S3 L1 Gas Exchange Flashcards
Kinetic Theory of Gases
- What is this theory?
- What is gas pressure?
- Term used to talk about how much individual gases exert?
- What is Dalton’s Law? How to work out the total pressure of the gases?
- Give an example of a specific gases partial pressure
Gas pressure is caused by the collisions of
gas particles with the walls of the container -
pressure is the force generated by the
collisions. There is a TOTAL gas pressure in
the box.
Partial pressure of the gas
Last two Q (see seperate pic)
Real world example:
- Composition of different gases in air
- Total atmospheric pressure at sea level is?
- Units of pressure?
- Have an idea of: partial pressure of oxygen, nitrogen and carbon dioxide
- Is atmospheric pressure always the same i.e. at sea level and on a mountain? Explain this?
Total Atmospheric Pressure at sea level =101 kiloPascals (kPa)
Partial pressure of O2 = 101 x .209 = 21.1 kPa
Partial pressure of N2 = 101 x .78 = 78.7 kPa
Partial pressure of CO2 = 101 x 0.03%= 0.03 kPa
- Why do we care about partial pressure?
- Partial pressure notations in the human body i.e. partial pressure in alveoli, arteries and veins
- In the body gases diffuse down their partial pressure gradient – From area of high partial pressure to area low partial pressure
Partial pressures – rather than concentration- used to describe gases in body
Upper Respiratory tract
- What molecules are added to the air?
- How does this change the partial pressure? (specific type of pressure)
- What is it affected by?
- Example of how this pressure affects partial pressure of oxygen, nitrogen and carbon dioxide, when the atm pressure is 101kPa
(last Q on Q side of FC)
Remember -TOTAL Pressure of gases in our URT = {101(kpa) – 6.28 (kPa)} = 94.7kPa
• The gases are still in the same ratios as in dry air:
therefore: pO2 = 94.7 kPa x 20.9% = 19.8 kPa
therefore pN2 = 94.7 kPa x 78 % = 73.8 kPa
therefore pCO2 = 94.7 x 0.03%= 0.03kPa
Review/recap of past flashcards: Gas partial pressures in the air we breath in and in air in our lungs
pic
The partial pressure oxygen in the upper respiratory tract vs alveoli is different:
URT: pO2 = 19.8 kPa
BUT: alveolar partial pressure oxygen = 13.3 kPa
- Why this difference?
pic
Note about maintaining a steady physiological state:
- How is it maintained?
Alveolar pO2 and pCO2 as a function of ventilation:
- draw a graph showing alveolar ventilation against alveolar pO2 and pCO2
- Use the graph to example how hypoventilation leads to hypercapnia
Alveolar Ventilation -oxygen - Rate of extraction and replenishment is constant -
therefore alveolar pO2 maintained at a steady physiological state
Alveolar Ventilation -carbon dioxide - Rate of extraction and replenishment is
constant - therefore alveolar pCO2 maintained at a steady physiological state
E.g. with exercise, make more carbon dioxide, so breath faster to get rid of it and keep carbon dioxide levels in the correct range
What is the relationship between alveolar gas pressures and systemic arterial gas pressure?
Gas dissolved in body fluids:
- How do dissolved gas molecules exert pressure in the liquid?
- when is equilibrium reached?
changes to alveolar partial pressure e.g. an increase, will increase arterial partial pressure for that particular molecule (due to concentration gradient)
Review of gas movement in an alveoli:
- between venous, alveolus and artery
Alveolar gas composition is steady because:
- amount of O2 (ml/min) brought in by ventilation = amount of O2 (ml/min) diffusing into the blood
- and amount of CO2 removed by ventilation = amount of CO2 diffusing from blood into the alveolus
Dissolved gas:
How is partial pressure different to the amount of a dissolved gas?
How to work out amount of gas that has been dissolved?
- equation
pic
Summary so far of last few slides (nothing new)
- How to work out total pressure in defined space?
- In order to calculate oxygen partial pressure in alveoli, need to take into account…
- How do gases move?
pic
- What does ‘total oxygen’ mean?
- If a gas chemically reacts, what does this mean for its partial pressure?
- 2 forms of oxygen in the blood
- What does equilibrium mean?
Blood contains both dissolved and Hb bound oxygen.
pO2 reflects the amount of dissolved oxygen in the blood
Dissolved oxygen is available to diffuse into tissues down its partial pressure gradient
As dissolved O2 leaves the blood for tissues, it will be
replaced by O2 bound to Hb unbinding from Hb and dissolving into the blood - and the process continues
This is how the oxygen bound to Hb will
What factors affect the rate of gas diffusion
- Equation
- State each factor
pic
Factors affect the rate of diffusion:
- Diffusion coefficient
- What factors (2) does the diffusion coefficient include?
