Unit 1 Lecture 6: Gas Exchange Flashcards
What are the three main gases that make up our atmospheric pressure?
- Nitrogen (79.04%)
- Oxygen (20.93%)
- Carbon Dioxide (0.03%)
Where does gas exchange occur and what about its structure makes it good for diffusion?
In the alveoli, and it is 1 cell thick (epithelial cell) which optimizes the diffusion of gases through the membrane
Smaller membrane = faster diffusion
What is unique about the space between the alveolus and pulmonary capillary?
The space is called the “interstitial space” which contains interstitial fluid and the purpose of the fluid is to optimize gas exchange amongst the alveoli and pulmonary capillary
- Interstitial Lung Disease causes scarring of the lung and the interstitial space will widen making it harder for gas exchange to occur
- Emphysema lowers SA of gas exchange because it destroys the alveoli making it harder for gas exchange to actually occur
What is the key takeaway when it comes to gas exchange in the pulmonary capillary and tissue capillary level?
O2 and Co2 are involved in simple diffusion down partial pressure gradients
What is partial pressure?
It is the amount of pressure of a gas in an entire system of different gases which in total add up to 100
Pgas = Ptotal x Fgas
Fgas is always the same for N2, O2 and CO2 in inspiration and whatever the atmospheric pressure changes to
Other than partial pressure, what additional factors affect diffusion of O2 and Co2
- Increased SA ==> More diffusion
- Thickness ==> Rate of diffusion
- Solubility of the molecule ==> How quick it diffuses
Solubility being the rate of gas exchange is directly proportional to solubility coefficient for a gas
CO2>O2 solubility - IMPORTANT!!!
What does Dalton’s Law of Pressure state? Explain how partial pressure works with the provided picture
In a mixture of gases, the pressure of each gas is independent of the others; total make up the atmospheric pressure
- Each gas in the picture has their own pressure which essentially makes up the total pressure. Each pressure is referred to as a partial pressure of the entire pressure
Total pressure (in lungs) = Atmospheric Pressure - REMEMBER!
When we move up towards higher altitudes, does our fraction of oxygen molecules change with the atmospheric pressure?
NO! The fraction of molecules (O2 being 0.2093) stays the same.
- However, as atmospheric pressure decreases due to increasing altitude the partial pressure of the gases will decrease (the partial pressure = total pressure = atmospheric pressure so ↓Atm = ↓Partial Pressure)
What is the key takeaway for partial pressures and changing altitudes?
Partial pressures of O2 are lower at higher altitudes along with the driving pressure of O2 being lower due to decreased atmospheric pressure
- Partial pressure of gas increases when you increase the fraction of gas
* Saturation of O2 is lower higher up because not enough oxygen molecules higher up to actually take in and saturate in the blood
Pressure differences in what drives gas movement?
Pressure differences in blood and alveoli; this is key when it comes to how oxygen and carbon dioxide move down and up through the blood and the alveoli
What does Henry’s Law state?
Volume of gas disolved in a liquid is proportional to the partial pressure of the gas & solubility of the gas (ability to go between gas and liquid)
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How easy is it to dissolve the gas into a liquid
Cgas = Pgas x K
K is the solubility coefficient
What does a higher K (more solubility) mean?
Less pressure is required to dissolve gas; not as much driving pressure required to facilitate diffusion
Co2 is far times more soluble in the blood than O2 which means that the rate at which it diffuses is much higher which allows for optimal gas exchange
Why is solubility important even though we have partial pressure gradients? - KEY TAKEAWAY
Partial pressure gradients are used for the diffusion of gases but when we have a gas moving into a liquid we need to know how easily and quickly it dissolves and diffuses into the liquid. This is important because we have gases moving from alveoli into blood (liquid)
A high solubility coefficient vs low solubility coefficient can have same diffusion rate but be different in partial pressure - EXACTLY LIKE O2 and CO2
Is oxygen higher in the atmosphere or the alveoli?
It is higher in the atmosphere because in the alveoli the air goes through condensation and moves through dead space which causes the partial pressure of O2 to decrease in the alveoli; higher O2 atmosphere
- CO2 is higher in the alveoli than in the atmosphere
Key thing: Alveolar Pressure O2 is 100mmHg and the arterial pressure O2 is 100mmHg which means diffusion is good between the two
Venous Blood Co2 is about 46mmHg. When it moves up back to the heart and diffuses through the lungs, how much Pco2 diffuses into the lungs?
6mmHg making it 40mmHg in the lungs
Explain the V/Q Ratio for all three examples
- In the first example where one alveoli is not getting airflow (0 ventilation) but blood is flowing to it is called - Interpulmonary shunt; Low V/Q and Low Arterial O2 due to not enough gas exchange. Ratio = 0/1 (V/Q)
- In the second one we have the alveoli wrapped with capillaries and deoxygenated blood flows around until it comes into contact with the oxygen and the ventilation and perfusion are perfectly synced to allow for normal arterial O2 and Co2
- In the third example we have airflow (100% Ventilation) but one of the alveoli does not have capillaries around it (0% Perfusion - pulmonary embolism in blood); this means that although ventilation may be 1 the perfusion is 0 so this is known as alveolar dead space - alveoli don’t take part in gas exchange
Explain what the picture is representing?
This image illustrates the dynamics of diffusion of oxygen across alveolar-capillary membrane. When blood flows through the capillaries, it goes at a rate that by the end of 1/4 of a second, full diffusion has occurred
If cardiac output is high then the blood flows too quickly to not have full diffusion occur so diffusion limitation and ↓PaO2
