Ch 3 Diffusion Flashcards
What is the primary mechanism by which gases move across the blood-gas barrier?
Diffusion
Diffusion is a passive process where gases move from areas of higher partial pressure to areas of lower partial pressure.
What does Fick’s law state regarding gas transfer through tissues?
The rate of transfer is proportional to tissue area and partial pressure difference, and inversely proportional to tissue thickness.
Fick’s law describes how the characteristics of the tissue and the gas affect diffusion rates.
What is the thickness of the blood-gas barrier in the lung?
Approximately 0.3 μm
The thinness of the barrier facilitates efficient gas exchange.
Which gas diffuses more rapidly through tissues, CO2 or O2, and why?
CO2 diffuses about 20 times more rapidly than O2 due to its higher solubility.
The diffusion rate depends on the solubility of the gas and its molecular weight.
Define diffusion-limited gas transfer.
Transfer of gas where the partial pressure in the blood hardly changes, limiting the uptake by diffusion properties.
An example is carbon monoxide transfer in the pulmonary capillaries.
Define perfusion-limited gas transfer.
Transfer of gas where the uptake depends on blood flow rather than diffusion properties.
An example is nitrous oxide transfer in the pulmonary capillaries.
What happens to oxygen transfer during severe exercise?
The time spent in the capillary decreases, challenging the diffusion process.
The time can be reduced from about 0.75 s to as little as 0.25 s.
What is the normal Po2 of blood entering the capillary?
About 40 mm Hg
This value can change based on conditions like exercise or altitude.
What is the significance of the slope of the O2 dissociation curve?
It determines the rate of rise of Po2 in blood for a given increase in O2 concentration.
A steep slope indicates a rapid increase in oxygen uptake.
What is the diffusing capacity of the lung (DL) for carbon monoxide?
The volume of carbon monoxide transferred in ml per minute per mm Hg of alveolar partial pressure.
It is measured to assess the diffusion properties of the lung.
What is the normal value of the diffusing capacity for carbon monoxide at rest?
About 25 ml·min−1·mm Hg−1
This value can increase significantly with exercise due to recruitment of pulmonary capillaries.
What is the single-breath method used for?
To measure the diffusing capacity of the lung using a dilute mixture of carbon monoxide.
The method involves measuring the rate of disappearance of carbon monoxide during a breath-hold.
True or False: Oxygen transfer can be both perfusion-limited and diffusion-limited.
True
This can happen under abnormal conditions when diffusion properties of the lung are impaired.
What is the normal value of the diffusing capacity for carbon monoxide at rest?
About 25 ml·min−1·mm Hg−1
How does the diffusing capacity for carbon monoxide change during exercise?
It increases to two or three times the resting value
Why is carbon monoxide used for measuring diffusing capacity?
Because the uptake of this gas is diffusion limited
What are the two stages in the uptake of O2 (or CO) in the blood?
- Diffusion of O2 through the blood-gas barrier
- Reaction of O2 with hemoglobin
What is the formula for the diffusing capacity of the lung (DL)?
DL = V·gas / (P1 − P2)
What does the inverse of DL represent in relation to diffusion resistance?
Pressure difference divided by flow, analogous to electrical resistance
What is the effect of breathing a high O2 mixture on the reaction rate of CO?
It reduces the reaction rate of CO with hemoglobin
How can the separate contributions of diffusion properties and capillary blood volume be derived?
By measuring the diffusing capacity for CO at different alveolar Po2 values
What is Fick’s law of diffusion?
The rate of diffusion of a gas through a tissue sheet is proportional to the area and partial pressure difference and inversely proportional to the thickness
What type of gases are carbon monoxide and nitrous oxide classified as?
- Carbon monoxide - diffusion-limited
- Nitrous oxide - perfusion-limited
What is the relationship between the reaction rate of O2 and its transfer rate into the blood?
The finite reaction rate of O2 with hemoglobin can reduce its transfer rate
True or False: CO2 transfer across the blood-gas barrier is likely diffusion limited.
False
What factors affect the measured diffusing capacity of the lung for CO?
- Area and thickness of the blood-gas barrier
- Volume of blood in the pulmonary capillaries
- Distribution of diffusion properties
- Alveolar volume
Fill in the blank: The uptake of O2 (or CO) can be regarded as occurring in two stages: diffusion through the blood-gas barrier and _______.
reaction with hemoglobin
What can cause a decrease in the diffusing capacity of the lung?
Reduction of capillary blood volume by disease
How does breathing oxygen affect the measured diffusing capacity for carbon monoxide?
It reduces the measured diffusing capacity
What would you expect the arterial Pco2 to be in a patient with pulmonary fibrosis?
Increased due to impaired gas exchange
In a normal person, what happens to arterial Pco2 during resting breathing when the diffusing capacity of the lung is doubled?
It decreases
What is the expected outcome on exercise testing for a patient with pulmonary fibrosis?
Decreased arterial PO2
What is the effect of emphysema on the diffusing capacity of the lung for carbon monoxide?
It increases due to loss of pulmonary capillaries
What is the significance of the thickness of the blood-gas barrier in the context of lung disease?
Thickening can reduce the measured diffusing capacity
What does a decrease in hemoglobin concentration indicate in terms of diffusing capacity?
Decreased diffusing capacity for carbon monoxide
