Diffusion and perfusion Flashcards
Define solubility coefficient
tendency of any molecule to dissolve in a liquid
what is the variable for O2 saturation coefficient
alphaO2
what is alphaO2 at 37C in isotonic NaCl solution (approximates blood)
alphaO2 = 0.0013 mM/Torr
how do you calculate dissolved [O2] in blood
what is typical value at a typical arterial PO2 of 100 Torr
[O2] = alphaO2 x PO2
[O2] = 0.13 mM at PO2 = 100 Torr
why is so little O2 carried in freely dissolved form in blood ([O2] in blood only = 0.13 mM) (2 reasons)
1) O2 doesn’t dissolve well in blood
2) O2 binds quickly to Hb and taken out of freely dissolved form rapidly
what is alphaCO2 typically
alphaCO2 = 0.03 mM/Torr
why is alphaCO2 > alphaO2
1) CO2 = 20x more soluble in blood than O2
CO2 dissolves more readily than O2
what does oxy-hemoglobin dissociation curve reflect?
what is SaO2 at PaO2 = 100 Torr?
ratio of O2 saturation of Hb (SO2) at equilibrium to PO2 in blood
ratio of 9 mM oxy-hemoglobin to 9.1 mM artial O2 content
@ PaO2 = 100 Torr
SaO2 = 98%
For PO2 = 100 Torr (normal arterial O2 sat), SO2 = ___
SO2 = 97.5%
so in arterial blood, Hb is close to full saturation
any further incr in PO2 would not incr amount of O2 carried by Hb
what is mixed venous blood
what is PO2 of mixed venous blood
what is SO2 of mixed venous blood
what is PO2 of mixed venous blood at half saturation
what is SO2 of mixed venous blood at half saturation
mixed venous blood = blood from RV
PO2 = 40 Torr
SO2 = 75%
PO2 at half saturation = 26 Torr
SO2 at half saturation = 50%
why is oxy Hb curve steep between 20 to 60 Torr
curve is steep at moderate PO2
because O2 binding to Hb is a “cooperative” process
what is significance of steep drop in SO2 with decrPO2
allows peripheral tissues that see relatively unsaturated Hb to withdraw larger amounts of O2 for small drops in capillary PO2
what are criteria during which standard Oxy-deoxy Hb curve functions under
pH = 7.40
PCO2 = 40 Torr
Temp = 37C
[2,3 DPG] = 15 umoles/g Hb
situations that can lead to reduced PO2 levels
why?
situations that could prevent reaching 100 Torr
severe disease or moderate disease + exercise
O2 impedes diffusion slightly, O2 needs to be in dissolved state –> from alveolus to interstitial space –> blood, poor solubility slows this process and makes O2 diffusion susceptible to disease
how does moderate disease and exercise affect PO2 as you pass through capillary bed
why?
if you have moderate disease or exercise that impedes diffusion, then curve of PO2 vs. % capillary bed passed shifts down, still able to reach 100% PO2
with exercise, blood flows more quickly through pulm circulation
___
why? O2 impedes diffusion slightly, O2 needs to be in dissolved state –> from alveolus to interstitial space –> blood, poor solubility slows this process and makes O2 diffusion susceptible to disease
how does severe disease (DIFFUSION DISORDERS) affect CO2 levels in blood?
SAME OR DIFFERENT FROM O2
WHY?
severe disease DO NOT affect CO2 levels in blood
STILL REACH 40 TORR
–> DIFFERENT FROM O2 CURVE
–> BECAUSE DIFFERENCE IN SOLUBILITY
CO2 passes READILY from blood to interstitium to alveolar space without trouble so diffusion disorders don’t affect CO2 diffusion sufficiently
Factors that promote rapid gas transfer across an arbitrary tissue plane (O2 diffusion from alveoli and pulm capillaries)
1) difference in partial pressure of gas on two sides
2) tissue plane area (A)
3) tissue thickness (d)
4) constant k reflects tissue solubility and molecular weight of gas
equation for gas transfer rate (VG) = Flux gas
what is the equation called
Flux gas = VG = (P1-P2) x (A/d) x k (Fick’s Law)
simplify equation for VG (Fick’s Law) to solve for diffusing capacity Dm
Dm = (A/d) x k
what does large surface area (A) refer to?
