Ventilation and Diffusion Flashcards
State Dalton’s Law of Partial Pressures.
In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
How does Dalton’s Law of Partial Pressures relate to atmospheric pressure ?
Each individual gas exerts a proportion of atmospheric pressure, according to
its partial pressure.
Nitrogen: 78.08% x 760 mmHg = 593 mmHg
Oxygen: 20.95% x 760 mmHg = 159 mmHg
Carbon Dioxide: 0.039% x 760 mmHg = 0.29 mmHg
What happens to partial pressures if atmospheric pressure changes ? Give a real life example of this.
If atmospheric pressure changes, then the partial pressure changes.
Underwater as pressure increases, so will partial pressure (since oxygen can be toxic at high levels, inert gases are used to replace some of the oxygen. For example trimix – nitrogen-oxygen-helium or Heliox – oxygen and helium.)
What happens to the partial pressure of a gas if its proportion changes.
If the proportion of gas changes, its partial pressure changes.
What is the value of atmospheric P ?
760 mmHg or 101.325 kPa
State Henry’s Law. How is this relevant to ventilation ?
-When a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion with its partial pressure (10x more gas will go into solution if its partial pressure is 10kPa than if it were 1kPa).
If the partial pressure in the liquid becomes greater than in the air, the gas will move out of the liquid, and vice versa. This gives the basis for O2 moving into the blood in the lungs, whereas C02 moves out of the blood in the lungs.
-This relationship only applies to the dissolved gas (not to the gas particles that are chemically bound in solution)
Keeping in mind Henry’s Law, is ventilation an active or passive process ?
Passive process, controlled by partial Ps in lungs and bloodstream.
Explain the effect of underwater diving on Nitrogen concentrations in the blood.
Normally, concentrations low because solubility of Nitrogen low (despite the fact that partial P high).
Underwater, pressure increases so partial P of Nitrogen also increases, N2 can be forced into blood, narcosis and bends!
What, besides partial pressure (i.e. Henry’s Law), influences level of gas dissolved in a liquid ? How is still relevant to ventilation ?
The absolute level of gas dissolved in a liquid also depends on the solubility of the gas. CO2 is the most soluble, oxygen is about 1/20th as soluble and N2 is barely soluble at atmospheric pressure
So although the partial pressure of N2 is the highest (593 vs 159 vs 0.29 mmHg) , the amount of N2 in the blood is negligible.
What are all the factors which affect level of gas dissolved in liquid ?
- Partial P (affects amount dissolved)
- Solubility in liquid (affects absolute amount dissolved)
What happens to the total amount of gas in the liquid if some of it enters a chemical reaction (e.g. CO2 in blood becomes bicarbonate) ?
If the gas enters into a chemical reaction (e.g. CO2 producing bicarbonate) the total amount of the gas in the liquid is the amount dissolved plus that chemically bound in solution (i.e. dissolved amount could be small but then amount sequestered could be high)
Graph the amount of gas dissolved in liquid depending on the partial pressure of the gas, for each of CO2, O2, and N2.
Refer to slide 7 in lecture on “Ventilation and Diffusion”
Describe the composition of alveolar gas, explaining it.
- Alveolar gas is not atmospheric air
- Conducting passageways of the respiratory system humidify the air – warmed and moistened air water vapour pressure of 47 mmHg. Hence 760-47=713 mmHg. 713 x 20.93% = 149 mmHg for O2 (=INSPIRED PO2)
- In fact in lungs the O2 is lower (104 mmHg), the CO2 is higher (40 mmHg) and the water vapour is higher (and as a consequence the proportion of nitrogen is lower) because alveolar air is made up of ‘fresh air’ plus the air that remained in the lungs after the last breath
- Rest of difference is due to constant flux - O2 is moving out of the alveoli into the blood and CO2 is moving the other way
What factors affect the rate of diffusion of gases in blood ? Briefly explain how each of these factors affects oxygen and CO2 diffusion specifically and therefore state which of CO2 or O2 moves more.
Directly proportional to:
• Surface area (large, 50 -100 m2 + 500 million alveoli)
• Solubility (CO2 diffusing 20 x more rapidly than O2)
• Concentration gradient (difference in partial pressure) (large, PO2 alveolar air is 104 mmHg, PO2 of venous blood is 40 mmHg. smaller gradient for CO2, alveolar PCO2 is 40 mmHg, venous PCO2 is 45 mmHg)
Indirectly proportional to:
• Tissue Thickness of Lung (small, 0.2 – 0.5 um)
• √molecular mass (insignificant between gases)
Overall, same amount of gas moves because CO2 has higher solubility (O2 has lower) but CO2 has much lower concentration gradient (O2 has higher) and the rest of the factors affects same in the same way (i.e. SA, tissue thickness, molecular mass insignificant).
What are the steps to oxygen getting into the blood once it is in the alveoli ?
1) dissolve in the aqueous layer coating the alveoli
2) diffuse across the membranes
3) enter the blood
