Lecture 4: Physical Principles Of Gas Exchange Flashcards
What is diffusion?
Random molecular motion of molecules
Movement of a gas in one direction is the effect of…
A concentration gradient
Direction of diffusion occurs from areas of ___ to ___
High to low concentration
Rate of diffusion is dependent on ___
Pressure
Total pressure of mixture =
The summation of partial pressures of gases in a mixture
Respiratory gases include (3)
- Oxygen
- Nitrogen
- Carbon dioxide
Composition of air
- 79% nitrogen
- 21% oxygen
Total pressure of air mixture =
760 mm Hg
1 atmosphere =
760 mm Hg
Nitrogen partial pressure
79% of 760 mm Hg = 600 mm Hg
Oxygen partial pressure
21% of 760 mm Hg = 160 mm Hg
Henry’s Law—Solubility of a gas in a liquid depends on what 4 things?
- Temperature
- Partial pressure of gas over the liquid
- Nature of the solvent
- Nature of the gas
When dissolved molecules are attracted by water more and can be accumulated without building up excess pressure in solution = ___ soluble
Highly soluble
Molecules which are repelled by water will dissolve less and have lower concentration = ___ soluble
Poorly soluble
Carbon dioxide is ___ times more soluble than oxygen
20 times
Rate of net diffusion of gases is determined by…
Difference of partial pressures (pp)
If pp of gas in alveoli > blood, then…
Gas moves into blood (i.e.: oxygen)
If pp of gas in blood > alveoli, then…
Gas moves into alveoli (i.e.: carbon dioxide)
What is vapor pressure?
Partial pressure of water escaping surface into gas phase
Vapor pressure is ___ mm Hg when gas mixture is fully humidified at 37 degrees Celsius
47 mm Hg
Vapor pressure depends on ___
Temperature
At greater temperature, the ___ the kinetic energy and ___ water escaping into the ___ phase
Greater the kinetic energy, more water escaping into the gas phase
How do you calculate the pp of O2 in the upper airway?
760 mm Hg (total) = 713 mm Hg (pp Air) + 47 mm Hg (pp H2O vapor)
Pp of O2 in upper airway = 713 mm Hg x 0.21 = 150 mm Hg
What are 6 factors that affect gas diffusion rates?
- Pressure differences
- Gas solubility in fluid
- Area of fluid
- Distance which gas must diffuse
- Molecular weight of gas
- Temperature of fluid (constant in body)
What are 2 characteristics of the gas which affect the ability and rate of net diffusion?
- Solubility of gas molecule
- Molecular weight
Diffusion coefficient is proportional to…
The relative rates at which different gases diffuse
D =
Diffusion coefficient
D is directly proportional with ___
Solubility
D is inversely proportional to…
The square root of the gas’ molecular weight
Respiratory gases are highly soluble in ___
Lipids (the main component of cell membranes)
Cell membranes are highly permeable to…
Respiratory gases
Rate of gas movement into tissues is limited by…
Diffusion rate of gas through tissue water
Movement of gas into and out of tissues =
Diffusion rate of gas through water
Does alveolar air have the same gas concentrations as atmospheric air?
No
Why are there differences in alveolar air vs. atmospheric air? (4 things)
- Alveolar air is partially replaced by atmospheric air during each breath
- Oxygen constantly absorbed into blood from alveoli
- Carbon dioxide diffused into alveoli from blood
- As air enters respiratory passages, it becomes humidified, diluting the inspired gases partial pressures
What is required to exchange alveolar air?
Multiple breaths
___ ccs of air per breath
350 ccs
FRC is roughly ___ ccs
2500 ccs
Each breath replaces a ___ of FRC
A seventh
Multiple breaths to exchange alveolar air prevents…
Sudden change in gas concentrations; allows respiratory control mechanisms to be more stable
Normal alveolar ventilation removes 1/2 gas in ___ seconds
17 seconds
Half normal ventilation removes 1/2 gas in ___ seconds
34 seconds
Twice normal ventilation removes 1/2 gas in ___ seconds
8 seconds
Partial pressure of oxygen is controlled by rate of ___ and ___
Absorption and ventilation
What affects alveolar PO2? (3 things)
- Rate of ventilation
- Oxygen pressure
- Exercise
Normal alveolar PO2 =
100 mm Hg
Normal alveolar PCO2 =
40 mm Hg
Alveolar PCO2 increases in proportion to…
CO2 excretion
PCO2 decreases in inverse to…
Alveolar ventilation
Expired air is a combination of ___ and ___
Dead space and alveolar air
First portion of inspired air is…
Dead space air—consists of humidified air
Second portion of expired air is…
Mixture of both dead space and alveolar air
Alveolar air is expired at ___ of exhalation
End
What is the overall thickness of the respiratory membrane?
0.6 micrometers
What is the total surface area of the respiratory membrane?
70 meters squared
RBC squeeze through ___ micrometer diameter capillaries
5 micrometer
___ diffusion rates for respiratory gases
Rapid!
