Gas transport in blood Flashcards
Define methaemoglobinaemia
◦ methaemoglobin is an altered state of Hb where ferrous ions (Fe2+) of haem are oxidised to the ferric state (Fe3+) and rendered unable to bind O2
◦ normal level is < 1.5%
What is the normal level of methaemoglobin
- Normal rate of autooxidation 0.5 - 3% of total Hb - due to oxygen acting as an oxidising agent where it becomes a superoxide radical (O2-) as it dissociates with Fe. It is normal that when Fe and O2 associate that iron is oxidised to its ferric form temporarily
What are the 3 divisions of causes of methaemoglobinaemia
Congenital enzyme deficiencies
Acquaired/toxins - indirect oxidants
Acquired direct oxidants
What acquired indirect oxidants cause methaemoglobinaemia
◦ Aromatic hydrocarbons - indirect oxidant of Hb
‣ aniline dyes
‣ benzene derivatives
◦ Sulfonamides - indirect oxidant of Hb
‣ Dapsone
‣ Bactrim
◦ Random antibioitcs - nitrofurantoin (indirect)
◦ Local anaesthetics - indirect
‣ Benzocaine
‣ Prilocaine
What direct oxidants cause methameoglobinaemia
◦ Methylene blue (direct oxidant)
◦ Nitrites (NO2-) - autocatalyst reaction where methaemoglobin catalyses the further oxidation of oxyhaemoglobin where nitrites accept 2 electrons (Lewis acid)
‣ NO
‣ Sodium nitrite
◦ Nitrates (NO3-) - reduced to nitrite by gut bacteria and therefore via above action also causes it
‣ GTN
‣ nitroprusside
◦ Antimalarials - rarely unless enzyme defects present
‣ chloroquine
How is methaemoglobin metabolised
How does methylene blue act to help methaemoglobin processing
What are the clinical features of methaemoglobinaemia
- cyanosis
- symptoms and signs of decreased oxygen delivery e.g. chest pain, dyspnea, altered metal state, end organ damage
- SpO2 reading 85-90%
- blood samples typically have a chocolate brown hue
- Normal PaO2
What factors may cause an incorrect high measurement of methaemoglobin
◦ Note that hyperlipidaemia via scattering can result in erroneous readings of high methaemoglobin due to the perceived absorbance being high; however it is purely scattering
◦ Additionally isosulfant blue or patent blue used for sentinal node biopsy also can cause spurious high measures
What is carboxyhaemoglobinaemia
CO - Hb
Why does carbon monoxide bind to haemoglobin instead of oxygen and how does it cause problems
◦ 210x affinity for Hb compared to oxygen - rate of binding 20% that of O2, and actively displaced O2
◦ Therefore renders haemoglobin oxygen carrying capacity and delivery to tissue reduced resulting in tissue hypoxia and ishcaemic injury
◦ It additionally interferes with cooperative binding of haemoglobin flattening out the oxyhaemoglobin dissocation curve so affinity for unaffected normal haem is increased, left shifts the curve and oxygen is not released to hypoxic tissues
◦ CO also beinds to intracellulra cytochromes impairing aerobic metabolism
◦ Triggers endothelial oxidative injury, lipid peroxidation and inflammatory cascade
Typical symptoms and concentrations of Carboxyhaemoglobin
◦ <10% (nil, commonly found in smokers) 3-10% common
◦ 10 – 20% (nil or vague nondescript symptoms)
◦ 30 – 40% (headache, tachycardia, confusion, weakness, nausea, vomiting, collapse)
◦ 50 – 60% (coma, convulsions, Cheyne-Stokes breathing, arrhythmias, ECG changes)
◦ 70 – 80% (circulatory and ventilatory failure, cardiac arrest, death)
What factors influence carboxyhaemoglobin absorption
◦ COHb concentration in blood is a function of CO contcentration in inspired air and itme of exposure
◦ Uptake increased by
‣ Decreased barometric pressure
‣ Increased activity
‣ Increased rate of ventilation
‣ High metabolic rate
‣ Anaemia
Distribution of carbon monoxide
Rapid
Metabolismm of carboxyhaemoglobin
<1% endogenously metabolised
Carboxyhaemoglobin excretion
◦ CO eliminated unchanged from the lungs in an exponential manner
◦ Biological half life in sedentary healthy adult 4-5 hours
◦ This half-life decreases with oxygen administration
◦ ~ 40–80 minutes with administration of 100% oxygen
◦ ~ 23 minutes with hyperbaric oxygen (2 atmospheres)
◦ elimination is affected by the factors as absorption (see above) and is likely faster in many CO poisoned patients due to compensatory measures (e.g. hyperventilation, increased cardiac output)
