Test 3: Wk11: 7.2 Gas Transport and Diffusion - Dasgupta Flashcards
alveolus PO2
100mmHg
Steps of O2 diffusion
- O2 dissolves in water layer of alveolus
- Diffuses through thin layer of surfactant
- alveolar epithelium
- interstitial space
- capillary endothelium
- diffuse through plasma and bind Hb
PO2 in deoxygenated blood
40mmHg
what drives O2 diffusion
Partial pressure gradient
Diffusion is governed by
Fick’s Law
Fick’s Law
V*Gas = (A x D x deltaP) / T
gas exchange factors
- Partial Pressure Gradient
- Thickness and Properties of Membrane
- Surface Area of Alveoli
what limits O2 uptake in pulmonary capillaries
perfusion limitation
perfusion limitation
situation where gas diffuses through the membrane well and equilibrates quickly
The amount of perfusion limited gas that gets taken up is determined by
how much blood you pump through the capillary
on the average, blood is fully saturated with O2 by the time it is about — of the way down a pulmonary capillary.
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The uptake of O2 in a pulmonary capillary is said to be perfusion limited
because
blood is saturated with O2 at the end of the pulmonary capillary.
Thus, the more blood you pump through, the more O2 is taken up
the amount of O2 is taken up into the blood is determined the
rate of
perfusion of pulmonary capillaries
Other gases which
are perfusion-limited are
N2O and CO2
The classic example of the diffusion limited gas is
CO
The partial
pressure of CO on the blood side of the capillary is always — because
0
because of the
high affinity of Hb for it
what limits CO uptake?
The ability of
the membrane to transfer CO by diffusion
The mean pulmonary capillary partial pressure of CO is zero
because the gas does not
get time to equilibrate in the blood
DLCO
Diffusing Capacity of Lung
Diffusing Lung Capacity Test is used
to measure gas exchange
DLCO Overview
pt breaths a known amount of a gas
determine how much gas gets taken up by pt
compare to normal values
To measure DLCO — and — are needed
CO measuring device and a spirometer
“Diffusing capacity” is not a capacity… it is a measure of
CONDUCTANCE
conductance will reflect anything that alters the ability of a lung to exchange gases
area available for exchange, and barrier thickness
Obstructive disease DLCO will be
reduced because of alveolar simplification
and loss of pulmonary capillary structures.
Restrictive disease DLCO will be
reduced because of thickening of the alveolar
capillary membrane, which impairs diffusion
Fibrosis will — DLCO because
decrease because of increased thickness
Emphysema will — DLCO because
decrease because it destroys areas of gas exchange
Pulmonary Edema due to ARDS will — DLCO because
decrease because of thickening of capillary membrane from inflammation and edema
Alveolar Hemorrhage will — DLCO because
increase from red blood cells in alveolar membrane
Normal DLCO
20-30ml/min per mmHg
Emphysema decrease DLCO By
changes effective area because alveolar simplification lowers effective
area, and poorly ventilated units don’t contribute much to gas exchange
Embolus decrease DLCO By
blocks perfusion to alveolar capillaries
Alveolar Fibrosis or lung Fibrosis decrease DLCO By
decreases effective area
Interstitial or Alveolar Edema decrease DLCO By
fluid accumulation increases thickness of diffusion barrier
Tumors/lung reseaction decrease DLCO By
decreased surface area
Sarcoidosis, Asbestosis decrease DLCO By
Lesions thicken alveolar walls
Collagen vascular disease decrease DLCO By
Conditions like scleroderma, systemic lupus erythematous
alter or obilterate capillary walls which block diffusion
Anemia decrease DLCO By
reducing pulmonary capillary hemoglobin, the rate of uptake of CO is reduced,
so DLCO is decreased
Pregnancy decrease DLCO By
cause is not known
Alveolar Proteinosis decrease DLCO By
Alveoli are filled with protein and phospho-lipid-rich material so
DLCO goes down
Smokers decrease DLCO By
High levels of carboxyhemoglobin due to smoking reduce the rate of uptake of DLCO
Pulmonary Hypertension decrease DLCO By
Reduction of pulmonary capillary volume due to thickening of arteries
Disease that dont change DLCO
Bronchitis
Neuromuscular Dz
Chest wall deformities
Asthma
Bronchitis doesn’t decrease DLCO because
chronic bronchitis only affects upper airways
Neuromuscular Junction/chest wall deformities doesn’t decrease DLCO because
do not affect the alveolar
capillary membrane
Asthma DLCO
The status of DLCO in asthma is variable. Sometimes it is normal and
sometimes increased.
Causes of increased DLCO (7)
supine position Exercise Polycythemia Asthma Interalveolar hemorrhage L-R Cardiac Shunts Extreme Obesity
Intracardiac Shunts or left-right cardiac shunts increase DLCO by
Increased pulmonary
capillary volume
Intra-alveolar hemorrhage increase DLCO by
refers to hemorrhage when blood comes
into the alveoli. The hemoglobin in the alveoli combines with CO to raise the
DLCO.
Polycythemia increase DLCO by
increase in hemoglobin and therefore the uptake of
carbon monoxide is higher
Extreme Obesity increase DLCO by
have higher pulmonary
blood volumes and therefore have higher DLCO
— observed in Goodpasture’s syndrome
Intra-alveolar hemorrhage
hemoglobin in the alveoli combines with CO to raise the
DLCO
How does DLCO help in dx lung dz
diffusion is governed by changes of partial pressure and characteristics of alveolar capillary membrane which are changed in restrictive or obstructive dz
