11 Diffusion / Hypoxemia and Hypercapnia Flashcards
1
Q
Bulk flow
- Bulk flow
- During expiration,…
A
- Bulk flow
- Oxygen and carbon dioxide are transported through the conducting airways by “bulk flow”
- That is, there is actual movement of the gas along a pressure gradient
- During expiration,…
- This process is reversed
- Gas flow progressively increases as it moves toward the mouth
2
Q
Bulk flow
- During inspiration, as gas moves from the mouth through the conducting airways, this pressure gradient (the difference between airway and alveolar pressure)…
- This occurs for two reasons
- This causes…
- At the level of the alveoli,…
A
- During inspiration, as gas moves from the mouth through the conducting airways, this pressure gradient (the difference between airway and alveolar pressure)…
- Progressively falls
- This occurs for two reasons
- (1) As gas flows through the airways, total gas or airway pressure decreases because of losses to viscous or frictional forces
- (2) As gas travels from the trachea to the respiratory bronchioles, the diameter of individual airways decreases, but total cross-sectional airway diameter increases dramatically
- This causes…
- A progressive drop in total airway resistance, which is accompanied by a decrease in airway pressure
- As airway pressure falls, so does the rate of gas flow
- At the level of the alveoli,…
- Bulk flow stops
- The movement of O2 and CO2 depend solely on the process of diffusion
3
Q
Diffusion
- Diffusion
- O2 and CO2 move…
- O2 dissolves in and diffuses through…
- CO2 diffuses…
A
- Diffusion
- The net movement of molecules from a region in which a particular gas exerts a high partial pressure to a region in which a lower partial pressure is present
- The process whereby O2 and CO2 are exchanged across the alverolar-capillary interface
- O2 and CO2 move…
- Between the alveoli and the pulmonary capillary blood along partial pressure gradients
-
O2 dissolves in and diffuses through…
- The alveolar epithelium
- The capillary endothelium
- The plasma (where some remains dissolved)
- The erythrocyte (where it combines with hemoglobin)
-
CO2 diffuses…
- In the opposite direction
4
Q
Fick’s law
- Fick’s law
- The surface area available for gas diffusion in an average size adult
- The rate of gas transfer will fall with…
A
- Fick’s law for diffusion
- Describe the factors that determine the rate at which O2 and CO2 move across the alveolar-capillary interface
- The more surface area of contact you have b/n gas and blood, the more O2 and CO2 will be able to move across it
- The greater the gradient of partial pressures b/n alveolar gas and capillary blood, the faster the molecules will move
- The thicker the membrane, the less the molecules will move
- Vgas = [A x D x (P1 - P2)] / T *don’t need to memorize*
- Vgas = the rate at which gas passes through the gas-liquid interface
- A = surface area of the interface
- D = diffusion coefficient of the particular gas
- P1 – P2 = partial pressure difference of the gas across the interface (between alveolar gas and plasma)
- T = thickness of the interface (diffusion distance)
- Describe the factors that determine the rate at which O2 and CO2 move across the alveolar-capillary interface
- The surface area available for gas diffusion in an average size adult
- ~ 100 square meters
- The rate of gas transfer will fall with…
- Any process that decreases the gas-liquid interface
- E.g. destruction of lung parenchyma or alveolar capillaries
5
Q
Diffusion coefficient
- Diffusion coefficient (D)
- DCO2 vs. DO2
A
- Diffusion coefficient (D)
- Directly proportional to the solubility of the gas in the tissues and fluids that it must traverse
-
Inversely related to the square root of the molecular weight (MW) of the gas
- I.e. the larger the molecule, the slower it moves
- D α solubility / √MW
- DCO2 vs. DO2
- Although CO2 has a higher molecular weight than O2 (1.17:1), its solubility is 24 times that of O2
- These factors cause DCO2 to be approximately 20 times DO2
6
Q
Partial pressure gradient
- Gas molecules
- May be considered to exist in 2 forms in a liquid
- The molecules in these phases are…
- The partial pressure gradient
- PO2
- PCO2
- The thickness of the alveolar-capillary barrier
- Gas transfer will be decreased by…
A
-
Gas molecules
-
May be considered to exist in 2 forms in a liquid
-
Gas –> partial pressure
- The partial pressure of a gas in a liquid is equal to the partial pressure of the gas outside the liquid
-
Dissolved in liquid –> concentration
- The concentration of a gas in a liquid is determined by both its partial pressure and its solubility
-
Gas –> partial pressure
- The molecules in the gas phase and the liquid phase are in equilibrium
-
May be considered to exist in 2 forms in a liquid
- The partial pressure gradient
- Important factor determining gas transfer
- PO2
- O2 moves from an alveolar PO2 of approximately 100 mmHg to a mixed venous PO2 of about 40 mmHg
- PCO2
- The partial pressure of CO2, however, only falls from about 46 to 40 mmHg during the movement of blood through the lungs.