ABg findings in carboxyhaemoglobinaemia
◦ HbCO (elevated levels are significant, but low levels do not rule out exposure) - 1% is normal. Note foetal haemoglobin interferes with its measurement
◦ lactate (tissue hypoxia)
◦ PaO2 should be normal, SpO2 only accurate if measured (not calculated from PaO2)
◦ MetHb (exclude)
What is the most important factor in CO2 transport in the blood being so effective in veinous environments
Haldane effect
What effect does the haldane effect have on the CO2 dissociation curve
upward shift, same PCO2 but more dissolved
What causes the Haldane effects
30% from the increased Buffering from de-oxyhaemoglobin allowing increased CO2 dissociation in water
Deoxyhaemoglobin is more effective at forming carbamino compounds than oxyhaemoglobin accounting for 70% of the Haldane effect
What proportion of CO2 transport is done by each mechanism arterially
HCO3 - 90% arterial
Carbamino 5%
Dissolved 5%
Of the arterioveinous increase in CO2 required for transport what proportion of this new CO2 is transported by each mechanism?
60% via HCO3
30% via carbamino compounds
10% via dissolved CO2
How much more soluble is CO2 than O2
20x
What is arterial blood CO2 content
480ml/L
What is veinous blood CO2 content
520ml/L
How is CO2 transported
Bicarbonate ions 70-90%
Carbamagtes or carbaamino compounds 10-20%
Dissolved CO2 10%
Explain why bicarbonate is so readily able to be a storage for CO2
◦ In RBC this process is accelarated by carbonic anhydrase where CO2 combined with water, forms carbonic acid, which in turn forms bicarbonate:
‣ CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+
‣ Very small amounts of carbonic acid, but nil in plasma
◦ The rise in intracellular HCO3- leads to the exchange of bicarbonate and chloride, the chloride shift. Chloride is taken up by RBCSs, and bicarbonate is liberated.
◦ Thus chloride concentration is lower in systemic venous blood than in systemic arterial blood
◦ In arterial blood 90% of CO2 is transported as bicarbonate; in veinous blood 85% of the new CO2 is carried as HCO3
What % of CO2 is carried by HCO3 in veinous blood
85%
What % of CO2 is carried by HCO3 in arterial blood
90%
Is chloride higher ina rterial or veinous conditions?
Arterial, as it is uptaken by RBCs in veinous blood
Carbamino compounds comprise what % of CO2 transport
10-20%
What is a carbamino compound
◦ Dissociated conjugate bases of carbamino acids, which form in the spontaneous reaction of CO2 with the terminal amine group of carbamino compounds (lysine and arginine side chains having R - NH2)- Hb the most important of these.
How does deoxyhaemoglobin compare in its CO2 carrying capacity to oxyhaemoglobin?
3.5x the affinity
Allowing 5% extra of the storage of CO2 in veinous blood
Are carbamino compounds important to carriage of CO2
Not remarkably in arterial blood but account for the greatest contribution tot he difference between arterial and ceinous CO2 concentration
Dissolved CO2 % of CO2 transport
10%
Henrys law
Amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid
For every 1mmHg of pCO2 the blood concentration increases by?
0.03mmol/L
At baseline is 1.2mmol/L
CO2 solubility vs oxygen?
24x
What is the Bohr reffect?
CO2 in blood affects oxygen binding affinity
The presence of carbamte groups in critical regions of the Hb stabilises the deoxygenated form decreasing the affinity for binding to O2 –> promoting release of O2
Draw a CO2 dissocation curve
Describe the association between CO2 content and pCO2
The CO2 dissociation curve describes the change in the total CO2 content of blood which occurs with changing partial pressure of CO2.
* This curve is more linear and steep than the oxygen-haemoglobin dissociation curve
* It has no plateau
* As the result of this, shunt has little effect on CO2 (increasing the ventilation of already well-ventilated regions will improve CO2 exchange, even though it will not improve oxygenation)
On a CO2 dissoication curve where is the arterial point at baseline? What is the CO2 content? Why is there a different curve for veinous blood?
