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Lecture 44

Physiology 3120 Lecture Notes - Lecture 44: Pulmonary Artery, Pulmonary Vein, Blood Gas Test

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Tom Stavraky

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Lecture 44 Lungs
Gas exchange
- Function of the lung is gas exchange
o Exchange of oxygen from air into blood and carbon dioxide from blood into air
- Gas exchange occurs in the capillaries
o Oxygen diffuses into the blood and is transported to the heart
o Carbon dioxide exits the capillaries, goes into the lungs and is exhaled
- Oxygenated blood from the lungs goes through the pulmonary vein, through the heart and
into the systemic arteries (where oxygen is high)
- Carbon dioxide goes through systemic veins, pulmonary artery (where carbon dioxide is
high) and then gas exchange occurs and it is removed
Blood gas values
- When talking about how well the lung is working and how much oxygen is in the blood, we
are referring to the systemic arteries (where blood goes after gas exchange occurs in the
- Arteriole blood = systemic arteries (not pulmonary)
- Gas exchange within the lungs at the blood-gas barrier occurs by: SIMPLE DIFFUSION
o For gas exchange to occur, we need oxygen AND air
- Diffusion occurs according to: FICK’S LAW OF DIFFUSION
Fick’s law of diffusion
- Volume of gas that is transported across the tissue per time
- Equation governs how much gas can be transferred into/out of the tissue/blood
- Simple diffuse is just molecules floating around in space
- E.g. inject gas molecules in a box
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o Molecules move around in the box and the gas becomes distributed equally
- E.g. if inject molecules in a box but rather than having the whole box open, there is only a
small hole
o In the same amount of time, the amount of oxygen in the other compartment would
be a lot lower
o This is an indication of the area by which diffusion can occur
o Area has an impact on the amount of gas being transported
- Larger the area for diffusion, the faster it occurs
- If oxygen has to go through tissue to get to the other side:
o Easier for it to diffuse if it is thinner
o Oxygen has to go through the cell and out through the other side
o Diffusion is slower if it is thick
- The higher the diffusion constant, the more diffusion occurs
- Diffusion constant is a property of the gas
- The diffusion constant is proportional to the gas solubility but inversely proportional to
the square root of its molecular weight:
o 1) Size of the molecule smaller molecule larger constant
o 2) Solubility of the molecule in tissue higher solubility higher constant
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- Difference in partial pressures in the tissue have an impact on diffusion
- Large pressure gradient = more rapid diffusion compared to a small gradient
In the lung…
- Partial pressure gradient exists because of the blood flow in the lung
o Blood flow is on one side and the alveolar air space of the lung is on the other side
- Because of continuous blood flow, there is always a partial pressure gradient that causes
- Blood gas barrier is very large and very thin
- Pressure gradient is because of blood flow and ventilation (air being brought into lungs)
- Constant is a property of the oxygen and carbon dioxide
Partial pressures
- What are partial pressures of gases? How does it apply to gas within a liquid?
o Partial pressure of a gas within a liquid
o E.g. partial pressure of oxygen in the blood
- Partial pressure of a gas is the pressure exerted by any one gas in a mixture of gases
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