pk 4 - Alveolar Gas Equation & Diffusion Flashcards
Normally only a small fraction of air is replenished within the respiratory zone with each breath. How much is this?
The alveolar partial pressure of O2 and CO2 are essentially stable at ….kPa and ….kPa respectively and these set the partial pressures found in ………….. …………..
13 and 5
arterial blood
The alveolar partial pressures of CO2 is related to …………….. ………. and …………. related to alveolar ventilation. Consequently, if VA is varied in proportion to VCO2, PACO2 and hence PaCO2 can be maintained ……………
metabolic rate (VCO2) inversely constant
What are the venous gas tensions in the pulmonary capillaries?
CO2 = 6 O2 = 5
The alveolar partial pressures of CO2 is related to …………….. ………. and …………. related to alveolar ventilation. Consequently, if VA is varied in proportion to VCO2, PACO2 and hence PaCO2 can be maintained ……………
metabolic rate (VCO2) inversely constant
What determines alveolar CO2?
What equation can be produced from this?
Which constant is used?
the relationship between metabolism and ventilation
alveolar CO2 = 115x (VCO2/ VA)
115kPa
What shape graph defines the relationship between alveolar CO2 and alveolar ventilation at any particular metabolic rate?
metabolic hyperbola
What is the definition of hyperventilation?
ventilating more than our metabolic needs
The more you hyperventilation the more PCO2 will move towards …… , (the level of CO2 in inspired air)
0
If metabolic rate is increased, how does the metabolic hyperbola change?
It shifts upwards and right, in order to maintain alveolar PCO2 at 5
Arterial CO2 is a clinical measure of ………… …………. adequacy
alveolar ventilation
What determines alveolar O2?
What has to be taken into account?
metabolism and ventilation
the fact that inspired O2 is not zero
What equation do you get if you combine the equations for alveolar O2 and CO2?
the alveolar gas equation
PAO2 = PIO2 - (PACO2/R)
When doing calculations with the alveolar gas equations, how do you calculate PiO2?
Needs to be of dry air at 37C
Need to remove water vapour pressure
How can the alveolar gas equation be used to see if there is a respiratory impairment?
calculate alveolar oxygen, using arterial CO2
compare to arterial O2
if there is more than 1kPa difference then there is an impairment
Gas movement within the ling depend crucially upon …….. within both liquids and gases and so is dependent upon …………….. …………, …………. and …………, Co2 is …..x more diffusive than O2, making ……….., rather than ……….., the first sign of diffusion limitation
diffusion concentration gradients molecular weight solubility 20 hypoxia hypercapnia
What three layers make up the alveolar-capillary membrane?
alveolar epithelium, basal lamina, capillary endothelium
In gas, grahams law states that bigger molecules diffuse ……… than smaller ones.
Info about CO2 and O2
slower
CO2 is bigger so diffuse 0.85 as fast as O2
In liquids, henry’s law says that a more soluble gas maintains a …….. concentration difference and diffuses …….
higher
easier
CO2 is 23x more soluble than O2 and so diffuses 23x as fast
What information do we get if we combine Henry’s Law and Graham’s Law?
0.85 x 23 = 20
CO2 diffuses 20 times more easily than O2
-
solubility
chemical combination
Which two gases are used to show the extremes of how gases move across a membrane?
N2O which is perfusion limited, low solubility and no chemical combination, the partial pressures with equilibrate very quickly, the only way to get more N2O into the body at 13kPa is to increase perfusion
CO which is diffusion limited, it is highly soluble and has a high chemical concentration. At an alveolar PCO of 13kPa, partial pressures of CO with not equilibrate and CO is therefore diffusion limited
Is O2 perfusion or diffusion limited?
perfusion limited
What does a high diffusion reserve for O2 allow us to do? How?
2/3 of transit time in which transit has occurred. This means we can increase the speed of blood through the pulmonary capillaries and decrease the transit time and still reach equilibration. This means we can get more oxygen into the blood and can therefore exercise
What can decrease diffusion reserve in a healthy person? Why?
Their oxygen is becoming more diffusion limited - failure to achieve equilibration of PO2. become hypoxic during exercise –> breathlessness. E.g. in pulmonary oedema, thickened membrane, altitude
What factors affect pulmonary diffusing capacity?
diffusibility of gas
area
thickness
pressure gradient
What can be measured using low conc CO? Why? Whats the equation?
diffusing capacity of the lung
Dl = V/(P1-P2).
CO combines with Hb so P2 is effectively zero.
DL = V/PACO
What 3 things increase diffusing capacity?
body size, exercise (efficiency), lying down (blood distribution)
Pulmonary Diffusing Capacity is reduced by:
Reducing effective surface area e.g. ………
Increasing diffusion path length e.g. ……….
loss of lung tissue, airway obstruction, capillary obstruction, V/Q mismatch
thickened alveolar-capillary membrane, accumulation of fluid, increasing intracapillary distance (anaemia)
How does a patient with diffusion limitation typically present?
hypoxiac with cyanosis that is aggravated by exercise. increased alveolar ventilation and decreased PaCO2
Normal ventilatory capacity
