Gas Exchange Flashcards
What is anatomical dead space?
Some inspired air remains in the airways where it is not available for gas exchange
What is the equation for pulmonary ventilation (PL)?
PL = tidal volume (l/breath) x respiratory rate (breath/min)
Calculate pulmonary ventilation when tidal volume = 0.5 l/breath and respiratory rate is 12 breath/min
PL = 6 l/min
What are the units for pulmonary ventilation?
L/min
Why is alveolar ventilation less than pulmonary ventilation?
Due to presence of anatomical dead space
What is the equation for alveolar ventilation?
AV = (tidal volume - dead space volume) x respiratory rate
Calculate alveolar ventilation when tidal volume is 0.5, dead space volume is 0.15, and respiratory rate is 12
AV = 4.2 l/min
What is pulmonary ventilation?
The volume of air breathed in and out per minute
What is alveolar ventilation?
The volume of air exchanged between the atmosphere and alveoli per minute
How is pulmonary ventilation increased (e.g. during exercise)?
Increasing both the depth (tidal volume) and respiration rate
Why is it more advantageous to decrease the depth of breathing rather than respiration rate?
Due to anatomical dead space
Deep, slow breathing increases alveolar ventilation
Shallow, rapid breathing decreases alveolar ventilation
What 2 factors does transfer of gases between the body and atmosphere depend upon?
- Ventilation - the rate at which gas is passing through the lungs
- Perfusion - the rate at which blood is passing through the lungs
What results from the fact that blood flow and ventilation vary from bottom to top of the lung?
Average arterial and alveolar pressures of oxygen are not exactly the same
At the bottom of the lung, what is V/Q (ventilation: perfusion) ratio?
V/Q = <1
There is greater blood flow than ventilation
At the top of the lung, what is V/Q (ventilation: perfusion) ratio?
V/Q = >1
There is greater ventilation than blood flow
Why is the match between air in the alveoli and blood in the pulmonary capillaries not always perfect?
Due to alveolar dead space
What is alveolar dead space?
Ventilated alveoli which are not adequately perfused with blood are considered as alveolar dead space
What is physiological dead space?
The sum of anatomical dead space + alveolar dead space
How does the body try to match airflow to blood flow?
Local controls acting on smooth muscles of airways and arterioles
What does accumulation of CO2 as a result of increased perfusion result in?
Decreases airway resistance (dilation of airways) leading to increased airflow
What does accumulation of O2 as a result of increased ventilation result in?
Pulmonary vasodilation which increases blood flow to match airflow
What does O2 decrease in the area result in?
Pulmonary vasoconstriction which decreases blood flow to match airflow
What is the effect of decreased O2 on pulmonary and systemic arterioles?
Vasoconstriction of pulmonary arterioles
Vasodilation of systemic arterioles
What is the effect of increased O2 on pulmonary and systemic arterioles?
Vasodilation of pulmonary arterioles
Vasoconstriction of systemic arterioles
What are four factors that influence the rate of gas exchange across alveolar membranes?
- Partial pressure gradient of O2 and CO2
- Diffusion coefficient for O2 and CO2
- Surface area of alveolar membrane
- Thickness of alveolar membrane
What is partial pressure of a gas (in a mixture of gases that don’t react with each other)?
The pressure that that gas exert if it occupied the total volume for the mixture in the absence of other components (at a given temperature)
How do gases move across cell membranes?
By a pressure gradient
What determines the pressure gradient for a specific gas?
The partial pressure of that gas
What is Dalton’s law of Partial Pressures?
The total pressure exerted by a gaseous mixture = the sum of the partial pressure of each individual component in the gas mixture
P(total) = P1 + P2 + …
If total pressure of the gas mixture is 100kPa, and half of the mixture is gas 1, what is the partial pressure of gas 1?
50 kPa
What is the value of total atmospheric pressure?
760 mm Hg (101 kPa)
What is the partial pressure of N2 in atmospheric air if nitrogen makes up 72% of air?
760 x 0.79 = 600 mm Hg (80 kPa)
What is the partial pressure of O2 in atmospheric air if oxygen makes up 21% of air?
760 x 0.21 = 160 mm Hg (21 kPa)
How can an average value for the partial pressure of O2 be worked out when oxygen is continuously diffusing from alveoli into blood?
Alveolar Gas Equation
What is the alveolar gas equation?
