Alveolar Gas Exchange Flashcards
Alveolar gas exchange depends on what 2 things?
Perfusion
Ventilation
What is the difference between perfusion and ventilation?
Perfusion = blood to the lungs
Ventilation = gas to the lungs (volume of air reaching the alveoli)
Perfusion refers to blood going to the lungs. This blood comes from the ____ ventricle at a rate of about _____ L/min.
Right; 5
T/F: the cardiac output from the right ventricle to the lungs is the same as the cardiac output from the left ventricle
True
On average, what is the ventilation rate (in L/min) referring to the volume of air reaching the alveoli?
4 L/min
In general, conduction of gas to alveoli occurs as air moves from areas of high pressure to areas of lower pressure. The pressure differences between 2 ends of the conducting zone occur due to changes in ____ _____
Lung volume
What factor affecting alveolar gas exchange can be changed most easily?
Resistance
What is the main determinant of resistance as it applies to alveolar gas exchange?
Radius of bronchioles (controlled by smooth muscle)
What are a few consequences of changes in airway resistance?
Difficult air flow
High resistance leads to slower air flow
What gas laws are important to consider when it comes to resistance affecting alveolar gas exchange?
Poiseuille’s law (R = 8nL/pi x r^4)
Ohm’s + Poiseuille’s law (Q = P1-P2 x pi x r^4 / 8nL)
These are dependent on: Resistance (R) Viscosity (n) Length of tube (L) Radius of tube (r)
How is perfusion of the lungs calculated? Hint: what gas law is used?
Ohm’s law
Q = P1-P2/R
Where P1-P2 is the pressure gradient and R is resistance
What is Dalton’s law?
Total pressure of a gas mixture = sum of pressures of each gas in it
State Dalton’s law of partial pressures as it applies to respiratory physiology
Partial pressure of a gas in a mixture of gases is the pressure that the gas would exert if it occupied the total volume of the mixture
How is the partial pressure of a respiratory gas calculated?
Multiply % of individual gas by total pressure
Barometric pressure is the sum of the partial pressures of what gases?
O2
CO2
N2
H2O
How would you calculate the pressure of a humidified gas?
Px = (barometric pressure - water vapor pressure at 37C) x fractional concentration of gas
Water vapor pressure = 47 mm Hg at 37 celsius
Fractional percentages at barometric pressure of 760 mm Hg = O2 21%, N2 78%, CO2 1%
What are the fractional percentages of respiratory gases at a barometric pressure of 760 mm Hg?
O2 = 21%
N2 = 78%
CO2 = 0%
What effect does water vapor (i.e., humidification) have on total pressure of a gas?
Decreases it
For example: dry inspired air has pressure of 160 mm Hg PO2, once humidified it is 150 mm Hg.
How is PO2 calculated of normal inspired air calculated?
Barometric pressure = 760 mm Hg
Fractional percent of O2 in dry air = 21%
Based on Dalton’s Law: 760 x 0.21 = 160 mm Hg
Partial pressure of respiratory gases in ______ air matches the needs of the body on an everyday basis; meaning that O2 transfer from this air equals the O2 consumption in the body, and CO2 transfer to this air equals the amount of CO2 produced
Alveolar
O2 transfer from alveolar air = O2 consumption by the body
CO2 transfer to alveolar air = CO2 production
What are the normal partial pressures for O2 and CO2 in alveolar air?
Partial pressure of O2 = 100 mm Hg
Partial pressure of CO2 = 40 mm Hg
Mixed venous blood = blood entering the pulmonary capillaries which has returned from tissues.
How have the partial pressures changed in comparison to alveolar air, which is 100 mm Hg for O2 and 40 mm Hg for CO2?
Partial pressure for oxygen: 40 mm Hg
Partial pressure for CO2 = 46 mm Hg
Within systemic arterial blood, there is exchange of O2 and CO2 between ______ air and ________ ____ blood
Alveolar; pulmonary capillary
Because diffusion across alveolar capillary barriers is rapid, blood leaving the pulmonary capillaries usually has the same values as _______ air
However, it may have a slightly lower PO2 because of the ________ __________
Alveolar
Physiologic shunt
What is the physiologic shunt?
