Principles of Gas Exchange Flashcards
What areas of the lung participate in gas exchange?
the alveoli and the respiratory bronchioles
What law governs the rate of diffusion?
Fick’s Law
the rate of diffusion is proportional to surface area multiplied by pressure gradient
What is the equation for Fick’s Law?
Rate of diffusion = A x delta P
A = surface area
delta P = pressure gradient
What other factors is diffusion dependent on?
the physicochemical properties of the gas and the nature of the membrane
What type of cells is are the alveoli walls made from?
What is the problem with this?
It is made from simple squamous epithelia cells and is only one cell thick
This means disease progress can rapidly change lung function
What is the pressure gradient?
the difference in partial pressures of the respective gases in the alveolus and the blood
What is the partial pressure of a gas?
the partial pressure of a gas in a mixture of gases is the pressure it would exert if it was the only gas in the equation
What does Dalton’s Law state?
The partial pressure of a gas in a gas mixture is equal to the total pressure multiplied by the fractional concentration of the gas
What is the Dalton’s Law equation?
Partial pressure of gas A = total pressure x fractional concentration of gas A
What is the fractional concentration, partial pressure at sea level and partial pressure at 5,000m of oxygen in the air?
fractional concentration - 21%
partial pressure sea level - 21.3 kPa
partial pressure 5,000m - 11.8 kPa
What is the fractional concentration, partial pressure at sea level and partial pressure at 5,000m of carbon dioxide in the air?
0
What is the fractional concentration, partial pressure at sea level and partial pressure at 5,000m of nitrogen in the air?
fractional concentration - 79%
partial pressure sea level - 80 kPa
partial pressure 5,000m - 44.2 kPa
What is atmospheric pressure in kPa, mmHg and cmH2O?
1 atmosphere
101.3 kPa
760 mmHg
1033 cm of water
Why does partial pressure decrease with higher altitudes?
In a given volume of air, there are fewer molecules present
The % of the gas is the same as at sea level but there are fewer molecules of every gas present
How does partial pressure of oxygen differ in the alveoli and the atmosphere?
partial pressure of oxygen in the alveoli is lower than in room air
Why is partial pressure of oxygen in the alveoli lower than room air?
- inspired air is humidified in the upper airway
- in the alveoli, oxygen is taken up while CO2 is released
- the body consumes more oxygen molecules than it produces CO2
Why is nasal breathing encouraged at rest?
The nose is highly efficient at humidifying inspired air
With how much water is the air humidified with?
the air is humidified with around 6.3 kPa of water
Why does the air need to be humidified before it reaches the lungs?
Dry air reaching the lungs would lead to rapid loss of fluid from the lungs
the lungs would stop working efficiently and would need higher pressures to expand and recoil
breathing becomes more difficult
How much more oxygen is consumed by the body than CO2 produced and why?
The body consumes 1.25 x as much oxygen than it produces CO2
This is due to the respiratory quotient
What is the body’s respiratory quotient? How could it be made to be closer to 1?
0.8
a diet of pure carbohydrate and no fat would produce a RQ closer to 1
this is because fats are oxygen poor and carbohydrates are oxygen rich
What is the saturated vapour pressure of water at 37oC?
6.3 kPa
this is the partial pressure of water vapour
What is the partial pressure of oxygen in fully humidified air at 37oC?
19.95 kPa
How is arterial blood gas CO2 related to the CO2 in the capillaries?
Arterial blood gas CO2 is the same as the capillaries
The blood has already passed out of the lungs at this point
What is the normal alveolar partial pressure of CO2?
5 kPa
The healthy range is 4.5 - 6kPa
What is the partial pressure of oxygen in the blood?
13.3 kPa
What is the expected partial pressure of oxygen in the blood?
Why is the actual value lower?
13.7 kPa
It is lower due to shunting where blood passes straight from the arterial to venous system without participating in gas exchange
What term is used to describe how molecules move around?
Brownian motion
They move in a random manner
What happens when molecules hit the surface of a solution?
Their motion is not changed but it is slowed down
Molecules diffuse in and out of the solution at a slower pace
It is a dynamic equilibrium across the surface of a solution
How does the partial pressure of a gas in solution relate to its solubility?
The partial pressure of a gas in solution is inversely proportional to its solubility
Why does a more soluble gas have a lower partial pressure in solution?
The lower solubility gas will equilibrate more quickly and generate its partial pressure
The greater the solubility, the more molecules can be accommodated for a given partial pressure
How many times more soluble is CO2 compared to O2?
What does this mean?
CO2 is 24 x more soluble than O2
A lot more CO2 molecules are needed in solution than oxygen molecules to generate the same partial pressure
Why is CO2 transfer more efficient than O2 transfer?
