Gas Transport in the Airways Flashcards
How do we calculate PiO2? (equation)
PiO2=(PB-47Torr) x .21
PB= barometric pressure (at sea level is 760 Torr; Denver is 620 Torr) 47Torr= partial pressure of water vapor .21= partial pressure of O2 in dry air
What is Pi02?
the partial pressure of O2 in inspired air at the point at which it has just entered the airways.
We know that dry air is about 21% O2. Why can we not just use this to approximate O2 in the airways when inhaled?
When air enters the lungs it becomes fully saturated with water vapor (at 37 degrees C). We use the partial pressure of water vapor at this temp and subtract that value from the barometric pressure to get a more accurate number.
Ok, he said we have to memorize the alveolar gas equation… what is it?
PAO2= PiO2 - (PACO2/R)
This just means that the O2 in the alveolus= the amount of O2 that you breathed in minus the amount of CO2 that went there from the gas exchange with the blood (divided by the respiratory exchange ratio R).
What is R in the alveolar gas equation and what does it mean?
respiratory exchange ratio
this tells us the relationship between O2 consumed and CO2 produced. It is the ratio of CO2 produced per amount of O2 consumed (for example 23O2–> 16 CO2; R= 0.7)
Normal is about 0.8
How do we determine R if patient is on 100% O2?
R=1 because normally when there is a deficit (meaning there is more oxygen produced than CO2 consumed) the extra space in the alveolus is filled by nitrogen (there is more N than O2 in breathing air). So if a rxn requires 23 O2 and produces 16CO2 there will be a deficit of 7 molecules in the alveolus (this space is usually filled by nitrogen). However, on 100% O2 the only available molecule to replace is more O2 so it will always balance out R if there was a deficit.
Why does R vary?
different for diff. metabolites. It is high for carbs (about 1) but low for fats (near .7). This just means that to breakdown fats the body requires more O2 than it produces CO2. normal diet results in R of 0.8
Which is the rate limiting step in CO2 removal, diffusion or ventilation?
ventilation- this is the process of CO2 transport between the alveoli and the outside air. This is bad because it will backup everything downstream from this process if it gets messed up and ultimately lead to increased levels of PaCO2 (which is what we really care about in the end).
Diffusion is the process of CO2 moving from the arterial supply to the alveolus. It happens almost instantly and reaches equilibrium such that at any given time PaCO2=PACO2.
Ok we were also told to memorize the alveolar ventilation equation… what is it?
PACO2=VCO2/VA x k
VCO2= CO2 production in one minute
VA=alveolar ventilation in one minute
k= constant
Because PACO2 approximately equals PaCO2, PaCO2 can be substituted into the equation.
Basically, this tells us that if CO2 production is constant and alveolar ventilation is decreased, PACO2 and PaCO2 is going to go up….. NARF!
How do we define hyperventilation vs. hypoventilation
hyperventilation- a decrease in PaCO2 (and an associated increase in VA).
hypoventilation- an increase in PaCO2 (and an associated decrease in VA).
If only VA changes but PaCO2 does not, this is not hyper/hypoventilation. Increased VA that matches CO2 production is just a normal response to exercise. This increase in ventilation during exercise is called hyperpnea.
common causes of hyperventilation?
- acute hypoxemia (low arterial oxygen)
- metabolic acidosis
- CNS stimulation
common causes of hypoventilation?
- obstructive/restrictive disease
- metabolic alkalosis
- CNS depression
T or F hyper and hypo ventilation refer to frequency of breathing?
F. they refer to ventilation. thank you.
So then what do we call a higher than normal frequency of breathing?
tachypnea
