Gas transport in the Airway Flashcards
what has to be considered when calculating air PIO2 (partial pressure of inspired air)
1) total pressure of inhaled air = Pa
2) sum of partial pressures of each gas in air = PB
3) each gas makes up certain percentage (dry air)
21% O2
0.03% CO2
79% N2
4) air immediately saturated with water when enter airway so water vapor pressure subtract from barometricP = 47 Torr @ 37C
Define PaO2
Normal value at sea level
Normal value at Denver
partial pressure of arterial O2
partial pressure of O2 in blood dissolved or bound to Hb in equilibrium with O2 in air
PaO2 = 90-100 Torr
PaO2 Denver = 80-85 Torr
air becomes ___ when it enters airway
consequences on water vapor pressure
saturated
so must subtract water vapor pressure from barometric pressure
what is equation for air PIO2 (Dalton’s Law)
** MEMORIZE
PIO2 = (PB - PH2O) x FO2
PIO2 = (PB - 47 Torr) x 0.21
what is partial pressure of water vapor
what is PB typically
47 torr
720 at sea level
620 at Denver
does partial pressure of water vapor vary with altitudes
no constant
does PB vapor vary with altitudes
does PIO2 vary with altitude
YES
decr at higher altitudes
equation for PB
PB = PIO2 + PIN2 + PICO2 + PIH2O
calculate PIO2 if patient breathing 100%
Use FO2 = 100%
PB = 760 Torr
PIO2 = 713 Torr
a
a
define respiratory exchange ratio in words
ratio of amount of CO2 generated per amount of O2 consumed
equation for respiratory exchange ratio
R = V(CO2) / V(O2)
why is respiratory exchange ratio significant
exchange of CO2 for O2 NOT ALWAYS 1:1
because metabolic reactions consume more O2 compared to CO2
carbs: R = 1.1
fats: R = 0.7
avg diet: R = 0.8
what are typical R values for
carbs
fats
avg diet
carbs: R = 1.1
fats: R = 0.7
avg diet: R = 0.8
what is factored into the calculation of alveolar PAO2
1) exchange of O2 for CO2 in alveoli based on R
when R decr with heavy fat diet, how does PAO2 change
substantial decrease
based on PAO2 = PIO2 - (PACO2/R)
when R decr with heavy fat diet, how does N2 change
increase in N2 as opposed to O2
because more N2 in air (4x more than O2)
so deficit created by low levels in CO2
made up by N2
equation for PAO2
PAO2 = PIO2 - (PACO2/R)
normal values in Denver for PIO2 PACO2 R R (if on 100% O2)
PIO2 = 120 Torr
PACO2 = 40 Torr
R = 0.8
R (if on 100% O2) = 1
difference between PACO2 and PaCO2
PACO2 = alveolar
PaCO2 = artery
normal values at sea level for PIO2 PACO2 R R (if on 100% O2)
PAO2
PIO2 = 150 Torr
PACO2 = 40 Torr
R = 0.8
R (if on 100% O2) = 1
PAO2 = 100 Torr
if patient is breathing 100% O2, why is R = 1
if breathe 100% O2, no N2
so deficit in CO2 made up by O2 and R drops out of equation and
PAO2 = PIO2 - PACO2
2 steps in CO2 removal
1) diffusion of CO2 from pulm capillaries to alveoli; THEREFORE PULM CAPILLARY EQUILIBRIUM WITH ALVEOLI CO2
2) CO2 transport from alveoli to the outside air.
what is rate limiting step between two steps of CO2 removal
why?
transport of CO2 to outside air (ventilation)
because diffusion of CO2 from blood to alveoli = assumed near instantaneous (FAST)
what does rate limiting step of CO2 removal depend on?
what does it affect?
step = transport of CO2 to outside air
depends on:
1) alveolar ventilation
affects
1) PACO2
2) PaC=O2
what is significance of change in alveolar ventilation on rate limiting step in CO2 removal?
decr alveolar ventilation
decr CO2 removal
incr PACO2
but other step, diffusion from blood to alveoli near instant so PaCO2 (artery) equilibrate with PACO2 (alveoli)
PaCO2 and PACO2 assumed to be ____
why?
equal
because rapid diffusion of CO2 from blood to alveoli
what do PaCO2 and PACO2 depend on?
