Exam #3: Gas Exchange Flashcards
What is effective respiration?
Enough oxygenation to meet tissue demand
What is the main driver of gas exchange?
- The gradient in partial pressure of gases
- Gases diffuse “downhill” along the gradient of decreasing partial pressure
What is Dalton’s Law of Partial Gas Pressures?
Pressure of a gas is directly proportional to the concentration of its molecules
Ptotal= P1 + P2 +P3 Pb= PN2 + PO2 + PCO2 + Pothers
Pb= 760 mmHg
Px=Fx (PB-PH20)
How is the the total dry gas pressure obtained?
Subtract water vapor pressure from the total barometric pressure
Px= Fx (PB- PH20)
What is Henry’s Law?
When gas is DISSOLVED in fluid & encounters a surface, it exerts its own partial pressure
Partial pressure= Concentration of dissolved gas/ solubility coefficient
How does the solubility of CO2 compare to O2?
CO2»>O2
- This means CO2 dissolves easily.
- At EQUAL concentrations, oxygen exerts a higher partial pressure than CO2
What is the ideal gas law?
PV=nRT
What is the functional implication of the ideal gas law?
If P1>P2 or vice versa, then gas exchange will take place until:
P1V1= P2V2
E.g. in anesthesiology, when gas is administered, gas exchange will occur until Palveoli = Pblood
How do gases diffuse in fluids & tissues?
Gas molecules diffuse proportionally to gas partial pressure difference
What is Fick’s law of diffusion?
The amount of gas that moves across a tissue sheet is:
- Proportional to area
- Inversely proportional to thickness
Write the equation for net rate of diffusion.
Diffusion is proportional to:
- Pressure
- Area
- Solubility
And inversely proportional to:
- distance through which the gas must diffuse
- square root of molecular weight of gas
Describe gas exchange at the respiratory membrane.
Oxygen
- 100 P02 in alveoli
- 40 PO2 in venous circ.
Net into circulation
What two things are required for adequate respiration?
Diffusion
Perfusion
What is the difference between PAO2 & PaO2?
A= alveoli a= arterial
What is the respiratory unit?
Location of gas exchange
- Bronchioles
- Alveolar ducts
- Atria
- Alveoli
How is diffusion effected in atelectasis?
Collapse of lung tissue
- Decreased dP
- Increased distance
- Decreased SA
How is diffusion effected in pneumonia?
Markedly increased distance = decreased diffusion
How is diffusion effected in pulmonary edema?
Increased capillary hydrostatic pressure
- Increased distance
- Diffusion effectively blocked
Hydrostatic pressure increases & can cause pink frothy sputum (capillary blood)
How is diffusion effected in pulmonary fibrosis?
Increased distance due to increase collagen & elastin deposition
Describe oxygen diffusion between the alveoli & pulmonary capillary.
Arterial end= pulmonary arterial end
- Low PO2 (40 mmHg)
- Alveoli= 100mmHg
Oxygen in
Equilibration occurs by the time the blood reaches the venous end i.e. to go into the left side of circulation
During exercise & tachycardia how does equilibration of oxygen change?
Takes longer
What happens to the diffusion of oxygen in the tissues?
Arterial PO2» Venous & capillary= net diffusion of oxygen into the tissues
- Opposite occurs with CO2
Draw the change in PO2 as blood travels through the circulation.
N/A
What causes the drop in PO2 from pulmonary capillaries to systemic arterial blood?
Pulmonary shunting occurs i.e. deoxygenated blood from bronchioles returns straight to the left side of the circulation
Draw the interdependence between tissue PO2, blood flow, & tissue metabolism.
Blood flow has to adapt to metabolic rate to maintain tissue oxygenation
Less metabolism= less flow
More metabolism= more flow
Why is the small pressure difference between capillary CO2 & interstitial CO2 sufficient to drive diffusion?
High solubility of CO2
What is the Diffusing Capacity (DlCO)
- Measurement of the lungs ability to transfer gases
- Patient breathes small amount of CO & initial value is measured
- Diffusion occurs
- Measure CO in alveoli afterward
Determine capacity to diffuse
How do you predict the diffusing capacity of the lung for oxygen?
DLCO x 1.23
What does a decrease in diffusing capacity tell you?
1) Thickened or damaged respiratory membrane
2) Anemia (less Hb)
- E.g. emphysema, pulmonary fibrosis, interstitial lung disease, pulmonary HTN, chronic PE
What causes an increase in diffusing capacity?
1) Polycythemia
What is the alveolar gas equation?
Relationship between oxygen in the alveoli, fraction of inspiratory oxygen & arterial pressure of Co2.
List the pathway that oxygen takes as it is inspired & then exchanged for CO2.
- O2 inspired into the alveoli
- Exchange across the respiratory membrane
- Pulmonary capillaries
- Pulmonary vein
- Left atrium
- Left ventricle
- Systemic circulation