- The arterial point corresponds to the CO2 content of arterial blood:
◦ PCO2 = 40 mmHg
◦ CO2 content is 480 ml/L (or, 48ml/dL)
◦ Notably on the veinous curve a corresponding PCO2 of 40 leads to CO2 content of 500ml/L due to the Haldane effect where deoxyhaemoglobin has a higher affinity for CO2
Mixed veinous CO2 point on the CO2 dissocation curve
PCO2 is 46 mmHg
◦ CO2 content is 520 ml/L. Because of the Haldane effect, if this blood were to be “arterialised” by the addition of oxygen while the total CO2 content remained the same, the extra CO2 liberated by the oxygenation of haemoglobin would produce an increase in PCO2 to something like 55 mmHg.
How do you draw a physiological CO2 dissociation curve
The physiological CO2 dissociation curve is a line which connects the venous and arterial points, and represents the normal physiological progression of blood on the way through the circulation
How does the proportion of carbon dioxide getting carried vary between arterial and veinous systems?
- Much of this difference is due to the increase in bicarbonate concentration (85%)
- Some of this difference is also due to the Haldane effect:
◦ Deoxyhaemoglobin has about 3.5 times the affinity for CO2 when compared to oxyhaemoglobin
◦ This increases the CO2 binding capacity of venous blood
◦ Deoxyhaemoglobin is also a better buffer than oxyhaemoglobin, which increases the capacity of RBCs to carry HCO3-
What is the Hamburger effect
The chloride shift
describes the movement of chloride into RBCs which occurs when the buffer effects of deoxygenated haemoglobin increase the intracellular bicarbonate concentration, and the bicarbonate is exported from the RBC in exchange for chloride.
* This results in a difference of 2-4 mmol/L of chloride between the arterial and venous blood (and a similar difference in bicarbonate concentration).
Demonstrate the reactions in systemic capillaries of CO2 intake
What is the protein implicated in the Hamburger shift
‣ The Band 3 exchange protein (transmembrare transporter) then faciitates the diffusion of bicarbonate out of the cell, and chloride into the cell. (passive process)
What is the reverse Hamburger effect
‣ Oxygen binds to Hb reducing the binding affinity of Hb for H+ causing pH to shift down –> bicaronate is converted to CO2 and water which is removed by alveolar ventilation
‣ Bicarbonate in ECF diffuses back into the red cell, and chloride diffuses out
‣ Carbonic anhydrase converts bicarbonate back into carbon dioxide and water
Why is the chlorie shift important? 3
◦ It mitigates the change in pH which would otherwise occur in the peripheral circulation due to metabolic byproducts (mainly CO2) were deoxygenated Hb not able to buffer the acid and sequester chloride
◦ It increases the CO2-carrying capacity of the venous blood
◦ It increases the unloading of oxgyen, because of the allosteric modulation of the haemoglobin tetramer by chloride (it stabilises the deoxygenated T-state) making O2 more availabel to tissues
Define the Bohr effect
- The Bohr effect describes the decrease in the oxygen affinity of haemoglobin in the presence of low pH or high CO2
What is the mechanism of the Bohr effect (2)
- pH and CO2 both have effects on the haemoglobin tetramer stabilising the deoxygenated form:
◦ At a low pH (pKa 7), the histidine residues on one haemoglobin dimer become protonated, which permits the fomration of a “salt bridge” between dimers
‣ i.e. the more acidic the environment the more this process is enabled
◦ The formation of this bond stabilises the deoxygenated T-state
◦ The bond cannot form at a higher pH
◦ At a high CO2, CO2 binds to terminal amino groups and forms negatively charged carbamate groups
◦ These carbamate groups also stabilise the deoxygenated T-state of the haemoglobin tetramer by forming bonds with the positively charged amino groups on the opposite dimer
How does CO2 binding affect a Hb structurally
- pH and CO2 both have effects on the haemoglobin tetramer stabilising the deoxygenated form:
◦ At a low pH (pKa 7), the histidine residues on one haemoglobin dimer become protonated, which permits the fomration of a “salt bridge” between dimers
‣ i.e. the more acidic the environment the more this process is enabled
◦ The formation of this bond stabilises the deoxygenated T-state
◦ The bond cannot form at a higher pH
◦ At a high CO2, CO2 binds to terminal amino groups and forms negatively charged carbamate groups
◦ These carbamate groups also stabilise the deoxygenated T-state of the haemoglobin tetramer by forming bonds with the positively charged amino groups on the opposite dimer
How does H+ binding to Hb affect its structure?
