Week 4 Physical Principles Of Gas Exchange Flashcards
What is the diffusion process in gas exchange?
-Random molecular motion of molecules
• Movement in both directions through the
membranes & fluids of the respiratory
structure
What is the net diffusion of gas?
- movement of a gas one direction is the effect of a concentration gradient
- diffusion goes from high to low
- rate of diffusion dependent on pressure
What is the partial pressure of a gas mixture?
- Each gas in a mixture contributes to the total pressure in proportion to its concentration
- respiratory gases mainly include: oxygen, nitrogen, and carbon dioxide
- each gas exerts it own individual pressure on the respiratory wall
What is the composition of air?
- 21% oxygen, 79% nitrogen
- total pressure of air mixture is 760mmHg = 1 atmosphere
What is Henry’s Law?
Solubility of a gas in a liquid depend on temperature, the partial pressure of a gas over the liquid, the nature of the solvent and the nature of the gas
-the colder it is, the more gas can be dissolved into a fluid
What is the solubility coefficient (D)?
- tells you how many molecules can be dissolved in a fluid
- highly soluble: when dissolved molecules are attracted by water more can be accumulated without building up excess pressure in a solution
- poorly soluble: molecules which are repelled by water will dissolve less and have a lower concentration
*CO2 is 20x more soluble than O2
What determines the rate of net diffusion between the alveoli and blood?
The difference of partial pressures (pp)
- if pp of gas in alveoli > blood then gas moves into blood (O2)
- if pp of gas in blood > alveoli then gas moves into alveoli (CO2)
What is the vapor pressure of water?
*vapor pressures = parietal pressure of water escaping surface into gas phase
Vapor pressure is 47mmHg when gas mixture is fully humidified at 37 degrees C
How does vapor pressure alter the partial pressure of the upper airway?
Part of the total pressure (760mmHg) is due to the vapor pressure of water
- 760mmHg(total) = 713mmHg (pp air) + 47mmHg (pp H2O vapor)
- so pp of O2 in upper airway = 713mmHg x 0.21 = 150mmHg
What factors affect gas diffusion rates?
- pressure difference main factor
- gas solubility in fluid
- area of fluid
- distance which gas must diffuse
- molecular weight of gas
- temperature of fluid (constant in body)
What are the characteristics of the gas which affect the ability and rate of net diffusion?
- solubility of gas molecule
- molecular weight
- the relative rate at which different gases diffuse are proportional to their diffusion coefficient
- D is directly proportional with solubility
- D is inversely proportional to the square root of the gas’ molecular weight-heavier gas will move more slowly
What allows for diffusion of gasses through tissues?
- respiratory gases are highly soluble in lipids (the main component of cell membranes)
- cell membranes are highly permeable to these gases
- rate of gas movement into tissues is limited by diffusion rate of gas through tissue water
- movement of gas into and out of tissues = diffusion are of gas through water
Why is the alveolar air composition different from atmospheric air composition?
- alveolar are is partially replaced by atmospheric air during each breath
- oxygen constantly absorbs into blood from alveoli
- carbon dioxide diffused into alveoli from blood
- as air enters respiratory passages it becomes humidified diluting the inspired gases partial pressures
What prevents sudden changes in alveolar gas concentration?
- multiple breaths are required to exchange alveolar air
- 350mL or air per breath
- FRC is roughly 2500mL
- each breath replaces a seventh of FRC
*allows respiratory control mechanism to be more stable
What is the rate of normal alveolar ventilation?
-normal ventilation remove 1/2 of gas in 17 seconds
