BSI: Respiratory Introduction
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What are the units typically used to express gas pressure in the lungs?
The old system used for centuries by physicians to measure blood pressure: millimeters of mercury (mmHg).
Which of the following can be compressed?
(this multiple choice question has been scrambled)
Are gases susceptible to temperature change?
Yes. Gases are susceptible to temperature change. For example, as heat rises, gas is more likely to escape into the atmosphere as more energy allows particles to move greater distances.
What is gas pressure?
- Gas pressure is caused by gas molecules, such as: nitrogen [N2]
- oxygen [O2]
- carbon dioxide [CO2]
These gas molecules are continuously hitting the wall of their container
Therefore, more gas
means there are more molecules hitting the walls
A higher temperature
results in the molecules moving faster and hitting the walls more frequently
- If you decrease the volume of the container, then the molecules hit the walls more frequently and also results in increased pressure.
What is the Gas Law?
The gas law
is a summary of gas pressure behavior
- (P1V1/ T1) = (P2V2/ T2)
- P = pressure
- V = volume
- T = temperature
- *Ignore units in this class!
Using the gas law, what happens to pressure if volume is decreased?
According to the gas law, pressure and volume are inversely proportional
. Therefore, if you decrease the volume
, you will increase the pressure
, and visa versa.
(P1V1/ T1) = (P2V2/ T2)
How does the gas law relate to the lungs?
With the lungs, a constant temperature is assumed, so a simpler gas law is used that eliminates temperature from the equation. This is called "Boyle's Law."
What is the atmospheric pressure at sea level?
At sea level and standard temperature, the Earth's atmosphere exerts a total pressure of 760 mm Hg
(would support a column of mercury 760mm high). This is equal to 1 atmosphere of pressure
What is the atmosphere or air we breathe composed of?
- The air we breathe is a mixture of the following gases:
- N2 (78.6%)
- O2 (20.8%)
- CO2 (0.04%) *and rising due to global warming
- Argon (1%) *a totally inert noble gas
What is the individual contribution of each component of the atmosphere to atmospheric pressure?
The individual contributions of each component of the atmosphere is directly related to the percentage of their composition to the atmosphere:
For N2 the "partial" pressure (Pn2) is therefore given by 78.6% of 760 mm Hg = 597 mm Hg
For O2 the "partial" pressure (Po2) is therefore given by 20.8% of 760 mm Hg = 158 mm Hg
For CO2 the "partial" pressure (Pco2) is therefore given by 0.04% of 760 mm Hg = 0.3 mm Hg
Argon will not be considered because it has no known effect on breathing.
How will gases "diffuse"?
As gas pressure depends on the number of molecules
hitting the walls of it's container these partial pressures
can be thought of as the gas equivalent to concentration
Gases will diffuse from areas of high partial pressure
to those of low partial pressure
Why is it harder to breathe the higher in elevation you go?
As you ascend up a mountain, the amount of atmosphere above you decreases and so does the atmospheric pressure. In other words, the higher you go, the less molecules of oxygen there are to breathe.
For example, at ~20,000 feet, atmospheric pressure is less than half normal (meaning at sea level) which means the Po2 is less than half and will not saturate hemoglobin. This can cause drastic consequences for the body (anoxia).
At normal pressures, which two components of the atmosphere have no known physiological effect?
Under what conditions can nitrogen negatively affect the human body?
If you breath pressurized air (SCUBA diving, etc.) you can force nitrogen into solution.
Therefore, if you ascend to the surface too quickly during SCUBA diving, it will cause a sudden ↓ pressure, causing the nitrogen to come out of solution (similar to opening a can of shaken soda in your blood vessels).
This condition is called the "bends" and is extremely painful.
What temperature, in Centigrade, is our body usually at?
37 degrees centigrade
What other pressure, besides the pressure exerted from dry atmospheric gases, must be taken into account when talking about the lungs?
Water does vaporize and therefore exerts a pressure. Our bodies are at 37oC and the air in our moist lung is saturated with water vapor, exerting a pressure of 47 mm Hg (Saturated Water Vapor Pressure: SWVP) which must be taken into account.
Why must the lungs be kept moist?
A moist environment in the lungs is necessary for diffusion and gas exchange.
What is the partial pressure of O2 in the lungs?
Between breaths, our alveolar pressure must equal the surrounding atmospheric pressure
(760 mm Hg
Therefore, to calculate the partial pressure of O2 in our lungs
, we must take into account both the moist environment of the lungs
(water vapor exerting a pressure of 47 mm Hg
) and the percentage of "dry" oxygen
in the atmosphere (20.8% of 760 mm Hg).
- Therefore, the calculations are as follows for finding the pressure of oxygen in moist lungs:
- 760 mm Hg - 47 mm Hg = 713 mm Hg
- 713 mm Hg x (0.208) = 148 mm Hg
- Compare that number the pressure of dry oxygen:
- 760 mm Hg x (.208) = 158 mm Hg
in the dry atmosphere
but only 148 mm Hg
in the moist lungs
Gases exert a pressure in solution. What two things affect this pressure?
- Gases exert a pressure when in solution (which is also denoted by partial pressure). Pressure in a solution is affected by:
- 1) concentration of the gas in solution
- 2) solubility coefficient of the gas
What happens to CO2 in water (H2O)?
CO2 reacts with water (H2O) to form H2CO3
Which is more soluble in water: O2 or CO2? Why does this matter?
CO2 is ~20x more soluble than O2 and therefore has a bigger solubility coefficient:
CO2 = 0.57 vs. O2 = 0.024
Which has a larger partial pressure in solution: CO2 or O2?
Although CO2 has a much larger solubility coefficient, it only has 1/20th of the partial pressure of an equivalent amount of O2 in solution.
What is the formula for partial pressure?
partial pressure = (concentration of dissolved gas/solubility coefficient)
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