A small fraction of pulmonary blood flow that bypasses the alveoli and therefore is not arterialized
What are the 2 sources of the physiologic shunt and where do they end up draining into?
Bronchial blood flow
Small portion of coronary venous blood
End up draining directly into left ventricle rather than going to lungs to be oxygenated
Fick’s law states that the rate of transfer of a gas through a sheet of tissue is proportional to what 3 factors, and inversely proportional to what other factors?
Proportional to:
Tissue area
Difference in partial pressure
Solubility of gas (based on constant)
Inversely proportional to:
Thickness
Molecular weight of gas (based on constant)
Under normal resting conditions:
______ mL O2 are exchanged per minute
_______ mL CO2 are exchanged per minute
250
200
T/F: under normal resting conditions, alveolar gas exchange of O2 and CO2 per minute are equal
False - diffusion of these 2 gases occurs independent of one another
T/F: limitations of alveolar gas diffusion will rarely cause hypoxemia when at rest at sea level, EVEN IF lung disease is present
True
What factors may act as limitations of diffusion?
Surface area
Thickness
Gas solubility
Pressure gradient
How might surface area act as a limitation of diffusion?
Varying number of alveoli in lungs that are available for gas exchange (account for 50-100 m^2 of surface area)
Also depends on blood pulmonary capillaries surrounding these surfaces - which vary upon demand
In general, as surface area decreases, what happens to the rate of diffusion?
Decreases
What pathological condition acts as a limitation for diffusion by directly affecting the surface area available for gas exchange?
Emphysema
Patients have a hard time getting sufficient O2 due to reduction in alveoli surface area (reduction of alveoli in general bc they are ruptured)
Thickness of various structures may act as a limitation to diffusion. Distance gases typically travel is about 0.3-0.6 micrometers. What structures must the gases pass through during the process of gas exchange?
Fluid layer
Alveolar epithelium
Interstitial space
Blood vessel wall
In general, as thickness/distance increases, what happens to diffusion rate?
Decreases
What condition might limit diffusion by directly affecting the thickness that gases must travel for gas exchange to take place?
Collagen deposition within interstitial spaces
Compare the solubility in water and the molecular weights of O2 and CO2
CO2 is more soluble and has higher molecular weight
Compare the lung diffusion capacity of CO2 to that of O2
Lung diffusion capacity of CO2 is 20x larger
The partial pressure gradient for oxygen is as follows:
In alveoli = 100 mm Hg
In venous blood = 40 mm Hg
Overall gradient = 60 mm Hg
Where will oxygen go based on this pressure gradient?
Oxygen will be driven into alveolus
The partial pressure gradient of carbon dioxide is as follows:
In alveoli = 40 mm Hg
In venous blood = 45 mm Hg
Overall gradient = -5 mm Hg
Where will CO2 go based on this gradient?
Carbon dioxide will be driven into capillaries
Gas exchange during strenuous exercise and in pathologic conditions such as emphysema or fibrosis is considered _________ - limited
Diffusion
In the lungs of a normal person at rest, oxygen transfer from alveolar air into pulmonary capillary blood is ______ limited
Perfusion
Perfusion limited means gradient for diffusion is maintained
Why does CO2 diffuse 20x more rapidly than O2?
Because of higher solubility and relatively low molecular weight
Diffusion capacity of a gas takes into account area, thickness, and diffusion properties of the tissue and the gas concerned. It also takes into account the time required for gas to combine with proteins in the _______ ______ (i.e., hemoglobin binding)
Pulmonary capillary
Which gas is used to measure the diffusion capacity of other gases like O2? Why?
Carbon monoxide (CO), because CO transfer across the alveolar/pulmonary capillary barrier is limited exclusively by the diffusion process
CO doesn’t diffuse well, and what does end up diffusing through will bind readily and rapidly to hemoglobin, so the pressure gradient is the same as the alveolar pressure
How would you go about measuring the diffusion capacity of O2?
Using the single breath method: subject breathes single gas mixture containing a low concentration of carbon monoxide. The rate of disappearance of CO from the gas mixture is proportional to the diffusion capacity.
