alveolar Ventilation and diffusion

Question 1

What is daltons law of partial pressure?

Answer 1

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.

Question 2

What is the formula of Ptotal?

What is the formula for partial pressure of a given gas?

Answer 2

Ptotal = P1 + P2 + P3 + … Pn

Fractional concentration of the gas
Pgas = %gas x Ptotal

Question 3

What is the composition of air?

What is the partial pressure of N2, O2 and CO2?

Answer 3

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

Question 4

What is the composition of inspired?

Answer 4

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

Question 5

Why is alveolar air composition different from atmospheric air?

Answer 5

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

Question 6

What is the ventilation system of the ventilation system?

Answer 6

Conducting zone
transitional zone
Respiratory zones

Question 7

What is the conducting zone?

What is the function?

Answer 7

trachea- primary bronchus-bronchus-bronchi-bronchioles

No alveoli
Air transport, warming, humidification, particle filtration
Anatomic “Dead” Space

Question 8

What is the respiratory zone?

What is the function?

Answer 8

Respiratory bronchioles-alveolar ducts- alveolar sacs

Surface area for gas exchange

Question 9

What is the dead space?

What are the dead spaces?

Answer 9

Volume of airways and lungs that does NOT participate in gas exchange

Anatomic dead space (fixed)
Alveolar dead space
Physiologic dead space

Question 10

What is the volume of conducting air in the anatomic dead space?

Answer 10

Volume of conducting airways (~150 ml)

Question 11

What is the alveolar dead space in the alveolar space?

what is it in normal people?

Answer 11

Alveolus not perfused, so no gas exchange, e.g., pulmonary edema

Normal people alveolar dead space = 0

Question 12

What is the physiologic dead space?

What is it in normal people?

Answer 12

Anatomic dead space + alveolar dead space

Normal people: Physiologic dead space = Anatomic dead space.

Question 13

Is the dead space a part of lung residual volume?

Answer 13

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.

Question 14

How is dead space measured?

Answer 14

all expired CO2 comes from the alveolar gas and none from dead space

VD/VT = Paco2-Peco2/Paco2= 0.3

Question 15

What are the minute ventilation (VE) and dead space ventilation equal (VD)?

Answer 15

MV = tidal volume * breathes/minute

Tidal volume= volume exhaled with each breath

Dead space ventilation (VD) = VD * f

Question 16

How is the alveolar ventilation rates measured?

Answer 16

All CO2 comes from alveolar gas

VCO2 = VA * FCO2

VA = VCO2/PCO2 * K

Question 17

How can PCO2(alveolar) be measured from PaCO2(arterial co2)?

Answer 17

they are identical in a normal person
So PaCO2 can be measured easily

Can be used to measure alveolar ventilation

VA = VCO2/PCO2 * K

Question 18

What is the relationship between alveolar PCO2 and PO2

Answer 18

PCO2 = Vco2/VA
PO2=VA/VO2

Increase VA can PAO2 closer to inspired PO2

Question 19

How can it be determined if a person is hypoventilating using PaCO2 arterial and alveolar PCO2?

Answer 19

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.

Question 20

How is alveolar O2 pressure calculated?

Answer 20

PAO2 = (PIO2) – (PaCO2/R)

R= respiratory gas exchange =0.8

PAO2 = 150 – 40/0.8 = 100 mmHg

PIO2=150

Question 21

How is gas diffusion determined in alveolar?

Answer 21

The rate of gas diffusing across the blood-gas barrier is determined by the process of Diffusion and Perfusion.

Question 22

What is perfusion limitation?

Why?

Answer 22

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.

Question 23

What is diffusion limitation?

Answer 23

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.

Question 24

What are the factors that affect the rate of gas diffusion?

Answer 24

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

Question 25

What are pathological factors that decrease diffusing capacity?

Answer 25

Pathology of air-blood barrier ( increase thickness or decreases surface area),
decrease capillary volume
decrease hemoglobin.

Examples: COPD, anemia, fibrosis, pulmonary edema, pneumonia.