- pH and CO2 both have effects on the haemoglobin tetramer stabilising the deoxygenated form:
◦ At a low pH (pKa 7), the histidine residues on one haemoglobin dimer become protonated, which permits the fomration of a “salt bridge” between dimers
‣ i.e. the more acidic the environment the more this process is enabled
◦ The formation of this bond stabilises the deoxygenated T-state
◦ The bond cannot form at a higher pH
◦ At a high CO2, CO2 binds to terminal amino groups and forms negatively charged carbamate groups
◦ These carbamate groups also stabilise the deoxygenated T-state of the haemoglobin tetramer by forming bonds with the positively charged amino groups on the opposite dimer
What is more important to the shape of the oxyhaemoglobin dissociation curve - CO2 or pH
- Quantitatively, the changes in pH play a greater role in changing the shape of the oxygen-haemoglobin disscoiation curve than do the changes in CO2
What are the acid and alkaline Bohr effects
- Alkaline Bohr effect: protons are released by haemoglobin when it is oxygenated at physiological pH
- Acid Bohr effect: protons are absorbed by haemoglobin when it is oxygenated at a low pH - only once pH <6
How are the Bohr effect and the Haldene effect different
The Bohr effect is what happens to oxygen when CO2 stabilises the deoxygenated haemoglobin molecule, whereas the Haldane effect is what happens to CO2 when the haemoglobin molecule is deoxygenated.
Haldane effect is?
he Haldane effect is a physicochemical phenomenon which describes the increased capacity of blood to carry CO2 under conditions of decreased haemoglobin oxygen saturation
What is the reverse Haldane effect
- Bound CO2 is released from haemoglobin when it becomes oxygenated
◦ This “reverse Haldane effect” facilitates the elimination of CO2
Haldane effect has two mechanisms
1) Increased binding affinity for CO2 as deoxyhaemoglobin by 3.5x
70% of the Haldane effect
2) Increased buffering capacity of deoxygenated haemoglobin accounting for 30% of the Haldane effect
Why does de-oxygenated haemoglobin have higher affinity for CO2
◦ Deoxygenated haemoglobin has a higher affinity for CO2
‣ This is due to the allosteric modulation of CO2-binding sites by the oxygenated haem
‣ CO2 binds to uncharged N terminal alpha amino groups of both alpha and beta subunits of haemoglobin - oxygenation of the haem iron atom is a heterotropic allosteric modulator of these CO2 binding sites because it introduces a confirmational change to the haemoglobin tetramer (postivie cooperativity
‣ As a result of this allosteric moledulation CO2 has a higher affinity for deoxygenated T state than the R state
How is the buffering capacity fo Hb higher in veinous blood
‣ Reduced (deoxygenated) haemoglobin becomes more basic
* Each Hb has 38 charged histidine residues of which 4 are attached to the haem group
* The dissociation constant of each is influenced by oxidation of haem
* When haem loses its O2 the haemoglobin tetramer becomes more basic removing hydrogen from slution
* Thus bicarbonate is made from CO2 in increasing amounts
* Thus the loss of O2 by buffering increases CO2 carried in the form of bicarbonate
* Equates to 30% of the Haldane effect
What % of the CO2 difference between arterial and veinous blood is due to the Haldane effect
1/3 of it
Describe the reverse PCO2 cascade
- Mitochondrial and cellular PCO2:
◦ Essentially the same
◦ The mitochondrial membrane is incredibly permeable to CO2
◦ Microscopic distances allow CO2 to equilibrate
◦ Depending on the metabolism of the tissue, PCO2 ranges from 20-100 mmHg - Tissue CO2:
◦ Drops slightly by diffusion distance no nearest capillary
◦ Diffusion is highly variable and tissue-specific
