Unit 2, L5 Diffusion and Perfusion Flashcards
Transfer of gas across the blood gas barrier occurs by what
Diffusion
What factors facilitate gas diffusion in the respiratory system?
Large surface area, short distances, and gases with advantageous diffusion properties
Capillary diameter value
< 10 uM
Erythrocyte diameter
7 um
Erythrocytes in the capillaries
Pass through capillaries in single file, and the average transit time is < 1 second
The path an O2 molecule takes from alveoli to hemoglobin
Starts in the alveolar air, then diffuses across into the water layer. From there, it diffuses across the first membrane of alveolar type I epithelial cells, then diffuses into the inside of the type I epithelial cells, then diffuses out of those cells into the interstitial space with matrix, then diffuses through the first membrane layer of the pulmonary capillary endothelial cell, then diffuses into the inside of the capillary endothelial cell, then diffuses out of that cell and into the blood plasma, then diffuses across the membrane of the RBC, and finally diffuses into the cytoplasm of the RBC
Fick’s law for diffusion through tissue
The amount of gas transferred is proportional to the area (A), a diffusion constant, and the difference in partial pressure. The amount of gas transferred is indirect proportional to the thickness (T)
How would pulmonary edema effect diffusion through tissue
A mild pulmonary edema would increase the amount of fluid at th ebarrier, thicken the barrier, and decrease diffusion. A massive pulmonary edema would lose oxygen in the alveolar space and drop the driving force
How would a collapsed lung effect the diffusion of gas through tissue
It would completely eliminate the area, so there would be no diffusion
How would high altitude effect diffusion of gas through tissue
Lower total pressure, lower oxygen getting into alveoli, so dropping partial pressure of O2, lower driving pressure
Diffusion through tissue with Graham’s law
Rate of diffusion is proportional to solubility coefficient of gas over square root of molecular weight
Define diffusion-limited
The amount of molecule X carried into circulation is limited by the diffusion across the alveolar membrane
With a diffusion limited molecule, what is the driving force like
The driving force will always be higher than the rate at which the molecules can pass through the barrier
Perfusion limited definition
The amount of molecule X carried into circulation is limited by the rate of perfusion (blood flow through pulmonary capillaries)
Would diffusion or perfusion limited samples have slow equilibration
Diffusion limited would have slow equilibration
Would diffusion or perfusion limited samples have fast equilibration
Perfusion-limited would have fast equilibriation
Insoluble gasses, such as N2O, is perfusion or diffusion limited
Perfusion limited, as these chemicals do not combine with proteins, so they equilibrate rapidly and the gas transfer is limited by the amount of blood perfusing the alveolus
Soluble gases, such as CO, are perfusion or diffusion limited?
Diffusion limited. CO diffuses rapidly into erythrocytes and has high affinity for hemoglobin, with no or little increase in partial pressure
How do we measure the diffusion properties of a patient’s lungs?
Modification of Fick’s Law and Carbon Monoxide
CO in capillary blood under normal condition should be what value
0
How do you do the measurement of diffusion capacity?
Single breath of carbon monoxide. Person will breathe CO, hold for 10 seconds, then breathe back out. Look at concentration of either CO or the tracer gas that comes back out. If the patient has a diffusion capacity problem, line will go up as CO goes it, sit there and not diffuse, then come back out when they breathe out
Pulmonary arteries are the only arteries in the body that carry ____________
Deoxygenated blood
Total blood volume of pulmonary arteries
500 mL
In the alveolar capillary network, what is the volume under resting conditions
70 mL
In the alveolar capillary network, what is the volume during exercise
150-200 mL
Characteristics of pulmonary arteries
Thin wall, minimal smooth muscle, 7 times more compliant than systemic arteries and easily distensible
Factors that influence lung perfusion
PVR, gravity, alveolar pressure, and arterial-venous pressure gradient
Pulmonary vascular resistance (PVR) equation
PVR = change in pressure (pressure in pulmonary artery - left atrial pressure) / blood flow
PVR ______ with increasing vascular pressures
Decreases
What two mechanisms decreases PVR with increasing vascular pressures
1) Recruitment through opening closed vessels and conducting in previously non-conducting vessels
2) Distension, widening of open vessels
For alveolar vessels, what is transmural pressure
Pressure difference between alveolar pressure and vessel lumen pressure
For extra-alveolar vessels, what is transmural pressure
Pressure difference between intrapleural pressure and vessel lumen pressure
PVR is the lowest when
At FRC
At high lung volumes, what happens to the alveolar vessels and extra-alveolar vessels
At high lung volumes, vessel stretching crushes alveolar vessels and pulls open extra-alveolar vessels
At low lung volumes, what happens to the alveolar and extra-alveolar vessels
At low lung volumes, extra-alveolar vessels are subject to increased intrapleural pressure. Alveolar vessels are no longer crushed by large alvolai
There is increased blood flow at what part of the lungs
Bottom of the lungs
In zone 1, what is the line up of PA, Pa, and Pv and what is the blood flow
PA > Pa > Pv, so there is no blood flow
In zone 2, what is the line up of PA, Pa, and Pv and what is the blood flow
Pa > PA > Pv, and there is the beginning of blood flow, this is the waterfall effect
In zone 3, what is the line up of PA, Pa, and Pv and what is the blood flow
Pa > Pv > PA, and there is the most blood flow in this zone
In hypoxic vasoconstriction, what is it determined by?
Determined by PO2 in alveolar gas (PAO2) but not by P arterial O2
Oncotic pressure
Movement of H2O due to increase plasma protein concentration
If hydrostatic pressure = oncotic pressure, what happens
No net flow in or out of vessel
In a healthy individual, hydrostatic _______ oncotic
Hydrostatic exceeds oncotic, so there is a constant flow from lung interstitial fluid to lymph
Alveolar lung edema
Fluid in the alveoli themselves, very bad. No gas exchange happening then
