Oxygen and Carbon dixoide in the blood Flashcards
What is the solubility factor for O2?
0.01mmol.L-1.kPa-1
[O2} dissoled = _____ X ____
[O2] dissolved= solubility X pO2
= 0.01 x 13.3
What is the pO2?
13.3kPa
How much O2 do we need at rest?
12mmol 02 per min
So we would need a CO of 92l.min-1 to meet needs.
What is the CO at rest and max?
5l. min-1 at rest
25l. min-1 max
List requirements of Oxygen binding
The rxn needs to be reversible
The oxygen must dissociate at the tissues to supply them
What are the two oxygen binding pigments?
Heamoglobin - present in blood. Tetramer (cooperative binding= sigmoid curve)
Myoglobin- present in muscle cells. Monomer.
List details of myoglobin
Present in muscle cells
contain haem
only a single sub-unit
Dissociation curve (saturation vs partial pressure kPa) not sigmoid
saturates because the amount of pigment is limited
List details of Hb
4 sub units- Tetramer
Cooperative binding- T state (low affinity for O2) –> r state (high affinity for O2) as oxygen binds increase affinty. High conc of O2 = high affinity for O2, low conc of O2 = low affinity for O2 therefore release at muscles
Dissociation curve is sigmoid
Hb saturated above 9-10Kpa, virtually unsaturated below 1 kpa
What is normal Hb conc?
~2.2mmol.L-1
If normal Hb conc. is 2.2mmol.l-1 then what is the O2 concentration?
2.2mmol.l-1 X 4 (as Hb binds 4 O2)
= 8.8 mmol.l-1
how much O2 is given up at the tissues?
depends on how active tissue is
if 35% is given up
8.8 X 0.35 = 3mmol.1-1
Desrcibe the Bohr shift?
pH affects Hb Decrease in pH promotes T-sate Hb Increase promotes R state CO2 and H+ released as products of metabolism so Hb release O2 at metabolising tisues. curve shifts to the right
Describe the difference in capillary density in very metabolically tissue compared to less metabolically active tissues and why?
Higher capillary density in very metabolic tissues, as the higher the density the lower the pO2 can fall, the lower the pO2 the lower the Hb affinity for O2, therefore, more O2 is released at tissues
Describe the affect of increased temperature on the dissociation curve and why?
Increase in temperature shifts the curve to the right.
Metabolically active tissues have a higher temperature, O2 is released.
Increase in temperature denatures/weakens bond between O2 and Hb
Describe maximum unloading of O2?
Maximum unloading occurs in tissues where pO2 can fall to a low level, also in tissues wih increased metabolic activity (low pH, high temp)
under these conditions about 70% of bound O2 can be given up
Why is there an oxygen reserve?
How much of the oxygen from arterial blood is given up/
So oxygen deposition can be increased during exercise
27%
What is 2,3 BPG?
2,3 BPG levels increase with anaemia or at high altitude
Increased 2,3 BPG shifts Hb dissociation curve to the right
allowing more O2 to be given up at tissues
(also, 2,3 BPG binds to B sub unit of hb, therefore does not bind to fetal Hb (y) and therefore fetal Hb has a higher affinity for O2 in the presence of 2,3 BPG compared to maternal Hb)
CO monoxide poisoning and Hb
CO reacts with Hb to form CO Hb
Fatal if COHb is about 50%
Hb has higher affinity for CO
Also increases affinity of unaffected subunits for O2 and therefore wont deposit O2 at tissues
Describe cyanosis.
Blue coloration due to unsaturated Hb
Deoxygenated Hb is less red than oxygenated
Can be:
- Peripheral due to poor circulation
- Central due to poorly saturated blood in systemic circulation
What is pulse oximetry and what does it detect?
Detects level of Hb saturation (by detecting the difference in absorption of light between oxygenated and deoxygenated Hb)
Only detects pulsatile arterial blood (ignores levels in tissues and non-pulsatile venous blood)
Does NOT say how much Hb is present
Describe the chemical properties of CO2
CO2 is more soluble than O2
reacts with water
also reacts with Hb, but at a different site to O2
How much more CO2 does arterial blood have than O2?
2.5x as much
more dissolved and more reacted with water (HCO3-)
What is the total content of CO2 in arterial blood?
21 mmol.l-1
CO2 in arterial blood is NOT there as a waste product
What is the arterial blood pH range?
7.35-7.45
CO2 has a major role in controlling blood pH
[CO2] dissolved = ______ X _____
[CO2} dissolved = solubility X pCO2
= 0.23 X 5.3 Kpa =
Discuss CO2 in plasma.
CO2 reacts with the water in plasma:
CO2 + H2O H2CO3 H+ + HCO3-
Carbonic acid (H2CO3) quickly dissociates into H+ and HCO3-
Reversible reaction
Rate depends on the reactants and products
What does the pH of plasma depend on?
How much CO2 reacts to form H+
[CO2} dissolved pushes the reaction to the right (i.e. increase in H+)
Also depends on conc. of HCO3- (pushes reaction to the left
What does the amount of dissolved CO2 depend on?