Why are diffusion rates rapid through the respiratory membrane?
Minimal transfer time and distance through plasma
What are 4 factors that affect the rate of gas diffusion through the respiratory membrane?
- Thickness of respiratory membrane
- Surface area of respiratory membrane
- Diffusion coefficient (D)
- Pressure difference across respiratory membrane
Rate of diffusion is ___ proportional to membrane thickness
Inversely
-Increasing thickness by 2-3 times interferes significantly with normal respiratory exchange
What 3 things increase thickness of the respiratory membrane?
- Edema
- Fluid
- Fibrosis
What is the effect of decreasing surface area to 1/4 normal?
Impedes gas exchange significantly
i.e.: emphysema, removal of lung tissue during surgery
Emphysema can decrease surface area as much as ___
5-fold
What 2 things determine diffusion coefficient (D) of gases?
- Solubility
- Molecular weight of gas
CO2 diffuses ___ times faster than oxygen
20
Oxygen diffuses ___ times as rapidly as nitrogen
2
Diffusion occurs across the membrane ___ the pressure gradient by ___
Down, simple diffusion
What is diffusing capacity?
“The volume of a gas that will diffuse through the respiratory membrane each minute for a pressure difference of 1 mm Hg”
AKA the ability to exchange gas between alveoli and pulmonary blood, expressed in quantitative terms
What can affect the diffusing capacity?
Factors which affect diffusion through the respiratory membrane
What is the diffusing capacity for O2 at rest?
21 ml/min/mmHg
What is the oxygen diffusing capacity during exercise?
Diffusing capacity increases three-fold, to max of 65 ml/min/mmHg
What is the diffusing capacity of CO2 at rest?
400 ml/min/mmHg
What is the diffusing capacity of CO2 during exercise?
1200 ml/min/mmHg
What is the diffusing capacity for CO?
17 ml/min/mmHg
Which gas has the highest diffusing capacity rate?
CO2
What is the V/Q ratio?
Highly quantitative concept of imbalance between alveolar ventilation and blood flow
V/Q is normal when…
Alveolar ventilation and blood flow are normal
V/Q is zero when…
When ventilation is zero but perfusion is present
V/Q is infinity when…
If ventilation is present but there’s no perfusion
If V/Q ratio = zero or infinity, there is ___ exchange of gases
No exchange
V/Q = zero
- Blood flow but no alveolar ventilation (i.e.: complete airway obstruction)
- Gases diffuse between blood and alveolar air
- Air in alveoli reaches equilibrium with deoxygenated blood returning to lungs in pulmonary arteries
In normal deoxygenated blood, the PO2 is ___ mmHg and PCO2 is ___ mmHg
PO2 = 40 mmHg PCO2 = 45 mmHg
V/Q = infinity
- There is alveolar ventilation but no blood flow (i.e.: pulmonary artery obstruction)
- Alveolar air becomes equal with humidified inspired air—no loss of oxygen into blood, no gain of CO2 from blood
Alveolar gas partial pressures—PO2 is ___ mmHg and PCO2 is ___ mmHg
PO2 = 150 mmHg PCO2 = 0 mmHg
Normal V/Q occurs when…
Ventilation and capillary blood flow are normal, gas exchange is optimal
Normal inspired air partial pressures—PO2 ___ mmHg PCO2 ___ mmHg
PO2 = 150 mmHg PCO2 = 0 mmHg
Normal venous blood partial pressures—PO2 ___ mmHg PCO2 ___ mmHg
PO2 = 40 mmHg PCO2 = 45 mmHg
Normal alveolar partial pressures—Alveolar PO2 is ___ mmHg
100 mmHg
Normal alveolar partial pressures—Alveolar PCO2 is ___ mmHg
40 mmHg
Physiologic shunt—V/Q is ___
Below normal
Shunt =
Perfusion but no ventilation (i.e.: airway obstruction)
In a physiologic shunt, blood is being shunted from pulmonary ___ to pulmonary ___ without…
Pulmonary artery to pulmonary vein without participating in gas exchange
There is inadequate ventilation with a fraction of deoxygenated blood passing through capillaries and not becoming oxygenated
Shunted blood is not ___
Oxygenated
Physiologic shunt is total amount of…
Shunted blood per minute
The greater the physiologic shunt, the greater the amount of…
Blood that fails to be oxygenated in the lungs
Physiologic dead space—V/Q is…
Greater than normal
Dead space =
Ventilation but no perfusion—ventilation to alveoli is good, but blood flow is low
More available oxygen in alveoli than can be transported away by flowing blood
Physiologic dead space includes (2 things):
- Wasted ventilation
- Anatomical dead space
When physiologic dead space is great, much of the work of breathing is ___ because ventilated air ___
Wasted effort because ventilated air does not reach the blood
V/Q is below normal in…
Physiologic shunt
V/Q is greater than normal in…
Physiologic dead space
