Gas Transport Flashcards
What is P?
partial pressure (kPa or mmHg)
What is F?
fraction (% or decimal)
What is S?
Hb saturation (%)
What is Dalton’s Law?
pressure of a gas mixture is equal to the sum of the partial pressures of gases in that mixture
(N, O then other)
Pgas mixture = ΣPgas1 + Pgas2 + … + Pgasn
What is Ficks Law?
molecules diffuse from high [] region to low [] at rate proportional to [] gradient (P1-P2), the exchange SA (A), diffusion capacity of the gas (D) and inversely proportional to the thickness of the exchange surface (T)
e.g. movement from alveolar space to blood
Vgas = A X D X (P1-P2) / T
What is Henry’s Law?
at constant T, [] of gas that dissolves in given type/V of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid and solubility of the gas
CDgas = agas x Pgas
What is Boyle’s Law?
at constant T, V of gas is inversely proportional to the pressure of the gas
P proportional to 1/Vgas
What is Charles’ Law?
at constant P, volume of gas proportional to T
Vgas proportional to Tgas
What is the composition of the air that we breathe in?
N - 78%
O - 21%
Ar - 0.9%
CO2 - 0.04%
oxygen therapy - increased O2
smoke (house fire) - increased CO2, CO
high altitude - lower barometric pressure, reduced oxygen intake (same proportion)
How do we modify inspiratory gases?
4 steps
warmed, humidified, slowed and mixed passing down the respiratory tree .
dry air at sea level
(PO2 = 21kPa, PCO2 = 0kPa)
conducting airways
(PO2 reduced slightly due to mixing - 20)
PH2O = 6.3 kPa
respiratory airways
PO2 = 13.5
PCO2 = 5.3
PH2O = 6.3
greater mixing effect - O2 diluted
CO2 higher as moving out of blood to be cleared
saturated with water to facilitate gas exchange
Why is gas slowed down by generation 23?
to facilitate gas exchange by increased cross sectional cumulative area at each generation
How to find total O2 delivery at rest hypothetically?
CDgas = agas x pgas
= 0.32mL/dL diffused across alveoli
CO is 5L/min therefore approx total oxygen delivery is 16mL/min (VO2) at rest
How is O2 delivered in reality?
but resting VO2 is 250mL/min so cannot rely on oxygen alone to deliver O2 to tissues
use haemoglobin
What haemoglobin?
monomers with ferrous iron (Fe2+ haem) at centre of tetrapyrole porphyrin ring that is connected to protein chain globin
covalently bonded at proximal histamine molecule
all haemoglobin has 2 alpha monomers
HbA?
Hb alpha and beta
HbA2?
Hb alpha and delta
HbF?
Hb alpha and gamma
How does haemoglobin work?
- Low O2 affinity initially when no O2 bound
- Binding of O2 increases oxygen affinity of Hb via COOPERATIVE BINDING
4th subunit has increased affinity for O2 of x3000 - Increased affinity relaxed state opens extra binding site for 2,3-DPG
- 2,3-DPG binding pushes relaxed Hb into a tense state by causing O2 to be ejected
What behaviour does Hb exhibit?
allosteric behaviour
= binding of oxygen to one of the subunits is affected by its interactions with the other subunits causing structural changes
These cause increase affinity of Hb for O2 - COOPERATIVE BINDING
What is methaemoglobin?
MetHb is 0.5-1% of haemoglobin at one point
- constant flux between MetHb and Hb
Fe3+ instead
does not bind O2
What does Hb do to the skin?
provides us with colour
Describe foetal haemoglobin?
higher affinity for O2 than adult Hb
O2 dissociation curve it has a left shift
- greater affinity for O2
- lower partial pressure of O2 required to generate 50% Hb saturation
What is given in surgical scenario if MetHb too high
Methylene blue -
What is the intrinsic enzyme to reduce MetHb?
MetHb reductase -
Why are linear O2 dissociation curves no good?
too great variability in binding in lungs
small binding range systemically to offload (no sufficient access to reserve O2)
How to track changes in oxygen dissociation curves?
P50
Find partial pressure of O2 at 50% saturation
This is assuming normal Hb [] of 150g/L
What causes right shift?
increased temp
acidosis
increase 2,3-DPG
hypercapnia
all lead to decrease affinity for O2 –> at given partial pressure less O2 bound to Hb
What causes left shit?
decreased T
alkalosis
decreased 2,3-DPG
hypocapnia
increased affinity (loading)
What causes upwards shift?
polycythaemia - tumour secreting erythropoietin - increase [] RBCs in blood increased O2 carrying capacity saturation same
What causes downwards shift?
anaemia
impaired oxygen carrying capacity
less O2 in blood
saturation same!