one adult lung = ____ alveoli
surface area of lung = ____
refer to alveolar membrane
300 million alveoli
50-100 m2
what is thin membrane width (d) of alveolar membrane
thin membrane ___ diffusion
0.3 um
maximizes
what exists between alveoli and capillaries
PAO2- PcapO2
any O2 that transfers from alveolus to capillary keeps free O2 levels low and maintain pressure gradient for diffusion from alveolar air into blood
what ensures that PcapO2 remains low and the gradient (PAO2 - PcapO2) large
1) low solubility of O2 in blood
2) tendency of O2 to bind quickly to Hb
If PO2 reaches alveolar values in 1/3 of the time it spends in the pulmonary circulation, then why have all of that extra time for diffusion to occur
Safety net for
1) exercise when faster blood flow reduces time diffusion occurs
2) disease when O2 and CO2 transfer slows
how does interstitial disease affect rate of diffusion
thickens alveolar wall (incr d),
slows rate of diffusion
how does emphysema affect rate of diffusion?
destroy alveolar bed –> decr surface area (A) for diffusion (decr A) –> decr rate of diffusion
how does polycythemia affect rate of diffusion
diffusion increases
how does anemia affect perfusion
anemia = perfusion decr
changes in Hb concentration exert ___ effects on tissue oxygenation
why?
much larger
because alters O2 delivery to tissue rather than through changes in O2 diffusion
variable for perfusion
define perfusion
perfusion = Q
perfusion = blood flow of pulm circulation available for gas exchange per minute
perfusion equals ____
what is value of Q (perfusion) at rest)
cardiac output
Q = 5L/min
why does pulmonary circulation have low blood pressure (2 reasons)
purpose?
1) vast number of vessels
2) normally dilated state
keeps work of right heart as small as possible for efficient gas exchange
what is PAO2?
alveolar O2 tension
significance of PAO2
most important factor in governing minute-to-minute regulation of pulm circulation
what is the mechanism of alveolar O2 tension
1) O2 diffuses through thin alveolar walls into smooth muscles of microvessels
2) in response to low arterial PO2 –> constricts nearby arterials = hypoxic pulm vasoconstriction
3) decr local blood flow –> shift to other regions of lung
define hypoxic pulm vasoconstriction
how does response to low O2 differ between lungs and peripheral circulation?
low alveolar PO2 constricts nearby arterioles
decr local blood flow –> divert O2 to other lung regions (that may not be diseased)
________
OPPOSITE OF LOW O2 IN REST OF CIRCULATION
IF LOW O2 IN LEG –> VASODILATION TO OFFSET low O2 in blood and provide O2 to leg
chemical agents affecting regulation of perfusion (2)
1) thromboxane = most important vasoconstrictor = localized to region of O2 release
2) prostacyclin = prostaglandin I2 = vasodilator
where does thromboxane exert its effect
regions of O2 release
how does capillary recruitment affect perfusion in exercise
1) moderate exercise = passive regulation
2) recruit new capillaries and distension of previoulsy open microvessels
3) incr up to 200 mL blood and incr CO 3x
incr in BP in pulm circulation due to incr CO with exercise, allows lungs to accomodate greater CO
effect of gravity on pulm blood pressure in lung
pulm blood presure low at apex of lung, high in base of lung
significance of pulm blood pressure higher at base of lung
allows more capillaries to open –> higher blood flow
perfusion incr from apex –> bottom of lung
what is V/Q ratio
significance?
ventilation/perfusion ratio
describes variability in ventilation and perfusion ratio through lung
define alveolar dead-space in lung
V/Q = ?
volume of lung that does not engage in gas exchange
ventilation = normal
NO PERFUSION
MOST EXTREME VERSION OF HIGH V/Q (V/Q = INFINITY)
define shunt
V/Q= = ?
volume of blood that does not engage in gas exchange
blood perfusion present
NO VENTILATION
most extreme version of low V/Q (V/Q = 0)
IS A SMALL amount of shunt normal?
why?
yes, because some venous blood enters LA and LV by way of bronchopulm venous anastamoses and intracardiac Thebsian veins (1-2%)
how does shunts affect arterial oxygenation? when does this happen?
