Gas Exchange Flashcards
What do partial pressures of O2 and CO2 at end pulmonary capillaries (c’) reflect?
their alveolar (A) counterparts, and determine partial pressure of these gases in arterial blood (a)
What does gas exchange result in?
end capillary blood gases (Pc’O2 and Pc’CO2) that are in equilibrium with partial pressure of gases in alveoli (PAO2 and PACO2)
Where does arterial blood go after gas exchange?
courses through pulmonary vein and is pumped to left side of heart, where it is then pumped to tissues where gas exchange can take place at level of tissue
What will end capillary blood ultimately be composed of?
partial pressure of gases in alveoli
Ventilation/Perfusion Match
Ideal Lung (V/Q = 1)
matching rate at which alveolus gets ventilated and rate by which blood goes past it, ends up with same PO2 and PCO2 as air in alveoli
Ventilation/Perfusion Match
Shunt-like (V/Q = 0)
(obstruction in airway to alveoli)
partial pressures in incoming gas is mixed venous in nature from tissues
- no ventilation, but blood passes
- mixed venous blood will equilibrate with gas in alveoli with time
- since nothing fresh is coming in, essentially it will leave in the same nature, and whatever air is in alveoli will equilibrate with end capillary
Ventilation/Perfusion Match
Dead Space (V/Q = infinity)
(complete obstruction in capillary) – if only partially blocked, V/Q value would vary
- no perfusion, but very ventilated alveoli with relatively high O2 and low CO2 compared to normal
- blood does not see it because blood can’t pass (blocked)
What is air composed of?
mixture of gases – each gas exerts partial pressure (Pgas), which is pressure it would exert if it alone occupied a given volume
total atmospheric pressure = 760 mmHg
nitrogen: 79%, PN2 = 600 mmHg
oxygen: 21%, PO2 = 160 mmHg
argon: 0.93%
carbon dioxide: 0.03%
neon methane helium krypton hydrogen xenon
What is atmospheric/barometric pressure?
total pressure exerted by this mixture of gases = 760 mmHg at sea level
How do you determine the partial pressure of a gas?
Pgas = PB x FIgas
where B = barometric
where I = inspired
Determining Partial Pressure of Gas in Mixture of Gases and Water Vapour
see notes
What does partial pressure reflect?
% of air that is not water vapour
What happens to partial pressure of oxygen as inspired air courses through airways?
decreases
conducting airways
- heated and humidified
- PO2 decreases due to subtraction of water vapour introduced
alveoli
- mixed in reservoir of air at FRC
- PO2 decreases further
- PCO2 is introduced – alveolus is seeing blood, and is being influenced by it, and CO2 from blood and enters alveolus
At equilibrium, what is the tension of gas in solution (equal pressure in solution)?
= partial pressure of gas in air pocket above it (equal pressure in gaseous phase)
What do gases dissolve, diffuse, and react according to?
their partial pressure – NOT concentration
- concentration of gas in solution is not same as partial pressure
- solubility of gas affects its concentration – gases dissolve differently in different solutions
What happens to the partial pressure gradient of O2 as O2 moves into blood from alveoli?
gradient gets smaller until PO2 and partial pressure in alveoli are the same
Fick’s Law of Diffusion
What are the factors that impact diffusion of gases across barriers?
Vgas = flow of glass across barrier
D = ‘diffusivity’ of ‘diffusion coefficient’
- individualistic to that gas
- proportional to (solubility / √ MW)
ΔP = pressure gradient
- more diffusion if larger
A = surface area of barrier
- more diffusion if larger
T = thickness of barrier
- more diffusion of thinner
Fick’s Law of Diffusion
How is both O2 and CO2 able to reach equilibrium between air in lungs and blood in pulmonary capillary (complete gas exchange) in a similar time frame?
CO2 has greater diffusivity, but less driving pressure
O2 has lower diffusivity, but greater driving pressure
What is pulmonary fibrosis?
scar formation, which decreases lung compliance and increases thickness
What is pneumonia?
inflammatory fluid/pus accumulation in and around alveoli that causes increase in thickness
What conditions can result in decrease in surface area?
- emphysema
- atelectasis in NRDS/ARDS
What conditions can result in increase in surface area?
exercise
What conditions can result in increase in thickness?
- pulmonary fibrosis
- pneumonia
- pulmonary edema
What is pulmonary edema?
abnormal buildup of fluid in interstitial or alveolar space
result of direct damage to lung tissue, or inability of heart to pump blood (left heart failure)
How long does gas exchange take to complete?
within ¼ second
What is the normal pulmonary transit time (PTT) during a normal quiet breath?
under 1 second
Gas Exchange Across Pulmonary Capillary
At Rest
¾ PTT > ¼ second required to complete gas exchange
plenty of extra time
Gas Exchange Across Pulmonary Capillary
In Exercise
PTT > ¼ sec required to complete gas exchange
- despite ↑ cardiac output (faster blood)
- PTT can be faster, as long as it stays for minimum ¼ sec for equilibrium to occur
Gas Exchange Across Pulmonary Capillary
In Pulmonary Fibrosis
reduced gas exchange can occur with exercise
- additional time spent in capillary at rest is often sufficient to compensate for thickened barrier
- although they have enough time for gas exchange to happen, if thickness of their alveoli was normal + add additional thickness, when they exercise, the extra time available is not sufficient due to thickened barrier and faster blood while exercising
Gas Exchange Across Pulmonary Capillary
In Elite Athletes
PTT < ¼ sec during intense exercise
- due to large ↑ cardiac output
- ## inadequate O2 exchange at lungs → hypoxemia, low arterial PO2
athletes trained well to reach high CO to get blood to tissues that need it and get O2, BUT CO is so fast that blood that needs to exchange at level of lungs to bring in O2 is not doing adequate job
- desaturation of O2
- only occurring during serious exercise, not when at rest