Normal is 21-25 mL/min/mm Hg
D(L) O2 = 1.23 x D(L) CO
In general, the O2 concentration in the alveoli is controlled by what 2 factors?
Rate of new air into lungs
Rate of absorption into blood (= 250 mL O2/min)
Why does heavy exercise push the “safety factor” of O2 gas exchange in the alveoli to its limits?
Oxygen diffusion requires 0.25 seconds to reach alveolar equilibrium, with heavy exercise, RBCs are spending less time in pulmonary capillaries to get adequate oxygen
Normal CO2 concentration in the alveoli is about 40 mm Hg. In general, what 2 factors does this depend on?
Rate of excretion (200 mL O2/min)
Rate of new air into lungs - decreases inversely proportional to alveolar ventilation
With carbon dioxide diffusion, equilibrium is almost _________. There is a huge safety factor before RBC leaves the pulmonary capillaries. The diffusion capacity of CO2 at rest = ______ mL CO2/min/mm Hg
Immediate
400
T/F: lung disease has to be extensive to have problems with CO2 retention
True
Alveolar gas exchange is slow. Functional residual volume is about ______ mL, and only about ______ mL of new air enters the alveolus with each breath. This means that 1/7 of alveolar air is replaced with new air with each breath
2300; 350
What structures make up the respiratory membrane?
Layer of surfactant Alveolar epithelial cells Alveolar epithelial basement membrane Interstitial space Capillary basement membrane Capillary endothelial cells Capillary lumen
What cells make up 95-97% of the total surface area of alveoli where gas exchange is occuring?
Type I alveolar cells
What type of cells secrete surfactant, as well as reabsorb water and sodium?
Type II alveolar cells
Other than type I and type II alveolar cells, what cell type is associated with alveoli?
Macrophages - involved in immune function and recycling of surfactant
What causes surface tension at the alveoli?
Air/water interface [air in alveolus, water in tissue]
Water exposed to air will generate pressure
What is LaPlace’s law?
Presure = 2T/r
Where T = tension, r = radius
What is the difference in alveolar pressure in large alveoli vs. small alveoli?
In large alveolus, radius is large, so pressure will be relatively low
In small alveolus, radius is small, so pressure will be relatively high
Pressure is relatively low in large alveoli, and relatively high in small alveoli. What does this mean for air flow?
Air will flow preferentially into larger alveoli, reducing the surface area being used for diffusion. Small alveoli tend to collapse under higher pressures
Small alveoli tend to collapse under higher pressure. What serves to reduces this collapsing pressure?
Surfactant
Describe differences in surfactant function in smaller alveoli vs. larger alveoli
Surfactant reduces surface tension in smallest alveoli more so than in larger alveoli (it is more concentrated in the smaller alveoli)
Surfactant is synthesized from _____ _____ and secreted by Type II alveolar cells secrete pulmonary surfactant with ______ ______ ______.
Fatty acids; mature lamellar bodies
Surfactant promotes lung expansion during inspiration by increasing lung ________
Compliance
What are the 3 major components of surfaactant?
Phospholipids (80%) Neutral lipids (10%) Surfactant apoproteins (10%)
Surfactant is made up of phospholipids, neutral lipids, and surfactant apoproteins. Which phospholipid is the most important, making up 60% of the phospholipids in surfactant? What makes it so important?
DPCC
It is amphipathic; molecules align themselves on alveolar surface with hydrophobic portions attracted to each other and hydrophilic portions repelled
Intermolecular forces between them break up attracting forces between liquids lining alveoli (which had been responsible for surface tension)
The major phospholipid in surfactant is DPCC, what other phospholipids are typically present?
Phosphatidylglycerol
Phosphatidylethanolamine
Phosphatidylinositol
Surfactant is made up of phospholipids, neutral lipids, and surfactant apoproteins. Neutral lipids make up 10% of surfactant; which neutral lipid is mostly responsible for that 10%?
Cholesterol
Surfactant is made up of phospholipids, neutral lipids, and surfactant apoproteins. What are the surfactant apoproteins? Which ones are hydrophobic vs. hydrophilic?
Hydrophilic:
SP-A
SP-D
Hydrophobic:
SP-B
SP-C