DECR ARTERIAL oxygenation significantly when well-ventilated blood mixes with shunt blood
how do shunts affect level of PCO2
NO INCR IN PCO2
because incr countered by central chemoreceptors that incr ventilation with incr in PCO2
because PaCO2 inversely related to VAdot –> total ventilation remains normal (VAdot) so PaCO2 stays same
V/Q mismatch affects arterial O2 evels but NOT AFFECT CO2
why is arterial PCO2 lower than normal
because additional hypoxemic stimulus to ventilation
causes of V/Q mismatch (3)
1) resistance problem (OBSTRUCTIVE DISEASE) = partial occlusion of bronchiole (CAN BE MILD, MOD, OR SEVERE)–> differ from hypoventilation that requires severe disease because body wants to maintain normal CO2
2) Pulmonary embolism (blood flow diverted to other parts of circulation, incr Q, decr V/Q)
3) gravity–> creates different V/Q ratio due to difference in V and Q at top and bottom of lung
why does gravity lead to regional variations in ventilation/perfusion in upright person
what is the ratio of Vbottom/Vtop and Qbottom/Qtop
gravity has different effects on V/Q in different parts of lung
V/Q HIGH IN APEX
V/Q LOW IN BASE
Vbottom/Vtop = 2.5
Qbottom / Qtop = 6
(V/Qtop) / (V/Qbottom) = 4
blood pools in base of lung, incr BP, open up blood vessels, incr perfusion (Q) in lung
what is significance of gravity’s effect on regional variations in V/Q in lung
creates 5-10 Torr difference between arterial PaO2 and alveolar PAO2
PAO2 > PaO2
(V/Qtop) / (V/Qbottom) = 4
O2 is ___ soluble in blood
poorly because O2 forms poor chemical interactions with water molec
ARterial O2 Content = CaO2 = ___ + ___
typical values for all variables?
O2 bound to Hb and freely dissolved O2
CaO2 = 20.7 mL O2/100 mL blood = 20.4 mL O2/100 mL Hb-Bound + 0.3 mL/100 mL free
PaO2 only reflects which form of O2
reflects freely dissolved O2 ONLY not including Hb-bound
Hb-bound O2 is ___ related to PaO2 via the oxy deoxy Hb curve
indirectly
___ keeps delta P
O2 binding to Hb
Define perfusion
Define minute perfusion (Qdot)
perfusion = blood flow in the lung
minute perfusion (Qdot) = blood flow in lung per minute = cardiac ouptut because heart directly enters pulm circulation
Factors that affect perfusion
1) O2 tension (amount of O2 in blood vessel)
if portion of lung poorly ventilated (low O2 in bronchi –> blood vessel), then blood vesel constrict
2) Capillary recruitment
3) Gravity
V/Q mismatch examples
1) if have obstructive disease
2) if have high ventilation or partial occlusion of blood vessel
1) Obstructive disease that obstructs bronchiole, low ventilation –> low V/Q
2) Ventilation is high or (Partial occlusion of blood vessel, low perfusion)–> high V/Q
Problems in oxygenation can arise even if total ventilation and perfusion is normal
a
ideal V/Q ratio
V/Q = 1
approximated by
VAdot = 4.2L
Qdot = 5L
situation with low V/Q
introduce partial occlusion of bronchiole, decr ventilation
situation with high V/Q ratio
regions of High V/Q result when you have low V/Q
body is interested in maintaing a near normal PaCO2 to maintain normal PH
if we have blood entry with CVO2 = 15.7 mL O2/100 mL (mixed venous blood O2 level), what is CaO2 after passes through circulation?
small incr blood through portion with high V/Q (amount of incr is small relative to normal level of O2 in V/Q =1)
large deficit in O2 through portion of circuit with small V/Q (more than incr in branch with high V/Q)
because even at typical O2 level with normal ventilation, 98% O2 bound to Hb so few O2 binding sites available, LIMITING AMOUNT OF O2 THAT CAN BE ADDED TO BLOOD–> O2 SATURATION OF HB THAT LIMITS HOW MUCH O2 THE HIGH V/Q BRANCH CAN ADD
Diagram with V/Q mismatch and how that affects O2 and CO2
Causes of Shunts (2)
1) Lung defects (pneumonia)
2) Heart defects
Pneumonia causes V/Q = 0.0 (accum leukocytes)
a
