alveolar Ventilation and diffusion Flashcards
What is daltons law of partial pressure?
Dalton’s law of partial pressures states that in a mixture of non-reacting gases, the total pressure is equal to the sum of the partial pressures of the individual gases.
What is the formula of Ptotal?
What is the formula for partial pressure of a given gas?
Ptotal = P1 + P2 + P3 + … Pn
Fractional concentration of the gas
Pgas = %gas x Ptotal
What is the composition of air?
What is the partial pressure of N2, O2 and CO2?
Pair = 760 mm Hg = PN2 + PO2
PN2 = 79% x 760 mm Hg = 600 mm Hg
PO2 =21%x 760 mm Hg = 160 mm Hg
Air has only 0.03% of carbon dioxide
PCO2 = 0.0003 x 760 mm Hg = 0.23 mm Hg
What is the composition of inspired?
PH2O (water vapor) at 37oC is 47 mm Hg
Pair = 760 mm Hg = PN2 + PO2+ PH2O
PN2 = 79% x (760-47) mm Hg = 563 mm Hg PO2 = 21% x (760-47) mm Hg = 150 mm Hg PCO2 = 0.03% x (760-47) mm Hg = 0.21 mm Hg
Why is alveolar air composition different from atmospheric air?
Alveolar air is partially replaced by atmospheric air during each breath
Oxygen constantly absorbed 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 is the ventilation system of the ventilation system?
Conducting zone
transitional zone
Respiratory zones
What is the conducting zone?
What is the function?
trachea- primary bronchus-bronchus-bronchi-bronchioles
No alveoli
Air transport, warming, humidification, particle filtration
Anatomic “Dead” Space
What is the respiratory zone?
What is the function?
Respiratory bronchioles-alveolar ducts- alveolar sacs
Surface area for gas exchange
What is the dead space?
What are the dead spaces?
Volume of airways and lungs that does NOT participate in gas exchange
Anatomic dead space (fixed)
Alveolar dead space
Physiologic dead space
What is the volume of conducting air in the anatomic dead space?
Volume of conducting airways (~150 ml)
What is the alveolar dead space in the alveolar space?
what is it in normal people?
Alveolus not perfused, so no gas exchange, e.g., pulmonary edema
Normal people alveolar dead space = 0
What is the physiologic dead space?
What is it in normal people?
Anatomic dead space + alveolar dead space
Normal people: Physiologic dead space = Anatomic dead space.
Is the dead space a part of lung residual volume?
no
Location is different. Dead space the upper respiratory area which does not involve in gas exchange. While RV is the space within alveoli which air is trapped in.
Dead space is ~150 ml while RV is ~500 ml.
How is dead space measured?
all expired CO2 comes from the alveolar gas and none from dead space
VD/VT = Paco2-Peco2/Paco2= 0.3
What are the minute ventilation (VE) and dead space ventilation equal (VD)?
MV = tidal volume * breathes/minute
Tidal volume= volume exhaled with each breath
Dead space ventilation (VD) = VD * f
How is the alveolar ventilation rates measured?
All CO2 comes from alveolar gas
VCO2 = VA * FCO2
VA = VCO2/PCO2 * K
How can PCO2(alveolar) be measured from PaCO2(arterial co2)?
they are identical in a normal person
So PaCO2 can be measured easily
Can be used to measure alveolar ventilation
VA = VCO2/PCO2 * K
What is the relationship between alveolar PCO2 and PO2
PCO2 = Vco2/VA
PO2=VA/VO2
Increase VA can PAO2 closer to inspired PO2
How can it be determined if a person is hypoventilating using PaCO2 arterial and alveolar PCO2?
Since arterial PaCO2 is the same as the alveolar PACO2, by measuring PaCO2 in clinic, we can determine whether or not the patient has hypoventilation.
How is alveolar O2 pressure calculated?
PAO2 = (PIO2) – (PaCO2/R)
R= respiratory gas exchange =0.8
PAO2 = 150 – 40/0.8 = 100 mmHg
PIO2=150
How is gas diffusion determined in alveolar?
The rate of gas diffusing across the blood-gas barrier is determined by the process of Diffusion and Perfusion.
What is perfusion limitation?
Why?
Perfusion Limitation (N2O,CO2) - Gas exchange across alveolar-capillary barrier is limited by blood flow through pulmonary capillaries.
Because N2O has very low affinity to Hgb, as soon as N2O enters blood capillaries the partial pressure of N2O rises rapidly. Therefore, the pressure gradient between the blood-gas barrier reaches equilibrium quickly (no more transfer). The moving force of N2O is dependent on blood perfusion, therefore, the transfer of N2O is said to be Perfusion Limited.
What is diffusion limitation?
Diffusion Limitation (CO) - Gas exchange across alveolar-capillary barrier is limited by diffusion process.
Because CO has extremely high affinity to Hgb, regardless how much CO enters blood capillaries the partial pressure of CO nearly changes. Therefore, the pressure gradient between the blood-gas barrier hardly changes, and the moving force of CO is dependent on gas diffusion Therefore, the transfer of CO is said to be Diffusion Limited.
What are the factors that affect the rate of gas diffusion?
Pressure difference across the respiratory membrane
Thickness of respiratory membrane
Edema fluid & fibrosis increase thickness
Surface area of respiratory membrane
Emphysema – dissolution of many alveolar
Transfer of gas through membrane depends on the Diffusion coefficient (D)
Solubility and molecular weight of gas determine D
What are pathological factors that decrease diffusing capacity?
Pathology of air-blood barrier ( increase thickness or decreases surface area),
decrease capillary volume
decrease hemoglobin.
Examples: COPD, anemia, fibrosis, pulmonary edema, pneumonia.
