1.3.3 Mechanism of Gas Exchange Flashcards
What is the equation for the pressure produced by a single gas?
Describe how partial pressure of gases combine to create a total pressure.
Ptotal = Pa + Pb
Find the partial pressures of gases a and b given that the Ptotal is 100 mm Hg; a, n=6 and b, n=14.
The air we breathe is a mixture of O2 and N2. In dry air, FO2 is ~ 21%, and N2 is ~ 79%
The total pressure is the barometric pressure, which at sea level it is 760 mmHg. Find the partial pressures of O2 and N2.
PO2 = 760 * 0.21 = 160 mm Hg
PN2 = 760 * 0.79 = 600 mm Hg
What must be subtracted from wet air to calculate the partial pressure of oxygen?
The partial pressure of water (PH2O) which is 47 mm Hg at 37 degrees celcius. This value is independent of the total pressure
At KUMC, PB =747. What is PIO2? (hint: this is wet air)
PIO2 = (747-47) * 0.21 = 147 mm Hg
The sum of the partial pressures of all gases must add up to what?
PB
What are the two ways to manipulate the partial pressure of a gas?
Changing total pressure (PB) or changing the concentration (FIO2)
What are the partial pressures of gases (O2, CO2, PAN2) in alveoli, veins, and arteries?
Describe how ventilation and blood flow affect O2 and CO2.
O2: Delivered by ventilation, removed by blood flow (metabolism)
CO2: delivered by blood flow (metabolism), removed by ventilation
Describe what hyper- and hypoventilation will do PAO2 and PACO2.
Hyperventilation: High PAO2, Low PACO2
Hypoventilation: Low PAO2, High PACO2
Describe the phases: pre-inspiration, inspiration, end-inspiration, and expiration. What does it mean that VE = VA + VD?
The equation, VE = VA + VD, refers to the fact that expired air is a composition of air that undergoes gas exchange in the alveoli and air that is trapped in dead space (doesn’t undergo exchange).
What causes the lag demarcated by the blue lines?
The lag is due to the dead space in the respiratory system where gas exchange doesn’t occur.
During expiration, what portion of gas can provide a good reflection of PAO2 and PACO2?
End tidal gas (air at the end of expiration)
What equation is beneficial for measuring gas exchange? What is the average value in a healthy individual?
VD/VE = (FACO2 - FECO2)/FACO2); 0.3
Doubling and halving ventilation will have what effect on FACO2?
Doubling of alveolar (not total) ventilation will halve PACO2 (and PaCO2); halving alveolar ventilation will double PACO2.
What is the alveolar gas equation?
PAO2 = PIO2 - (PACO2 * 1.2)
If hyperventilation results in a 25% decrease PACO2 (from 40 to 30 mm Hg) and PIO2 =147 mm Hg, describe how all of the other gas concentrations will be effected.
If hypoventilation results in a 25% increase PACO2 (from 40 to 50 mm Hg) and PIO2 =147 mm Hg, describe how all of the other gas concentrations will be effected.
What is the equation for the diffusion of a substance across a membrane?
What is the equation for difussion capacity?
Are these gases diffusion or perfusion-limited: N2O and CO.
N2O = perfusion-limited; Partial pressure of N2O in the capillary raises rapidly because N2O is an “inert” gas, i.e. a gas that does not combine with any element of blood, and because it has very low solubility in blood. N2O crosses the membrane and, since it remains in solution in the blood, it rapidly builds a blood partial pressure which decreases the alveolo-capillary gradient and limits diffusion. As blood PN2O equals alveolar PN2O, net diffusion stops. The only way in which more N2O can be loaded into the blood is if blood without N2O enters the lung, thus creating a gradient again. Poorly soluble, inert gases like N2O are called perfusion limited because transfer depends on providing of adequate supply of fresh blood free of the gas in question.
CO = diffusion-limited; CO is the opposite, since it combines with Hb. Hb affinity for CO is over 200 times that of O2. Molecules of CO entering blood combine with Hb and only a few remain in solution, in this manner PCO of blood does not increase appreciably as blood flows through the capillary. This means that the alveolo-capillary gradient remains essentially unchanged during the entire time the blood remains in the capillary. Gases like CO are called diffusion-limited because the only factor that would limit its transfer is an increased “resistance” of the membrane to diffusion. CO is an excellent candidate to use in measurement of diffusion capacity, as we will see later
Which has a greater capacity for diffusion O2 or CO2?
