Physiology 3120 Lecture 45: Phys 3120 - Lecture 45
2 May 2018
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Human Physiology Lecture 45
Gas Exchange
- MAIN function of the lung = gas exchange between the blood and the external environment
o Exchange of oxygen from the air (external environment) into the blood, and carbon dioxide
from the blood into the air (external environment)
o Do not need an exchange of oxygen with carbon dioxide
o Occurs by: diffusion and blood-gas barrier
- Oxygen is inhaled from the air into the lungs and diffuses into the blood
o Oxygen transported by with blood to tissues is used for chemical reactions within cells
- Carbon dioxide is the major end product of those reactions
o Carbon dioxide is transported via blood and diffuses into the lung from the pulmonary
capillaries
o CO2 exhaled into the air
- Other functions of the lung:
o Reservoir and filter for blood
o Involved in metabolism of some compounds and providing airflow for speech
Gas Exchange
- Gas exchange in the capillaries of the lung, where blood comes through
o Oxygen diffuse into the blood, transported to the heart
o Carbon dioxide exists the blood, enters the lungs, and is exited
- Oxygenated blood (just went through the lungs where it got oxygen) goes through the pulmonary
veins, to the heart, and gets into the systemic arteries
o Oxygen is high
- Carbon dioxide goes from the systemic veins, to the pulmonary arteries (where carbon dioxide is
high) and gas exchange occurs in lungs and carbon dioxide is removed
- How much oxygen in blood – talking about the systemic arteries
o Arterial blood – NOT PULMONARY ARTERY, IT IS SYSTEMIC ARTERIES
o Easier to get blood from a person from here
Function: GAS EXCHANGE
- Gas exchange within the lung occurs by: DIFFUSION
o Occurs at the blood gas barrier
o Need oxygen to move in, carbon dioxide to move out
- Diffusion occurs according to: Fick’s Law of Diffusion
Fick's law of diffusion
Vgas = (constant * (P1-P2))*A/T
- Vgas = Volume of gas transferred per unit time
- (P1-P2) = difference in partial pressures (of the gas)
- A = Area of tissue sheet
- T = Thickness of the tissue sheet
- Constant = the diffusion constant
- This guides how much gas could be transferred into the tissue or out of the tissue/blood
- Rate of gas transport across a tissue sheet is:
o Proportional to: area of the sheet, diffusion constant, differences in partial pressure
o Inversely proportional to: thickness
- If you take a box, and inject a little bit of oxygen on one side of the box
o True diffusion – these molecules move throughout the box
o If you look at the box after a while, gas is distributed equally throughout the box
o Simple diffusion- molecules moving around in that space
- Repeat the experiment, expect now with a small hole rather than the entire box being open, in the
same amount of time it took to equally distribute in the first experiment, the amount of oxygen in the
right hand side is A LOT lower
o Indication of the area by which diffusion can occur has an imapct on the amount of gas being
transported to the right end half of the box
- Why in Frick’s law of diffusion, we have the area being one of the factors that imapcts it
o THE LARGER THE AREA DIFFUSION CAN OCCUR = THE FASTER DIFFUSION CAN OCCUR
