BIOL 2P93 Lecture Notes - Partial Pressure, Vasoconstriction, Pulmonary Circulation

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Diffusion & Pulmonary Circulation
I. Diffusion
a. Definitions
i. Diffusion: process by which a net transfer of molecules takes place from
zone of higher partial pressure to lower
ii. Partial Pressure—Ppartial = Ptotal x (fractional concentration of gas)
iii. Static Diffusion—reaches equilibrium w/ no net transfer of gases
b. Fick’s Law
( )
gasoffluxPPD
T
A
Vgasgas __21** =
=
i. DCO2 = 20DO2; DCO = DO2
c. Diffusing capacity (conductance) = 1/resistance
DL(gas) = (net rate of transfer) / (partial pressure gradient alv – cap)
i. Indicator of how well the lung can carry out diffusion
d. Oxygen Diffusion
i. Normally, capillary blood equilibrates completely w/ alveolar air even
when capillary transit time is shorted by exercise
ii. Diffusion defect—seldom causes systemic hypoxia at rest but may cause
hypoxemia during exercise (less time for O2 equilibration)
iii. Increasing PAO2 => increased gradient & better capillary oxygenation
e. Measuring Diffusion Capacity
i. Often use small amount of CO and look at diffusion during 10s breath
hold (b/c mean partial pressure of CO in pulmonary cap. bl. is 0)
ii. CO has same diffusivity as O2 => used as surrogate
iii. Useful for measuring reductions in O2 diffusing capacity
iv. Sensitive but non-specific for changes in gas exchange (anemia also => ↓)
f. CO2 Diffusion
i. Occurs 20x faster than O2 => never have clinical hypercapnia (elevation
in arterial CO2 tension) due to diffusion defect
II. Pulmonary Circulation
(Raymond has removed an image blood pressures in various parts of the circulatory
system.)
a. Cardiac output: ranges from 5-25 L/min BUT pulmonary circulation rises
minimally b/c pulmonary capillaries are distended/recruited to accommodate
additional flow
b. Pulmonary Vascular Resistance (PVR)—slope of pressure flow relation
PVR = (Ppa – Pla) / cardiac output = pressure gradient across lung / CO
c. Passive control of Pulmonary Circulation
i. Lung volume alters PVR
ii. Alveolar vessels—become smaller w/ lung expansion & become larger w/
lung deflation; mainly capillaries & arterioles
iii. Extra-alveolar vessels—pulled open w/ lung expansion & pushed closed
w/ lung deflation; larger arteries & veins
1
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