What causes downwards and leftwards shift?
increase HbCO
decreased capacity for release, increased affinity (cannot release O2 already bound)
Describe myoglobin O2 dissociation curve?
steeper - provide O2 for early stages of exercise
increased affinity to store O2
Myoglobin has an increased affinity than adult HbA to extract O2 from circulating blood and store it
Saturation of Hb in capillary after returning to lungs? What is the PO2 in capillary and alveolar space?
75% PO2 capillary = 5.3kPa lung = 13.5kPa HbO2 = 15ml/dl dissolved O2 lower (0.14)
How is oxygen transported in the lungs?
O2 moved to capillary along [] gradient till plasma PO2 = 13.5kPa
Some binds to Hb so SO2 (saturation) is 100% but some dissolved (0.34)
HbO2 = 20.1 ml/dl
Why does blood arriving at tissues have lower PO2 than in lungs?
lung tissue has 2 circulations
- some bronchial drainage from bronchial circulation drains into pulmonary veins to provide haemodilution
How do values change at the tissues?
PO2 decreased - 12.7
SO2 slightly decreased
HbO2 - 20 (down slightly)
dissolved O2 content slightly decreased
What does sigmoidal curve mean?
despite large decrease in PO2 entering and leaving tissues
the saturation of O2 decreased by a much smaller %
What is oxygen flux?
difference between the total dissolved and Hb bound O2 entering and leaving the tissues
(unloaded O2)
= HbO2 leaving - HbO2 entering (negative _ mL/dL)
What are the standard values for oxygen flux?
delta = -5mL/dL
multiplied by cardiac output (5L/min) (x50)
gives -250ml O2/ min
this is the resting VO2 as well as oxygen flux
What does [CO2] equal?
[H+]
What is tissue PCO2?
6.3 kPa
Where is CO2 converted to H2CO3?
in RBC
by carbonic anhydrase
How is HCO3- removed from RBC?
by AE1 transporter
via chloride shift
to maintain RMP of RBC
How is CO2 transported in the blood?
What is major form?
- solution
- as bicarbonate
- bound to Hb
AS BICARBONATE
Where does CO2 bind on Hb?
to amine end of globin chains
4 CO2 / Hb
to form carbaminohaemoglobin
How is increased in RBC H+ addressed?
negatively charged amino acids on globin chain of Hb (especially histadine) bind to protons to avoid lowering pH dramatically
What are the significant changes in PCO2 across tissue?
less significant increase in PCO2 compared to decrease in PO2
not sigmoidal shape as O2 dissociation curve
What is the CO2 flux?
difference between total dissolved/bicarbonate CO2 (leaving - entering)
52 - 48 = +4 mL/dL
+200mL CO2 / min
Overall consumption of O2 and CO2?
200 ml of CO2 released for 250 ml of O2 used / min
What is the Haldane effect?
oxygenation of blood in the lungs displaces carbon dioxide from haemoglobin which increases the removal of carbon dioxide
oxygenated blood has a reduced affinity for carbon dioxide
when Hb bound 4 O2 (100% saturated) will not bind any CO2
What is the pulmonary transit time?
amount of time that blood is in contact with the respiratory exchange surface
0.75s / erythrocyte
time for PO2 to equilibrate between alveolus and plasma
gas exchange time (0.25s for PCO2 to equilibrate between tissue and plasma)
What happens if CO increases?
more pulmonary capillary beds recruited
if very intense exercise, blood flows through capillary beds faster
Explain the regional differences in ventilation in the lung?
- Airways at top of lung more stretched, larger and less compliant due to reduced gravity effect
- Ppl more negative (-8cmH2O) so greater transmural pressure gradient
LESS VENTILATION - greater pressure required to inflate alveoli more
-At bottom alveoli are smaller and more compliant
- Smaller gradient (Ppl is less negative at -2cmH2O)
MORE VENTILATION
Explain regional differences in perfusion of lung?
TOP Lower intravascular pressure (gravity effect) less recruitment more resistance lower flow
BOTTOM Higher intravascular pressure more recruitment less resistance higher flow rate
Are the difference in ventilation and perfusion the same? Why?
No
Greater impact on perfusion than ventilation
Blood flow is denser and so more susceptible to effects of gravity
What is V/Q?
ventilation-perfusion ratio
at base tends toward 0, at apex tends towards infinity
at base there is wasted perfusion because there not as much ventilation, at apex there is wasted ventilation because there is not as much perfusion
IDEAL - where lines cross
