Zhong-Pulmonary Gas Exchange

Front 1

Describe the composition of air

Back 1

Atmospheric and inspired air: Pair=760mmHg = PN2 + PO2
PN2=79%; PO2 =21%
Air is only 0.03% CO2

Front 2

Describe alveolar air composition

Back 2

Alveolar air does not have same gas concentrations as atmospheric air composition
1. alveolar air is partially replaced by atmospheric air during each breath
2. O2 constantly absorbed into blood from alveoli
3. CO2 diffused into alveoli from blood
4. air entering respiratory passages becomes humidified diluting inspired gases partial pressures

Front 3

Rank the gas partial pressures in the lungs and blood from highest to lowest O2 partial pressure

Back 3

1. dry inspired air
2. humidified bronchial air
3. alveolar air and systemic arterial blood
4. mixed venous blood coming to the lungs (CO2 pressure is higher than O2 pressure)

Front 4

Describe the anatomic dead space of the lungs

Back 4

Volume of airways and lungs that do NOT participate in gas exchange
1. Anatomic dead space (conducting airways)
2. Alveolar dead space-alveolus not perfused, so no gas exchange
3. Physiologic dead space-Anatomic dead space + alveolar dead space
measuring dead space-all expired CO2 comes from alveolar gas, none from the dead space (Bohr method)

Front 5

Describe what the different ventilation rates are

Back 5

1. minute ventilation rate-tidal volume * breaths per minute
2. dead space ventilation rate-physiologic dead space
3. alveolar ventilation rate (VA)-(tidal volume - dead space) * breaths per minute
because all expired CO2 comes from alveolar gas, VA can be measured using VCO2
VA=(VCO2/PCO2) * K

Front 6

Explain the relationships b/t alveolar ventilation and alveolar PCO2 and PO2

Back 6

alveolar PCO2=VCO2/VA
alveolar PO2=VA/VO2
(ex. if PCO2 is halved and CO2 production stays the same, then VA has doubled)

Front 7

What is the respiratory exchange ratio?

Back 7

respiratory exchange ratio (R)=pulmonary CO2 elimination rate (VCO2)/pulmonary O2 uptake rate (VO2)

Front 8

Describe the factors that affect the rate of diffusion in gas exchange

Back 8

1. Thickness of respiratory membrane-rate of diffusion inversely proportional to membrane thickness; edema fluid & fibrosis increase thickness
2. Surface area of respiratory membrane-decreases of surface area to ¼ normal impedes gas exchange significantly-emphysema, surgical removal of lung tissue
3. Diffusion coefficient (D)-solubility and molecular weight of gas determine D (CO2 faster than O2 which is faster than N)
4. Pressure difference across respiratory membrane-difference in partial pressures of gas in alveoli & pulmonary blood; measure of net tendency for gas molecules to move through the membrane down the pressure gradient (simple diffusion)

Front 9

Describe perfusion limitation vs diffusion limitation

Back 9

perfusion limitation-because N2O has very low affinity to Hgb, as soon as N2O enters blood capillaries the partial pressure of N2O rises rapidly; pressure gradient b/t the blood-gas barrier reaches equilibrium quickly
moving force of N2O is dependent on blood perfusion (Perfusion Limited)
diffusion limitation-because CO has extremely high affinity to Hgb, regardless how much CO enters blood capillaries, partial pressure of CO barely changes; pressure gradient b/t the blood-gas barrier hardly changes, and the moving force of CO is dependent on gas diffusion (Diffusion Limited)
O2 diffusion limitations-abnormal lungs, low alveolar PO2 (high altitude); diffusion limitation is most noted at exercise (more mild at rest)

Front 10

Describe diffusing capacity

Back 10

volume of a gas that will diffuse through respiratory membrane each minute for a pressure difference of 1 mmHg
Difficult to measure O2 and CO2 diffusing capacity directly
Carbon Monoxide (CO) method: Partial pressure of CO is measured in an alveolar gas sample
CO binds tightly to hemoglobin (blood partial pressure = Zero)
Pressure difference of CO = alveolar partial pressure
Diffusing capacity of CO is similar to O2

Front 11

Describe the diffusing capacity for O2

Back 11

A. oxygen
pressure difference * diffusing capacity = total quantity of O2 diffusing across the membrane per minute
Exercise increases pulmonary blood flow & alveolar ventilation; oxygenation of blood is increased
Diffusing capacity increases three-fold to max in exercise-caused by recruitment of capillary fields (increased surface area of blood for O2 to diffuse); better ventilation/perfusion match with blood

Front 12

Describe the diffusing capacity for CO2

Back 12

CO2 diffuses very rapidly through respiratory membrane; minimal concentration differences between blood & alveoli
difficult to measure CO2 diffusing capacity; estimates based on diffusion coefficient
Diffusing capacity of CO2 is 3 times higher in excerise conditions compared to resting conditions

Front 13

Describe the factors that influence diffusing capacity

Back 13

1. Exercise-increases Diffusing Capacity due to recruitment and distension of pulmonary capillaries & better matching of blood flow and ventilation
2. Body Position-From standing to sitting, then to supine position increases DL because of increase in pulmonary capillary volume and more even distribution of pulmonary blood flow
3. Body Size-Lung size=surface area=higher Diffusing Capacity
Pathology of air-blood barrier (increased thickness or decreased surface area) decreased diffusing capacity-decreased capillary volume and hemoglobin (COPD, anemia, fibrosis, pulmonary edema, pneumonia)