- Explain more details about the factors (2) affecting diffusion coefficient
- Compare the diffusion coefficient of oxygen and carbon dioxide, explain the difference between them
Diffusion also depends on properties of the individual gas:
• The solubility of the gas in the liquid : greater the solubility, faster the rate of
diffusion
• Molecular weight of gas:
– Higher the molecular weight slower the rate of diffusion
Diffusion coefficient (D) = Solubility / (square root of molecular weight)
Diffusion barrier
- What are the diffusion barrier of alveolar air to RBC in capillary
Diffusion from alveolar air to RBC in capillary must cross:
– epithelial cell of alveolus
– Interstitial space
– endothelial cell of capillary
– plasma
– red cell membrane
- 5 cell membranes
- 3 layers of cytoplasm
- 2 layers of tissue fluid +plasma
Factors affecting rate of gas diffusion:
- Reserves we have in the body
- Disease examples of factors affecting rate of gas diffusion
Reserves:
Oxygen exchange complete in 1/3 of time blood spends
capillary
• Plenty of reserve – for exercise
Clinical link:
First time people with interstitial lung disease (etc) get short of breath, will be with exertion. Exercise: blood is following by alveoli more quickly, if oxygen takes longer to diffuse than it should, then will see problems of SOB, and oxygen partial pressure dropping in the blood. As disease processes, will become SOB and have low partial pressure of oxygen at rest too.
Summary of diffusion impairment in:
- Fibrotic lung disease - Pulmonary oedema - Emphysema
Review of last few slides
- Link between alveolar partial pressure and oxygen partial pressure, and the effect of haemoglobin bound oxygen
- Read the rest of it (just a recap)
- Alveolar partial pressure oxygen determines arterial partial pressure of oxygen BUT because oxygen binds to haemoglobin (and this bound oxygen does not contribute to blood oxygen partial pressure) the functional partial pressure gradient is
maintained at a very high level - Alveolar air has pO2 =13.3 kPa and pCO2 =5.3 kPa; remembering the point made above this means that Arterial pO2 =13.3 kPa and pCO2 =5.3 kPa
- Mixed venous blood (blood arriving at alveoli in capillaries) has pO2 = 6 kPa and pCO2 = 6.1 kPa
- Diseases in which the diffusion distance is increased – Interstitial lung disease; or the
fluid in the interstitial space is increased – congestive heart failure; or there is a loss of
surface area for diffusion to occur – emphysema – will all decrease arterial oxygen, CO2 typically normal until very late stage disease because it is more soluble c/w O2
Ventilation/perfusion ratio
- What is ventilation and perfusion?
- Normal V/Q ratio
- What is this ratio like at different parts of the lungs and why?
pic
In normal lungs:
- When can you have V=Q (ventilation perfusion match)
- Ideally: When have increased ventilation should have _____ perfusion
When have decreased ventilation, should have _____ perfusion
- What compensatory mechanism occurs in the alveoli and why?
increased
decreased
Compensatory mechanism:
When alveolar PAO2 is low, hypoxic vasoconstriction of pulmonary
arterioles occurs
– This diverts blood to better ventilated alveoli
– This compensation not complete - can’t divert all the blood
What is a Ventilation-Perfusion Mismatch?
Why does it occur?
What does V/Q mismatch mean for the body?
What are the consequences of low V/Q?
- How can this affect oxygen and carbon dioxide levels?
pic
Why can’t over-ventilated alveoli compensate for under-ventilated alveoli?
- focus on oxygen
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Pulmonary embolism and V/Q mismatch
- How does this affect V/Q?
- 2 ways
pic
Shunt -
- What is this?
- How does it affect V/Q?
- Clinically how could you tell if shunt was occuring?
pic
Summary of V/Q match and mismatch
- Read and understand
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Decompression sickness
- Also called?
- What is this, explain the condition?
Pressure below sea level = atmospheric
pressure + weight of water
Swimming up too quickly doesn’t give the nitrogen enough time to leave the blood - instead it forms bubbles. Medical condition caused by dissolved gases emerging from solution as bubbles due to depressurisation. Can cause gas embolus
Bits from group work:
- How is total oxygen calculated?
- How to find values from sigmoid curve?
- What is ventilation?
- What is the affect of hyperventilation on alveolus oxygen and carbon dioxide?
- How does all the types of partial pressures effect each other?
- Why are oxygen levels affected before carbon dioxide levels?
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