- The thickness of the alveolar-capillary barrier
- Normally between 0.2 – 0.5 μm
- Gas transfer will be decreased by…
- Any process that increases this diffusion distance
- E.g. interstitial fibrosis, interstitial edema
7
Q
Gas partial pressure and concentration
- The partial pressure of O2 and CO2 in the end-capillary blood is normally equal to…
- The concentration of dissolved O2 and CO2
A
- The partial pressure of O2 and CO2 in the end-capillary blood is normally equal to…
- The partial pressure in the alveolar gas
- The concentration of dissolved O2 and CO2
- Dissolved O2 (ml/dl) = 0.003 x PaO2
- Dissolved CO2 (ml/dl) = 0.067 x PaCO2
8
Q
Diffusion of O2
- Time an erythrocyte spends in alveolar capillaries
- O2 during this time in the lungs
- PO2
- PO2 under resting conditions
A
-
Time an erythrocyte spends in alveolar capillaries
- An average of 0.75 seconds
-
O2 during this time in the lungs
- O2 diffuses across the alveolar-capillary interface along its partial pressure gradient
- Most of the O2 enters RBCs and combines with hemoglobin
- Only a small proportion remains in a gaseous form or dissolved in the plasma
- PO2
- Only the gaseous O2 contributes to the PO2 of the blood
- A large amount of O2 can be transferred before alveolar and capillary PO2 become equal
- Even so, this equilibration occurs very rapidly
- PO2 under resting conditions
- Alveolar and capillary PO2 normally equilibrate within 0.25 second
- Equilibrium occurs even when transit time is reduced
- That is, the maximum amount of oxygen has normally been transferred to the erythrocytes and plasma by about one-third of the total blood transit time
9
Q
Diffusion of O2
- During strenuous exercise
- The time required for alveolar-capillary equilibration increases in the presence of disorders that…
- If the diffusion impairment is severe, there may be…
A
- During strenuous exercise
- Transit time is markedly reduced by the increase in cardiac output
- Alveolar and blood PO2 equilibrate long before the blood leaves the alveoli
- The time required for alveolar-capillary equilibration increases in the presence of disorders that…
- Decrease the surface area available for diffusion
- Increase the thickness of the diffusion barrier
-
If the diffusion impairment is severe, there may be…
- Insufficient time for equilibration to occur
-
A gradient between alveolar and end-capillary PO2
- This gradient will be further increased during exercise
10
Q
Diffusion of CO2
- Equilibration of PCO2 between the capillary blood and alveolar gas takes…
- In the lungs, CO2 molecules move…
- Most CO2 molecules come from…
- Only a small percentage…
A
-
Equilibration of PCO2 between the capillary blood and alveolar gas also takes approximately 0.25 second
- This may seem strange given the much greater diffusion coefficient for CO2
- However, the partial pressure gradient for CO2 is much less than that for O2
- Incomplete equilibrium of PCO2 is not clinically relevant because of the small partial pressure gradient
- In the lungs, CO2 molecules move along the partial pressure gradient from the blood to the alveolar gas
-
Most CO2 molecules come from the RBCs
- Only a small percentage leave the dissolved form in the plasma
11
Q
Clinical assessment of diffusion
- In the pulmonary function laboratory, gas diffusion between the alveoli and the capillary blood is assessed by…
- As capillary blood moves through the tissues, the movement of oxygen and carbon dioxide…
A
- In the pulmonary function laboratory, gas diffusion between the alveoli and the capillary blood is assessed by measuring the diffusing capacity for carbon monoxide (DLCO)
- As capillary blood moves through the tissues, the movement of oxygen and carbon dioxide are reversed
12
Q
Partial pressure vs. concentration
- When O2 or another gas enters the blood…
- Partial pressure
- Concentration of the gas in the blood
A
- When O2 or another gas enters the blood…
- Some molecules remain in gaseous form
- Others actually dissolve and enter the liquid phase
- Partial pressure
- The pressure exerted by the gaseous molecules
- Concentration of the gas in the blood
- The number of dissolved molecules is reflected
13
Q
Partial pressure vs. concentration
- Gas molecules move…
- At equilibrium,…
- Gases that are highly soluble will have…
A
- Gas molecules move…
- Between the alveoli and the blood
- Between the blood and the tissues
- Along their partial pressure gradient until equilibrium occurs
- At equilibrium,…
- The partial pressure of a gas is the same in the alveoli, blood, and all tissues and organs
- The concentration of a gas, however, depends both on its partial pressure and its solubility
- Gases that are highly soluble will have…
- A much higher blood and tissue concentration than poorly soluble gases, even if their partial pressures are the same
14
Q
Partial pressure vs. concentration
- The rate at which the partial pressure of a gas equilibrates between the alveoli, blood, and tissues
- For example, O2 and CO2 molecules
A
- The rate at which the partial pressure of a gas equilibrates between the alveoli, blood, and tissues
- Inversely related to its solubility
- For example, O2 and CO2 molecules
- Diffuse between the alveolar gas and the pulmonary capillary blood along their partial pressure gradients
- Both are relatively insoluble
- Equilibration between the partial pressure in blood and alveolar gas occurs very rapidly
- Equilibration occurs quickly between the partial pressure of O2 and CO2 in the blood and tissues
15
Q
Partial pressure vs. concentration
- The greater the gas solubility…
- For example, clinical anesthesia depends on…
- Therefore, the onset of action of an anesthetic gas…
A
- The greater the gas solubility…
- The longer it takes to reach partial pressure equilibrium
- For example, clinical anesthesia depends on…
- Achieving an adequate partial pressure (not concentration) of the anesthetic gas in the CNS
- Therefore, the onset of action of an anesthetic gas…
- Varies inversely with its solubility
- Insoluble anesthetics act quickly, whereas more soluble drugs have a more delayed effect