Depends directly on the Partial pressure of CO2
If pCO2 rises plasma pH falls (increase in CO2 –> increase in H+)
if pCO2 falls plasma pH increases
pCO2 of alveoli is the determining factor and this is controlled by controlling rate of breathing
How is the pCO2 of the alveoli determined and what does this affect?
controlled by controlling the rate of breathing
The pCO2 of the alveoli affects the amount of dissolved CO2 which in turn affects plasma pH
What is the concentration of HCO3- in the plasma?
Why is this important?
25mmol.l-1
high [HCO3-] prevents nearly all dissoled CO2 from reacting (pushes reaction to the left)
maintains the alkaline pH of plamsa
What is the cation mostly associated with HCO3-?
Na+
not enough C02 to have concentration of 25mmo.l-1
What is the henderson-Hasselbach equation?
pH= pK + Log ([HCO3-]/ (pCO2 X 0.23)
pK= 6.1
Use the henderson-hasslebach equation to calculate the pH of plamsa.
pH= pK + Log ([HCO3-]/ (pCO2 X 0.23)
pK= 6.1
HCO3- = 25mmol.l-1
[CO2} dissolved = solubility X pCO2 (5.3 X 0.23) = 1.219
25/1.22 = log 20 = 1.3
there is 20 X as much HCO3- as there is CO2 dissolved
pH= 6.1 + 1.3
= 7.4
How is the high [HCO3-] established?
In the RBC:
CO2 + H2O H+ + HCO3-
CA increases R of Rxn
The H+ is then bound to negatively charged Hb inside the RBC
The HCO3- is then transported out of the RBC by the chloride bicarbonate exchanger
(The RBCs control the production of HCO3- but not the concentration of HCO3- in the plasma, that is controlled by H+ binding to Hb)
How do the kidneys control [HCO3-]?
by varying excretion
therefor pH is dependent on both how much CO2 (so rate of breathing) and how much bicarbonate is present (controlled by kidneys)
How does [HCO3] buffer extra acids produced
e.g lactic acid?
Acids react with the HCO3- to produce CO2
therefore [HCO3-] decreases
CO2 is removed by breathing and pH changes are minimised
Why is pCO2 higher in venous blood?
Comes from metabolically active tissues
Describe how the buffering of H+ by Hb depends on level of oxygenation?
H+ ions re always bound to Hb but the amount depends on state of Hb molecule
If in T-state more H+ ions bind as less oxygen (at tissues)
In R state less H+ ions bid as more O2 bound (at lungs)
How does the binding of H+ to Hb affect the CO2 in the plasma and therefore transport of CO2?
If Hb binds more H+ in the RBC then more HCO3- can be produced
Therefore more CO2 is present in the plasma (both in dissolved and reacted from)
Consider H+ and Hb in venous blood.
In venous blood there is less oxygen present, therefore more Hb in the T state, so H+ can bind.
This allows more HCO3- produced from RBCs
Therefore more CO2 can be converted to HCO3-
Dissolved CO2 increases a little
Very small change in plamsa pH because both [HCO3-] and pCO2 have increased
What happens when venous blood arrives at the lungs?
Hb picks up O2 and goes into Rstate
This results in the release of H+
H+ reacts with HCO3 to form H2O and CO2
CO2 is breathed out
Discuss the formation of Carbamino compounds
CO2 can bind directly to proteins, binds directly to amine groups on globin of Hb
binding of molecular CO2 onto Hb is not part of the acid base balance but contributes to CO2 trasnport
More carbamino compounds are formed at the tissues because pCO2 is higher and unloading of O2 by Hb facilitates binding of CO2
This is CO2 given up at the lungs
What do Carbamino compounds contribute to?
CO2 transport but NOT acid base balance
What 3 forms is CO2 transported in?
8% Dissolved CO2
12% Carbamino coumpounds
80% HCO3
Describe the buffering actions of Hb
Hb ‘mops up’ the H+ produced in the reaction of CO2 + H20 –> H2CO3 –> H+ + HCO3- (carbonic anhydrase)
in RBC
Rapid
If only 8% of the total CO2 is transported what is the rest of the CO2 doing?
Part of the pH buffering system
Why can hyperventilation significantly reduce the CO2 content in the blood but cannot significantly increase the O2 content?
The CO2 dissociation curve has a relatively straight slope, with a linear relationship between the pCO2 and the content of CO2 in the blood.
Therefore as the alveolar pCO2 drops, correspondingly more CO2 is given up at the pulmonary capillaries
O2: Hb becomes saturated, reaches a plateau
In venous blood there is slightly more _____?
Carbamino compounds
What is the concentration of CO2 in venous blood compared to arterial blood?
- 5 mmol.L (venous)
21. 5mmol.L (arterial)
What is the proportion of transported CO2?
Total in venous blood – total in arterial blood
= 23.3- 21.5mmol.l-1
= 1.8mmol.l-1
Therefore only 8% of total is transported
In plasma dissolved CO2 undergoes a reaction with water to produce Bicarbonate ions, why (compared to RBC) is this reaction slow?
Low presence of carbonic anhydrase to catalyse the reaction
