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Resp Test 1 (Lecture 10: Alveolar Ventilation_Zhong)

Terms in this set (52)

How does compliance relate to elasticity?
Compliance is the inverse of elasticity.
A more elastic lung (with more elastic recoil) is less compliant.
Less change in V / delta P.
What is the major cause for hysteresis (different compliances for inspiration & expiration)?
Surface tension
The elastic forces can be divided into what 2 parts again?
1) Elastic tissue recoil
2) Surface tension of the alveoli
Therefore, what are the 2 major effects of surface tension?
1) Decreased compliance of the lung
2) maintains hysteresis
What are the 2 main effects of surfactant?
1) Lower surface tension of the lung
2) Stabilize the alveoli
Effect of surfactant on compliance?
Surfactant decreases surface tension, making the lung more compliant.
In air, which is composed of various gases, what is the Ptotal?
Sum of the various partial pressures:
Ptotal = P1 + P2 + .... Pn
The partial pressure of a gas depends on what?
The fractional [] of the gas.
What are the 2 major components of air?
Nitrogen (N2): 79%
Oxygen (O2): 21%
NOTE: CO2 is only 0.03%
So, what are the partial pressures of N2 and O2?
P air = 760mmHg = PN2 + PO2
- PN2 = 79% x 760 = 600mm Hg
- PO2 = 21% x 760 = 160mm Hg
So are these the same partial pressures that you'll find in your alveoli?
No
Why not?
Because of P(H2O) - water vapor
- at 37 degrees C, the P(H2O) is 47mmHg
Where is the water coming from?
Remember, air is humidified before it goes into our lungs.
So what are the resulting partial pressures in your alveoli?
Pair = 760mmHg = P(N2) + P(O2) + P(H2O)
- PN2 = 79% x (760-47) = 563mmHg
- PO2 = 21% x (760-47) = 150mmHg
- PCO2 = 0.03% x (760-47) = 0.21mmHg
What's the main point?
Alveolar air does not have the same gas []'s as atmospheric air composition.
"Dead space" is the volume of your airways and lungs that does NOT participate in gas exchange. What are the components of dead space?
1) Anatomical dead space (FIXED)
- volume of conducting airways = ~150ml
2) Alveolar dead space (alveoli that have lost function)
- alveolus not perfused, so no gas exchange
Physiological dead space?
Sum total = Anatomical dead space + alveolar dead space.
Symbols and abbreviations:
- C, F, S, V?
C: [] of gas
F: Fractional [] (eg. if a gas mixture)
S: Saturation of hemoglobin with O2
V: Volume of gas
Any time one of these letters has a dot over it, what does this mean?
/ per unit time
Large case letters mean what?
- eg. A
Gas phase
- A = alveolar
Lower case letters mean what?
- eg. a
Blood phase
- a = arterial, c = capillary
Calculating dead space: Bohr method. How does it work?
Approach that all expired CO2 comes from the alveolar gas and none from the dead space.
What is the final equation for V(D): physiologic dead space?
V(D) = V(T) x (PaCO2 - PECO2) / PaCO2
V(T): tidal volume
PaCO2: arterial P(CO2)
PECO2: expired air P(CO2)
Normally, the ratio of V(D) / V(T) is what?
0.3
~One third of your tidal volume is dead space.
There are different ways to express ventilation rate. What is minute ventilation rate?
V(dotE) or V(dotT) = tidal volume x breaths per minute
= V(T) x f
f = breathing frequency/minute
What is dead space ventilation rate (VdotD)?
May refer to either anatomic or physiological dead space (equal in a normal person).
V(dotD) = V(D) x f
Most important: what is alveolar ventilation rate (VdotA)?
V(dotA) = (Tidal Volume minus Dead Space) x breaths per minute
= V(A) x f, where V(A) = V(T) - V(D)
So: V(dotA) = (V(T) - V(D)) x f
Because all the expired CO2 comes from alveolar gas, V(dotA) can be measured using V(dotCO2).
V(dotA) = V(dotCO2)/P(CO2) x K
K: constant
Why is this equation very important?
Very important relation btw alveolar ventilation and alveolar PCO2 & PO2.
- easy to measure clinically and can be used to determine alveolar ventilation.
What happens to V(dotA) if PaCO2 is halved (and CO2 production remains unchanged)?
V(dotA) will double
The diffusion process is dependent on what?
Physics (Fick's Law) and the partial pressures of the gases involved.
What is the equation for the volume of gas that diffuses through a membrane per unit time - V(dot)?
V(dot) = A/T x D x (P1 - P2)
where D = Sol / square root (MW)
So, what are the factors that affect the rate of gas diffusion?
1) Thickness of resp membrane
2) Surface area of resp membrane
3) Diffusion coefficient (D)
4) Pressure different across membrane
Name some disease conditions that affect the membrane thickness.
Both increase thickness:
- Edema
- Fibrosis
Name a disease condition that decreases the surface area of the respiratory membrane.
Emphysema
other: removal of lung tissue during surgery
What's the difference btw diffusion vs perfusion?
Diffusion: molecules move from area of high [] to low [].
Perfusion: blood from pulmonary circulation through the alveolar capillary bed.
Why is this important?
There are 2 ways to stop gases from getting into the blood:
1) Perfusion limitation
2) Diffusion limitation
Why is the diffusion of nitrous oxide (N2O) perfusion limited?
Nitrous oxide is highly soluble in blood and is rapidly taken up by pulmonary capillary blood when a patient has inhaled nitrous oxide. Since it equilibrates so quickly in the capillary blood, the only way additional uptake can occur is by increased perfusion.
For N2O: what is P(A) compared to P(C)?
P(A) = P(C) --> perfusion limitation
This property of nitrous oxide is used for what?
To measure pulmonary blood flow (cardiac output)
What gas is used to explain a diffusion limitation?
CO: in contrast to nitrous oxide, carbon monoxide NEVER equilibrates in the pulmonary capillaries. This is because CO is bound immediately to hemoglobin and therefore does not accumulate in the plasma to exert a back pressure.
For CO: what is P(A) compared to P(C)?
P(A) > P(C) --> diffusion limitation
Why does CO always have a diffusion limitation?
The partial pressure in capillary blood stays very low due to rapid combination with hemoglobin.
What is the only way to transfer more CO into the blood?
Have a thinner barrier or more surface area.
What's the situation for O2?
Perfusion limitation.
- once O2 passes the membrane, it is readily taken up by the blood.
- normal healthy lungs are not diffusion limited, even during exercise.
What happens if you lower alveolar PO2 (eg. high altitude or hypoventilation)?
This slows diffusion.
- normal lungs during excercise can become diffusion limited when PO2 is low.
Why is the diffusion capacity for CO2 much higher than CO or O2?
CO2 is about 20 times more soluble than the others.
What are the factors that influence diffusing capacity?
- exercise
- body position
- body size
How does exercise influence diffusing capacity?
Due to recruitment and distension of pulmonary capillaries (opening more blood vessels) and better matching of blood flow and ventilation.
How does body position influence diffusing capacity?
Going from standing to sitting, then to supine position increases DL because of the increase in pulmonary capillary volume and a more even distribution of pulmonary blood flow.
How does body size influence diffusing capacity?
Lung size = surface area = higher diffusing capacity.
What are some pathological conditions that decrease diffusing capacity?
- Pathology of air-blood barrier (increased thickness or decreased surface area)
- decreased capillary volume
- decreased hemoglobin
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