L4 Principles of Gas Exchange

Question 1

what is ECMO?

Answer 1

Extracorporeal membrane oxygenation (ECMO) is a treatment that uses a pump to circulate blood through an artificial lung back into the bloodstream of a very ill baby. This system provides heart-lung bypass support outside of the baby’s body.

Question 2

what is the calculation for rate of diffusion?

Answer 2

Rate of diffusion = A x ΔP

A = surface area
ΔP = pressure gradient

Question 3

what is the universal gas equation?

Answer 3

PV = nRT

Question 4

what is partial pressure of a gas in a mixture of gases?

Answer 4

the pressure that it would exert if it was the only gas in the container

Question 5

what is dalton’s law (equation)?

Answer 5

the partial pressure of a gas in a gas mixture = total pressure x fractional concentration of the gas

Question 6

what is 1 atm in: kPa, bar, mmHg and cmH2O?

Answer 6

1 atm =
101.3 kPa
1.013 bar
760 mmHg
1033 cmH20

Question 7

what are the partial pressures of O2, CO2 and N2 at fractional concentration, at sea level and at an altitude of 5000m?

Answer 7

Oxygen:
21 %
21.3 kPa at sea level
11.8 kPa at 5000m

CO2:
0, 0, and 0

N2:
79 %
80 kPa sea
44.2 kPa 5000m

total % = 100, sea level = 101.3, 5000m = 56

Question 8

why is partial pressure of oxygen lower in alveoli than room air?

Answer 8

• inspired air is humidified in upper airway
• in alveoli o2 is taken up while co2 is added
• body consumes more o2 molecules than it produces co2 molecules (1.25x)

Question 9

why do we consume more o2 than we produce co2?

Answer 9

to do with the respiratory quotient which is dependent on diet. carbohydrate diet = respiratory quotient of 1, so for every o2 consumed, one co2 is produced. western diet as a quotient of 0.8 = so for every co2 produced, 1.2 molecules of o2 are consumed (less fat in fat)

Question 10

what is the saturated vapour pressure of water at 37*C?

Answer 10

6.3 kPa

Question 11

what is the partial pressure of oxygen at 37*C in fully humidified air?

saturated water vapour pressure = 6.3 kPa
oxygen saturation = 21%
sea level = 101.3 kPa

Answer 11

(101.3 - 6.3) x 0.21 = 19.95 kPa

Question 12

what is the typical value of alveolar partial pressure of co2?

Answer 12

5 kPa

Question 13

what is henry’s law?

Answer 13

solution in equilibrium with gas on top of it, the partial pressure of gas in solution is equal to the pp of gas above its surface

Question 14

how does a partial pressure of a gas in solution relate to its solubility?

Answer 14

partial pressure of a gas in solution is inversely proportional to its solubility

greater the solubility, the more molecules can be accommodated for a given pp

(anaesthetic is given as a low solubility gas so it when it gets to brain it comes out of solution faster)

Question 15

which one is more soluble, co2 or o2?

Answer 15

co2 is approx. 24 times more soluble in water than is oxygen

therefore co2 exerts a less pp than o2 for given number of molecules in solution

Question 16

what is the relative density, relative water solubility, relative diffusing capacity and pp gradient (kPa) of o2 and co2?

Answer 16

relative density: o2 = 1.00, co2 = 1.37

relative water solubility: o2 = 1.00, co2 = 24.0

relative diffusing capacity: o2 = 1.00, co2 = 20.5

pp gradient: 02 = 8.3 kPa, 6.1 kPa

(co2 diffuses quicker than o2)

Question 17

what is the difference between type I and type II respiratory failure?

Answer 17

Type I respiratory failure involves low oxygen, and normal or low carbon dioxide levels. Type II respiratory failure involves low oxygen, with high carbon dioxide.

Question 18

how do you measure oxygen diffusing capacity?

Answer 18

oxygen uptake / (alveolar pO2 - mean pulmonary capillary PO2)

Question 19

how do you measure carbon monoxide diffusing capacity and why is this used more than oxygen diffusing capacity?

Answer 19

carbon monoxide uptake - alveolar PCO (effectively 0 bc of very high affinity of CO for haemoglobin)

used more often as alveolar PO2 is very hard to measure

Question 20

what is alveolar fibrosis?

Answer 20

thickening of the alveolar wall

leads to decreased rate of diffusion and inc time for diffusion

blood is not fully oxygenated, can lead to type I respiratory failure

Question 21

what is pneumonia?

Answer 21

alveolar consolidation

bacterial infection fills lung

alveolus is blocked

blood goes around alveolus and is not oxygenated, is not participating in gas exchange

Question 22

what is pulmonary edema?

Answer 22

frothy secretions

mismatch of left and right sides of heart. left supplies body, right supplies lungs. mismatch means right keeps pushing into lungs but left does not pull out of lungs (if non functional). need to offload left side of heart

Question 23

what is interstitial edema?

Answer 23

systemic inflammatory response.

Question 24

what is emphysema?

Answer 24

alveolar-capillary destruction

easy to ventilate as has no tissue inside the lungs. lungs damaged by smoking. alveolus destroyed. also by vit deficiency. insufficient gas exchange.

Question 25

what is atelectasis?

Answer 25

alveolar collapse

older or larger patient who has lost elasticity in lungs. lung collapse.

Question 26

what is compliance?

Answer 26

ability of us to expand our lungs, the amount of gas we remove for each change in pressure/force in lungs.

opposite of elasticity

= 1/elasticity