CO2 has a greater capacity for diffusion. It takes less of a gradient to push CO2 across the alveolus.
What determines the diffusion of O2?
VO2 = (PAO2- PcapilO2) * DLO2
How does increasing (PAO2- P capil) O2 effect O2 flux?
Increases flux
What are some of the factors that determine DLO2?
Gas properties: O2 solubility and diffusivity
Lung properties: surface area for diffusion, distance alveolus - red cell
Hb-O2 characteristics: Time for O2 to combine with Hb
What is the normal duration for partial pressures of O2 and CO2 to reach equilibrium?
~0.25 sec
What happens if DLO2 is reduced?
A is normal
B is moderate reduction of DLO2
C is severe reduction in DLO2
Diffusing capacity of the lung for CO
A young person with normal lungs has a resting tidal volume of 600 ml and a respiratory frequency of 12 breaths/min. What would happen to the alveolar PO2 and PCO2 if this person would double the frequency and lower the tidal volume to one half for a few minutes?
In normal conditions during breathing at rest about one fourth to one third of the air entering the lungs remains in the airways (VD). In this case, this would be about 200 ml
So, when the person breathes normally, alveolar ventilation is VA = (600-200) x 12 = 4800 ml/min
when the person halves the tidal volume, the air volume that occupy the airways does not change much, so in this case
VA = (300- 200) x 24= 2400 ml/min.
Alveolar ventilation has halved, so PaCO2 should rise and PaO2 should fall.
a. decrease
b. increase. Diffusion capacity is influenced by the amount of Hb contained in the pulmonary capillaries, with DLO2 increasing as Hb increases. Basically, the total amount of Hb in the lungs can change if blood Hb concentration changes, or if capillary blood volume changes.
c. decrease
Indicate in which direction the following will change when DLO2 is abnormally low:
a. Difference between alveolar and arterial PO2
b. Arterial PO2 in exercise, compared to resting PaO2
c. Resting arterial PCO2
a. A-aPO2 will increase
b. Arterial PO2 will be lower in exercise than at rest
c. Resting arterial PCO2 will depend on the severity of the problem. If DLO2 is low enough to significantly lower arterial PO2 and induce hyperventilation, then PaCO2 will be low. If PaO2 at rest is normal, PaCO2 will also be normal.
A patient with alveolar hypoventilation has a PaCO2 of 60 mmHg. Assuming that PaCO2 will not change, calculate how much you would have to increase the inspired PO2 to maintain alveolar PO2 within normal range.
A person with normal lungs and normal blood gas values, with a tidal volume of 600 ml, breathes through a 200 ml snorkeling tube. Tidal volume and respiratory rate remain constant during the time the person breathes through the tube. Calculate the approximate values that PAO2 and PACO2 will reach once a steady state is achieved.
Snorkeling
Let’s assume that, before snorkeling, dead space was 200 ml and alveolar ventilation 400 ml (1/3 VD, 2/3 VA). If we add 200 ml to VD (snorkel) and VT remains unchanged, then VA goes down to 200 ml (i.e.“true”VD200ml +snorkel200ml+VA200ml= 600ml).This means that VA decreased to 50% of the original value. Other things being equal, this means that PACO2 will double (i.e. from a normal of 40 mm Hg to 80 mm Hg).
The new PAO2 will be:
PAO2 = 147–(80x1.2)=51mmHg
You may get slightly different results depending on the VD/VT value you use; the one I used is on the rather high side of normal but makes calculations easier. The point is that when you add dead space you must increase tidal volume by a proportionate amount, or you effectively end up hypoventilating. This is always taken into consideration in the design of respirators, respiratory equipment and anesthesia machines, which usually add very little dead space.
What are the respective values for FIO2 and FIN2?
FIO2 - .21
FIN2 - .79
What is PO2?
Partial pressure of O2 in a system
What is PIO2?
The partial pressure of O2 in inspirated air
What is PAO2?
That partial pressure of alveolar O2
What is PaO2?
Partial pressure of arterial oxygen
What is PvO2?
Partial pressure of venous O2
What is FACO2?
The fraction of CO2 in alveoli
What is VE?
Volume exhaled into the spirometer
