Lecture 18 resp physio O2 and CO2 exchange Flashcards
How do you Quantify individual atmospheric gas?
Partial Pressure
whats Daltons law?
Partial Pressure = fraction of individual gas x total gas pressure
Pgas= Fgas x Ptotal
For atmospheric air Ptotal = Pbarometric
(~760 mmHg) at sea level
For atmospheric air Ptotal =
Pbarometric
(~760 mmHg) at sea level
Dalton’s law of partial pressures
What is the partial pressure of O2, CO2 and N2 at sea level?
O2 = 20.93 % FIO2= 0.2093 CO2 = 0.04 % FICO2 = 0.0004 N2 = 78.6 % FIN2 = 0.786
Barometric pressure PB at sea level today = 760 mmHg.
P = F x PB
PO2 = 159 mmHg PCO2 = 0.3 mmHg PN2 = 597 mmHg
How do we exchange the gas?
Gases move across the alveolar capillary membrane by Diffusion (down pressure gradients)
what does Fick’s Law of Diffusion explain?
gas exchange through the membranes
F = (A / T) x D x (P1-P2)
Where: F = Flux (Amount flowing)
A = surface area
T = thickness
D = Diffusion constant
P1 - P2 = pressure difference
F = (A / T) x D x (P1-P2)
F = Flux (amount flowing) A = surface area T = thickness D = Diffusion constant P1 - P2 = pressure difference
what does the diffusion constant for substance depend on?
gas solubility (S) and its molecular weight (Grahams law).
on a per molecule basis, CO2 diffuses about 20x faster than O2 due to CO2 higher solubility
whats emphysema disease?
dilation of alveolar spaces and destruction of alveolar walls.
decrease SA
decrease PO2 in blood
F = (A / T) x D x (P1-P2)
whats the T?
Blood-gas barrier in lung is large, the thickness is 0.3μm in many places
whats pulmonary fibrosis?
thickening and scarring of alveolar membranes - arise from chronic inflammation, drugs, industrial chemicals.
increase thickness
decrease PO2 in blood
what does gas transfer by diffusion depend on?
- alveolar area for diffusion (emphysema)
- thickness of alveolar membrane (fibrosis)
- pressure difference drives diffusion
alveolar - arterial
PAO2 vs PaO2
PACO2 vs PaCO2
Alveolar partial pressure of O2 (PAO2)
~100 mmHg –
reduced from PB (159 mmHg) by water vapour and CO2 in alveoli
what does PAO2 depends on?
- Partial pressure of oxygen in inspired air - PIO2
- Alveolar ventilation - VA
- Oxygen consumption - VO2
atmospheric PO2 is usually constant, so it is the balance between _____ and _____ that is most important
oxygen consumption
alveolar ventilation