◦ Slowly equilibrating tissues (bone, fat) will take up to 30-60 minutes to equlibrate with the rest of the body
◦ Fast tissues (brain, blood, kidney) equilibrate over minutes and seconds - Capillary PCO2
◦ Highly variable, depending on tissue perfusion and metbaolic activity
‣ Anywhere from 42mmHg to 100mmHg
◦ PCO2 drops slightly because of storage in deoxyhaemoglobin and as bicarbonate - Mixed venous CO2
◦ Usually said to be ~ 46 mmHg
◦ usually 6mmHg higher than arterial PCO2 - Alveolar capillary PCO2
◦ Theoretically, should be higher than mixed venous because of reverse Haldane effect (release of CO2 from haemoglobin and bicarbonate stores)
‣ High oxygen environment causes CO2 to becarried more porely
‣ Oxygenated haemoglobin a poorer buffer affecting the bicarbonate equation
◦ Practically, trends towards arterial values because of rapid diffusion into the alveolus - Expired CO2
◦ Alveolar CO2 essentially equal to pulmoanry end capillary PCO2 50mmHg
◦ End tidal CO2 - Atmospheric PCO2
◦ 0.3 mmHg - Arterial CO2 6mmHg lower than venous
What is the oxygen content of blood with normal Hb
200ml/L
What is the normal oxygen delivery per minute
1000mL/min
How much of the DO2 per minute is delivered as dissolved oxygen
15ml
DO2 per kg
15ml/kg/min
Carriage of oxygen in the blood in 2 forms
Oxyhaemoglobin
Dissolved
What % of oxygen delivered per minute is from oxyhaemoglobin
97%
Describe the structure of haemoglobin as relevant to the binding of oxygen
A. Consists of 2 alpa and 2 beta chains, each chain is formed from an iron-porphyrin molecula called haem. Each molecule of Hb can bind 4 oxygen molecules )20.1ml oxygen per 100ml of blood or 15ml oxygen per 100ml in venous blood)
How much oxygen can be bound by haemoglobin per 100ml of blood
A. Consists of 2 alpa and 2 beta chains, each chain is formed from an iron-porphyrin molecula called haem. Each molecule of Hb can bind 4 oxygen molecules )20.1ml oxygen per 100ml of blood or 15ml oxygen per 100ml in venous blood)
Oxygen binding capacity per g of Hb
1.34ml/g
The reaction fo Hb binding to oxygen is described by what? Why is it not straight
Cooperative binding
Oxyhaemoglbin dissociation curve
What is the p50
The pressure at which haemoglobin is 50% saturated
p50 is 26.6mmHg
How much oxygen is dissolved in blood per mmHg
0.003 ml/mmHg/100ml of blood
or 0.03ml/mmHg/L of blood
What is Henrys law
Amount idssolved proportional to partial rpessure x solubility coeffciient
What ist he oxygen content equation
- Oxygen content = (sO2 × ceHb × BO2 ) + (PaO2 × 0.03), where:
What is ceHb
◦ ceHb = the effective haemoglobin concentration
‣ i.e. concentration of haemoglobin species capable of carrying and releasing oxygen appropriately
How is oxygen stored
70kg male contains 1.5L of oxygen
850ml in blood –> 20ml per 100ml with 97% as haemoglobin 3% as dissolved
200ml is bound to myoglobin
450ml in FRC (21% of 2.4L)
50ml in tissue fluids
What is oxygen demand per minute
3.5ml/kg/min
250ml/min
Explain the concept of tense and relaxed state
◦ This refers to two distinct states of the haemoglobin tetramer molecule
◦ The T (“Tense”) state is the deoxygenated form (with 0 O2 molecules)
◦ The R (“Relaxed”) state is the oxygenated state (with 4 O2 molecules)
‣ A ferric Fe (FE 3+) is locked in its R state and cannot revert to T state in presence of hypoxia
◦ One pair of αβ subunits in the fully oxygenated R-state appears rotated by 15° with respect to the other pair of subunits
◦ The binding of each oxygen molecule changes the state of the tetramer, changing the equlibrium contant for the next O2 molecule to bind the next subunit more easily
Why is the sigmoidal shape fo the oxyhaemoglobin dissociation curve useful?
