways)

Question 13

How do you calculate alveolar ventilation?

Answer 13

f x (VT-VD) VT>VD, usually VT = 3 X VD

Question 14

What is the effect of decreasing respiratory frequency on alveolar ventilation?

Answer 14

It will increase V dot A (this advantage is limited by mechanical factors, etc though)

Question 15

What is alveolar dead space?

Answer 15

Alveolar gas volume that is ventilated but not effectively perfused, so it does not participate in gas exchange. Reasons - hydrostatic failure, PE, lung injury (ventilation of non-vascular air space), external obstruction of pulmonary circulation (Ex: tumor).

Question 16

What is the physiologic dead space?

Answer 16

The total volume of inhaled gas that doesn’t participate in gas exchange - sum of anatomic + alveolar dead space. In health, anatomic = physiologic dead space.

Question 17

What is the respiratory quotient?

Answer 17

R = V dot CO2/V dot O2

Question 18

What does RQ depend on?

Answer 18

Diet and metabolism

Question 19

Rank these diets from highest to lowest RQ: lipid, carbohydrate, protein

Answer 19

Carbohydrate (RQ = 1) > Protein (RQ = 0.83) > Lipid (RQ = 0.7)

Question 20

What is a normal RQ?

Answer 20

0.8

Question 21

What are two reasons that alveolar air is different from ambient air?

Answer 21

1) Water vapor 2) CO2 from circulation

Question 22

What is the partial pressure of water vapor at 37 C (body temperature)?

Answer 22

47 mm Hg

Question 23

Why can we say that inspired O2 = alveolar O2 and alveolar CO2?

Answer 23

1) No net exchange of N2 by body (partial P equation becomes PIO2 + PICO2 = PAO2 + PACO2) 2) Basically no CO2 in room air (PICO2 = 0)

Question 24

What is the normal range for PAO2?

Answer 24

90-105 mm Hg

Question 25

What is the consequence of having an RQ\<1?

Answer 25

Expired volume is less than the inspired volume. (This is why we must divide PACO2 by RQ in the alveolar gas equation)

Question 26

What is the relationship between alveolar ventilation (V dot A) and PACO2?

Answer 26

Inversely hyperbolically related (with constant metabolic rate). However, ventilation increases proportionally with metabolic rate (ex: with exercise) which keeps PACO2 nearly constant.

Question 27

How do we define hyperventilation and hypoventilation?

Answer 27

Hyperventilation - more alveolar ventilation than needed to maintain normal PCO2 HypOventilation - Less alveolar ventilation than needed to maintain normal PCO2

Question 28

What is the hallmark of hyperventilation? Hypoventilation?

Answer 28

Hyperventilation: hypocapnia Hypoventilation: hypercapnia \*need to know metabolic rate to define these though

Question 29

What is hyperpnea?

Answer 29

Appropriate increase in alveolar ventilation with increased metabolic rate to maintain constant PaCO2.

Question 30

What is hypopnea?

Answer 30

Appropriate decrease in alveolar ventilation with decreased metabolic rate to maintain constant PaCO2.

Question 31

What is the relationship between alveolar ventilation and PAO2?

Answer 31

Hyperbolically proportional

Question 32

What does venous admixture mean?

Answer 32

Mixing of shunted, non-oxygenated blood with normal oxygenated blood

Question 33

What does the rate of gas diffusion depend on?

Answer 33

Proportional to area and pressure gradient, inversely proportional to thickness. Also depends on the diffusion coefficient D, which is proportional to gas solubility and inversely proportional to molecular weight.

Question 34

What influences the diffusion path for O2 between the lung and blood?

Answer 34

Normally none of the first 4 pose significant barriers (path worsens): Crossing gas space within alveolus (emphysema, loss of lung tissue Crossing alveolar-capillary membrane (thickening from fibrosis)' Through plasma from capillary wall into RBCs Across RBC membrane into cell O2 combining with Hb \*\* reaction rate is signicant

Question 35

What limits the rate of O2 transfer from alveolar gas into blood?

Answer 35

Reaction rate of O2 combining with Hb.

Question 36

What is the normal transit time for blood in the pulmonary capillaries (at rest)?

Answer 36

0.75 sec

Question 37

What type of gas transfer exchange does CO follow?

Answer 37

Diffusion limited

Question 38

Why does a rise in CO concentration in the blood only increase its partial pressure slowly?

Answer 38

The effective solubility (capacitance) is very high because of its tight binding with Hb, and there is normally not any baseline CO present. Partial P = concentration/capacitance

Question 39

What does the gas transfer rate of diffusion limited gas depend on?

Answer 39

The diffusional characteristics of the membrane and the gas

Question 40

How does increasing blood flow rate affect diffusion limited gas transfer?

Answer 40

It does not.

Question 41

In general, how does partial pressure correlate with gas solubility?

Answer 41

Dissolving more soluble gases raise partial pressure slowly (they need more gas dissolved to raise partial P) Dissolving insoluble gases can raise partial P very quickly. (Partial pressure = concentration/capacitance, capacitance = effective solubility)

Question 42

What type of gas transfer exchange does N2O follow?

Answer 42

Perfusion limited

Question 43

What determines gas exchange rate for perfusion limited gases?

Answer 43

The rate of blood flow. The partial pressure of such gases increases very fast, so it mainly depends on perfusion rate.

Question 44

What type of gas transfer exchange does O2 follow?

Answer 44

Perfusion limited (less rapid than N2O though)

Question 45

What is the diffusing capacity (DL) of the lung?

Answer 45

Rate of gas transfer/pressure gradient across alveolar-capillary membrane. Summarizes both diffusion through blood-gas barrier and the chemical reactions of O2 and CO2 in the RBCs.

Question 46

What is the equational analogue of resistance to diffusion in the lung?

Answer 46

1/DL

Question 47

What limits the rate of CO2 exchange (preventing it from being super fast)?

Answer 47

Chemical reactions of CO2 1) formation of bicarbonate (HCO3-) 2) formation of carbamino compounds with blood proteins (esp Hb)

Question 48

What is the solubility of O2 in blood?

Answer 48

0.003 mL/100 mL blood/mm Hg

Question 49

What is the oxygen capacity?

Answer 49

Maximum amount of oxygen that can bind Hb

Question 50

What is the p50 of Hb?

Answer 50

PO2 when 50% O2 capacity is bound to Hb (indicates affinity of Hb for O2). Normal is about 27 mm Hg.

Question 51

What does a low P50 for the Hb-O2 curve mean about the affinity of Hb for O2?

Answer 51

High affinity.

Question 52

How does a right shift in the Hb-O2 curve affect P50?

Answer 52

P50 increases

Question 53

What is the percent saturation (SO2) equation?

Answer 53

SO2 = (O2 content/O2 capacity) \* 100

Question 54

What are three things that cause a right shift of the Hb-O2 curve?

Answer 54

Increased temperature, decreased pH/increased PCO2, increased 2,3-DPG

Question 55

What is the expression for O2 delivery to tissues?

Answer 55

O2 delivery = arterial O2 concentration x blood flow

Question 56

What are the forms of CO2 in the blood?

Answer 56

1) dissolved CO2 2) bicarbonate 3) carbamino compounds

Question 57

What enzyme catalyzes the conversion of CO2 to H2CO3?

Answer 57

Carbonic anhydrase