Carbon dioxide has a higher relative diffusing capacity (20.5)
What are the partial pressures of oxygen and carbon dioxide in venous blood entering the alveolus?
pO2 - 5.3 kPa
pCO2 - 6.1 kPa
What are the partial pressures of O2 and CO2 within the alveolus?
pO2 - 13.7 kPa
pCO2 - 5.3 kPa
What are the partial pressures of O2 and CO2 in venous blood exiting the alveolus?
pO2 - 13.3 kPa
pCO2 - 5.3 kPa
How long does it take for the partial pressure of oxygen to increase when passing through the alveoli?
0.25 seconds
This is the time taken for the blood to equilibrate with the alveolus
How long does it take for CO2 to equilibrate when passing through the alveoli?
0.1 seconds
Why does CO2 equilibrate more quickly than oxygen?
CO2 diffusion is more efficient
There is less partial pressure to equilibrate
There are more molecules of CO2 as it is more soluble than oxygen
What happens to blood flow during exercise?
The blood passes through the lungs more quickly
It doesn’t have as much time to equilibrate as it passes through the alveoli
During exercise, how long does the blood take to pass through the alveolus and what does this mean?
0.25 seconds
There is still enough time for oxygen to equilibrate
CO2 still equilibrates in good time as it takes less than 0.25 seconds
How would oxygen deficit be caused?
If the blood passed through the alveoli any quicker than in 0.25 seconds, it would not have enough time to equilibrate properly
What is the pO2 in pulmonary fibrosis patients and why?
They do not have enough time during exercise to equilibrate their oxygen across the alveoli
pO2 can only reach 9.5 kPa
What happens to CO2 equilibration in pulmonary fibrosis patients and what does this lead to?
CO2 still has enough time to equilibrate
Type I respiratory failure as there is not adequate oxygenation but there is adequate expiration of CO2
How is oxygen diffusing capacity measured clinically?
oxygen uptake is divided by (alveolar pO2 - mean pulmonary capillary pO2)
Why is direct measurement of oxygen diffusing capacity not used in practice?
pulmonary capillary pO2 is very difficult to measure
What mechanism is used in practice to measure diffusing capacity?
Carbon monoxide diffusing capacity
transfer factor
How is carbon monoxide diffusing capacity measured?
Carbon monoxide uptake is divided by alveolar pCO
What is the value for mean pulmonary capillary pCO?
0
Hb has a very high affinity for CO, so when bound to Hb the CO cannot exert a partial pressure as it is not in solution
How is carbon monoxide diffusing capacity measured in practice?
A concentration of carbon monoxide is given to a patient and the amount of CO taken up is measured
What happens in alveolar fibrosis and how does this impact diffusion?
Thickening the alveolar wall increases the diffusion distance
Decrease in diffusion capacity which can lead to respiratory problems
What happens in pneumonia?
Consolidation pneumonia involves the creation of a shunt
The alveolus no longer participates in gas exchange even though blood flows through it
What happens to oxygen concentration in pneumonia patients?
venous blood pools with the arterial blood that was successful in gas exchange
This leads to a dilution of the oxygen concentration
What happens in pulmonary oedema?
Frothy secretions within the alveolus
Works in a similar way to consolidation pneumonia
Patients cough up pink frothy sputum
What happens in interstitial oedema?
Fluid around the base of the alveolus, between the alveolus and the capillary
What happens in emphysema?
Reduction in the number of alveoli leads to a reduction in the total surface area for gas exchange
The lungs are less able to oxygenate the blood
What is atelectasis?
Collapse of the alveoli
How is atelectasis related to age?
As you get older, the elasticity of the lungs and thoracic wall is lost
The lungs begin to close and can no longer participate in gas exchange
how does atelectasis affect gas exchange?
As more and more alveoli begin to collapse, there is less capacity to oxygenate the blood
What is a common cause of pulmonary oedema?
Left heart failure leads to a lot of back pressure in the pulmonary vein
The right heart is still working efficiently
A lot of fluid is being pumped into the lungs but this is not being efficiently removed
How can pulmonary oedema lead to respiratory failure?
Thickening of the basement membranes and the alveoli filling with fluid means oxygen transfer is not as efficient
What happens in adult respiratory distress syndrome?
Lungs are filled with water which prevents gas from entering the lungs, and consequently the blood
how does obesity affect FRC?
A large mass on top of the chest prevents the thorax from springing out and reduces FRC
What can obesity lead to?
The chest is pulled inwards due to the decrease in FRC, leading to closure of alveoli
This is atelectasis