amount of CO2 being produced relative to alveolar ventilation rate
alveolar ventilation equation ***MEMORIZE
PACO2 = PaCO2 = (VCO2dot/VAdot) x k k = constant
VCO2 dot = Co2 production per minute
VAdot = alveolar ventilation per minute
what is ideal PaCO2
40 Torr
why do we care about having particular PaCO2 = 40 Torr
affects pH
define hypoventilation
Low VAdot and High PaCO2
Constant VCO2dot
–> incr PaCO2
normal value of PaCO2 in Denver with hypoventilation
> 40 Torr
Causes of hypoventilation
1) severe Obstructive diseases = because if mild-moderate obstructive disease, decr VAdot body has compensatory mechanism to maintain total ventilation to maintain normal PaCO2 and pH
2) restrictive diseases
3) metabolic alkalosis
4) CNS depression
define hyperventilation
High VAdot and Low PaCO2
Constant VCO2 dot
–> decr PaCO2
Causes of hyperventilation
1) high altitude - ventilating more to compensate for low enviorn O2, decr CO2
2) acute hypoxemia
3) metabolic acidosis
4) CNS stimulation
define hyperpnia
High VAdot and normal PaCO2
–> PaCO2 unchanged
Causes of hyperpnia
moderate exercise (incr CO2, incr VACO2dot, to match incr VAdot)
what is partial pressure of water vapor
what is PB typically
47 torr
720 at sea level
620 at Denver
does partial pressure of water vapor vary with altitudes
no constant
does PB vapor vary with altitudes
YES
decr at higher altitudes
difference between PAO2 and PaO2
A= alveoli
a = arterial
why does respiratory exchange occur
1) Air entry mainly N2 and O2
2) Air in alveolus also has N2 and O2 BUT ALSO HAS CO2 because pulm capillaries
feeding CO2 into alveolus
3) for total pressure of air in alveolus similar to air coming in, with incr CO2 in alveolus then less N2 and O2
4) therefore, alveolar O2 less than PIO2 because CO2 replaces and exchanges
define respiratory exchange ratio in words
ratio of amount of CO2 PRODUCED to amount of O2 CONSUMED in metabolic reactions
equation for respiratory exchange ratio
R = V(CO2) / V(O2)
why is respiratory exchange ratio significant
*** memorize R for normal diet
exchange of CO2 for O2 NOT ALWAYS 1:1
because metabolic reactions consume more O2 compared to CO2
avg diet: R = 0.8
what are typical R values for
carbs
fats
avg diet
carbs: R = 1.0
fats: R = 0.7
avg diet: R = 0.8
what is factored into the calculation of alveolar PAO2
1) exchange of O2 for CO2 in alveoli based on R
equation for PAO2 (alveolar GAS EQUATION)
** MEMORIZE
PAO2 = PIO2 - (PACO2/R)
therefore, PAO2 less than PACO2 because more CO2 in alveolus
normal values in Denver for PIO2 PACO2 R R (if on 100% O2)
PIO2 = 120 Torr
PACO2 = 40 Torr
R = 0.8
R (if on 100% O2) = 1
difference between PACO2 and PaCO2
PACO2 = alveolar
PaCO2 = artery
PaCO2 and PACO2 assumed to be ____
why?
equal
because rapid diffusion of CO2 from blood to alveoli
what do PaCO2 and PACO2 depend on?
amount of CO2 being produced relative to alveolar ventilation rate
Path for O2 transport
1) O2 entry into lungs
2) entry into pulm capillaries
3) pulm capillaries diffuse into arterial blood
Is PaCO2 inversely related to alveolar ventilation?
if 50% drop in scoop-out rate or 50% decr in CO2 concentration, then you reach new equilibrium where CO2 concentration is doubled (50% drop in scoop out)
so now pull out 2 CO2 molec per scoop
What is PaCO2 if VAdot decr by 50%
PaCO2 = (VCO2dot) / (VAdot) x k
PaCO2 will double
General solution for change in VAdot
PaCO2(new) / PaCO2 (old) = VAdot (old) / VAdot (new)
why can we substituTe PACO2 for PaCO2
amount of CO2 essentially same because diffusion is RAPID
DECR IN PAO2 WITH CHANGE IN VENTILATION
Blood CO2 is ___ regulated by alveolar ventilation
directly
___ is directly regulated by alveolar ventilation
Blood CO2
Blood O2 is ___ regulated by alveolar ventilation via its effects on alveolar CO2
indirectly
___ is indirectly regulated by alveolar ventilation via its effects on alveolar CO2
Blood O2