- Buffering of oxygen content - oxygen content remains high even if PaO2 falls initially e.g. due to decreasing FiO2 with altitude
- Maintenance of diffusion gradient to tissues - steep section allows rapid oxygen unloading in low PaO2 regions –> steep blood:tissue partition especially if icnreased oxygen demand◦ A change in oxygen tension from 100mmHg to 20mmHg results in the release of 66% of O2
- Cooperative binding
- Curve can be right or left shifted by change in temperature, pH, CO2 and 2,3 DPG allowign modulation of oxygen unloading especially in different tissues despite a similar PaO2
What is p50
- = PO2 which oxygen carrying protein (Hb or myoglobin) is 50% saturated
Why do we care about p50
It is the steepest area of the curve, therefore the most sensitive spot to detect a curve shift or a change in the binding affinity of O2
What does 60mmHg correspond to in saturations
◦ most sensitive point to detect a curve shift
◦ 60mHg = Sats 90%
◦ Mixed venous point - PvO2 40mmHg –> Sats of 75%
◦ Arterial point - PaO2 100mmHg = Sats 98%
What does PvO2 of 40mmHg correspond to
◦ most sensitive point to detect a curve shift
◦ 60mHg = Sats 90%
◦ Mixed venous point - PvO2 40mmHg –> Sats of 75%
◦ Arterial point - PaO2 100mmHg = Sats 98%
How is a p50 derived/measured
the partial pressure of O2 required to achieved 50% Hb saturation - extrapolated on an ABG machine from measured PaO2 and SO2
What does an increase in p50 suggest
Reduction in affinity of Hb for O2
HbF p50
18
Myoglobin p50
2.75
In the absence of allosteric modulators p50 is?
◦ In the absence of allosteric modulators (zero 2.3 DPG, pH 7, CO2 0, 21 degrees p50 10mmHg)
What is 2,3 DPG
A byproduct of glycolysis in red cells
Requires the ability to unload oxygen so with time in stored blood it decreases
What is the MOA of 2,3 DPG
Stabilises the T state of deoxyhaemoglobin
What isthe concentration fo 2,3 DPG at baseline
5mmol/L
Is the binding of O2 endothermic or exothermic
Exothermic
So increases with cool environemtn
How does increasing H+ stabilise deoxyhaemoglobin
‣ acidic environment causes protonation of histidine residues faciltates formation of a bond between one Beta subunit and the alpha subunit of the other alpha/beta dimer
What modifies 2,3 DPG release
‣ increaseing acidosis inhibits the production of 2,3 DPG causing a reduction in the expected right shift from the acidosis
What are the 3 features of a red cell which facilitate oxygen transport
Structural features
Metabolic features
Function of red cells
- O2 transport, protecting Hb from the body and protecting the body from Hb
- Buffering - protein and bicarbonate
- Mitigation of pH change with the Hamburger effect
- Regional blood flow regulation by nitric oxide and nitrate balance
Describe red cell metabolism
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
What are the 3 shunts used in metabolism in the red cell
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
What is the embden Meyehof pathway
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
What is the Luebering Rapaport shunt
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
How does the red cell produce 2,3 DPG
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
How does the red cell reduce oxidative damage
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
What is the hexose monophosphate shunt
Glycolytic (Mebden Meyerhof pathway) main mechanism of ATP synthesis and also generates NADH
- Methaemoglobin reductase pathway borrows NADH to reduce methaemoglobin back to haemoglobin
- Luebering Rapaport shunt producing 2,3 DPG as a shunt off the glycolytic pathway
- Hexomonophosphate shunt produces NADPH which is used to convert oxidised glutathione to reduce glutathione allowing ongoing antioxidant
What structural features optimise the red cell for O2 transport 3
- 6-8 microm biconcave disc
- maximising surface area to volume to maximise area for diffusion, minimising diffusion distance throughout the cell - Deformability
- Enahnced deformability with paraboloidal transformation in capillariies
- Maximises laminar flow for O2 delivery and discourages atherogenesis
- Flexible membrane to squeeze into tighter spaces - Maximised internal area for oxygen transport
- No organelles
- No nucleus
- Main intracellular content is Hb
Myoglobin structure
Monomeric haem protein - one binding site
Where do you find myoglobin
Muscle - skeletal and cardiac
Myoglobin synthesised by? In response to
t is synthesised locally by muscle polysomes
◦ Its synthesis is thought to be stimulated by hypoxia.