PAO2 = PiO2 - (PaCO2/0.8)
PAO2 - partial pressure of O2 in alveolar air
PiO2 - partial pressure of O2 in inspired air
PaCO2 - partial pressure of CO2 in arterial blood
0.8 is the Respiratory Exchange Ratio (RER)
What is the air in the respiratory tract saturated with?
Water
How much does the water vapour pressure contribute to total pressure in the lungs?
Around 47 mm Hg
What is equation for pressure of inspired air?
Inspired air = atmospheric pressure - water vapour pressure
What is the value of pressure of inspired air?
Inspired air = 760 - 47 = 713 mm Hg at sea level
If inspired air pressure is 713 mm Hg, calculate the value of PiO2
713 x 0.21 = 150 mm Hg
What is the normal value of PCO2
40 mm Hg
If PiO2 = 150 mm Hg, and PaCO2 = 40 mm Hg, calculate PAO2
PAO2 = 150 - (40/0.8)
= 100 mm Hg at sea level
In which direction do gases move?
From higher to lower partial pressure gradients
What is oxygen’s partial pressure gradient?
Partial pressure gradient higher in atmosphere than in alveoli
Compare the partial pressures of O2 and CO2
Partial pressure is much higher for O2 than CO2
What is O2 partial pressure gradient from alveoli to blood?
100 (in alveoli) - 40 (in blood) mm Hg = 60 mm Hg
What is CO2 partial pressure gradient from alveoli to blood?
46 - 40 mm Hg = 6 mm Hg
What is O2 partial pressure gradient from blood to tissue cell?
100 - <40 mm Hg = >60 mm Hg
What is CO2 partial pressure gradient from blood to tissue cell?
46 - 40 mm Hg = 6 mm Hg
If partial pressure gradient for CO2 is much smaller than O2, what offsets the difference in partial pressure gradient?
CO2 is more soluble in membranes than O2 - the solubility of gas in membranes is known as the Diffusion Coefficient of the gas
What is the Diffusion Coefficient of a gas?
The solubility of gas in membranes
Compare the diffusion coefficient of O2 and Co2
Diffusion coefficient of CO2 is 20 times that of O2
Why does a small gradient normally exist been alveolar PO2 (PAO2) and arterial PO2 (PaO2)?
Ventilation-perfusion match is usually not perfect
What would a big gradient between PAO2 and PaO2 indicate?
Problems with gas change in the lungs or a right to left shunt in the heart
How do the lungs facilitate effective gas exchange?
Provide a very large surface area with thin membranes
Describe the effect of surface area and membrane thickness of gas diffusion (Fick’s Law of diffusion)
The amount of gas that moves across a sheet of tissue in unit time is proportional to the area of
the sheet but inversely proportional to its thickness
How do the airways increase the surface area for gas exchange in the lungs?
- They divide repeatedly
* Small airways form alveoli to increase surface area for gas exchange
How is it ensured that the lungs receive a good blood supply for gas exchange?
The pulmonary circulation receives the entire cardiac output
What are alveoli?
Thin-walled inflatable sacs
What is the function of alveoli?
Gas exchange
Describe the structure of alveoli
Walls consist of a single layer of flattened Type I alveolar cells
What encircles each alveolus?
Pulmonary capillaries
Describe the effect of partial pressure gradient of O2 and CO2 on rate of gas transfer
Rate of transfer increases as partial pressure increases (major influence in rate of gas transfer)
Describe the effect of diffusion coefficient on rate of gas transfer
Rate of transfer increases as diffusion coefficient increases
Describe the effect of surface area of alveolar membrane on rate of gas transfer
Rate of transfer increases as surface area increases
Describe the effect of thickness of alveolar membrane on rate of gas transfer
Rate of transfer decreases as thickness increases
What can increase surface area of alveolar membrane?
Exercise (deeper breathing expands alveoli and pulmonary capillaries open up when cardiac output increases)
What can decrease surface area of alveolar membrane?
Emphysema, lung collapse, pneumonectomy
What can increase thickness of alveolar membrane?
Pulmonary oedema, pulmonary fibrosis, pneumonia
List non-respiratory functions of respiratory system
- Route for water loss and heat elimination
- Enhances venous return
- Helps maintain normal acid-base balance
- Enables speech, singing, and other vocalizations
- Defends against inhaled foreign matter
- Removes, modifies, activates, or inactivates various materials passing through the pulmonary circulation
- Nose serves as the organ of smell