◦ It is degraded in muscles
What is the myoglobin dissociation curve like
- The oxygen-myoglobin dissociation curve is hyperbolic rather than sigmoid
◦ This is because the haemoglobin molecule is a tetramer with positive cooperativity betwen oxygen binding sites, which changes the shape of its oxygen dissociation curve
◦ Myoglobin has a very high affinity for oxygen: the p50 is ~ 2.7 mmHg
Why does a high binding affinity make sense for myoglobin
◦ In the tissues, PO2 is low
◦ Haemoglobin has low affinity for oxygen at this PO2, and it releases bound oxygen into the tissue fluids
◦ Myoglobin has a high oxygen affinity at this PO2, and it collects the released oxygen
◦ In this fashion, oxygen is transferred between haemoglobin and myoglobin and means at very low PaO2 values oxygen is bound and transported at these sites maintaining intracelluilar PO2 above the Pasteur point
* The main role of myoglobin is to maintain the oxygen supply to exercising muscle
* The total oxygen store of myoglobin in the human body is around 200-300ml (15% of total stores on room air), equivalent to about 7-10 seconds of muscle activity
What is arterial blood CO2 content
480ml/L
What is mixed venous blood CO2 content
520ml/L
What is the pressure difference between arterial and mixed venous CO2
2-6mmHg for a 40ml/L difference
What are the 3 main mechanisms of CO2 transport in the blood?
Bicarbonate
Carbamates
Dissolved gas
What % of CO2 transport in the blood does bicarbonate account for
70-90%
◦ In arterial blood 90% of CO2 is transported as bicarbonate; in veinous blood 85% of the new CO2 is carried as HCO3
WHat % of CO2 transport in the blood do carbamates account for?
10-20%
What % of CO2 transport in the blood does dissolved CO2 account for
10%
Does chloride concentration vary between arterial and venous blood
◦ The rise in intracellular HCO3- leads to the exchange of bicarbonate and chloride, the chloride shift. Chloride is taken up by RBCSs, and bicarbonate is liberated.
◦ Thus chloride concentration is lower in systemic venous blood than in systemic arterial blood
What is a carb amino compound? How is it formed? What reacts with what? What conformation is more able to perform this?
• As carbamates, the conjugate bases of carbamino acid (about 10-20%)
◦ Dissociated conjugate bases of carbamino acids, which form in the spontaneous reaction of CO2 with the terminal amine group of carbamino compounds (lysine and arginine side chains having R - NH2)- Hb the most important of these.
◦ Deoxyhaemoglobin has 3.5x the affinity for CO2 compared to oxyhaemoglobin allowing for carbamino compounds to store 5% of the extra CO2 in veinous blood; total 5% in arterial and veinous blood
◦ Makes the greatest contribution to the difference between arterial and venous CO2 concentration
Which law designates that the amount of dissolved Gas in a liquid is proportional to its partial pressure above the liquid?
Henry;s law
For every 1mmHg increase in CO2 how much does blood concentration change?
◦ Thus, for every 1 mmHg of pCO2 the blood concentration increases by about 0.03 mmol/L - at baseline 1.2mmol/L
◦ Thus, CO2 is 10-20 times more soluble than oxygen
◦ In arterial blood = 5% of oxygen transported this way; in veinous blood 10% of the new CO2 is carried this way
The difference between arterial and venous concentration of CO2 is made up in carriage of CO2 how?
Bicarbonate concentration accounts of 85% of CO2 carriage (reduced total %), carbamino compounds increased proportion of carriage
PCO2 rises by less than would be expected for the increase in CO2 content due to the Haldane effect
1. 2/3 of the effect due to increased affinity of deoxyhaemoglobin for CO2 (3.5x more)
2. Increased buffering capacity of deoxyhaemoglobin increasing capacity for bicarbonate dissociation
Oxygen storage
1.5L (Hb 1L, FRC 250ml, dissolved 50ml,
myoglobin 200ml)
CO2 stroage
> 100L (blood 2.5L, rest in bone/fat)
Arterial oxygen content and pressure
100mmHg
200 mlO2/L (SpO2 98-100%)
Veinous oxygen content and pressure
40mmHg
150 mlO2/L (SpO2 75%)
Respiratory quotient? How and why does it vary?
the ratio in steady state of volume of CO2 produced per volume of O2 consumed per
unit time (Carbohydrate: 1, Protein: 0.8, Fat: 0.7)
What is Huffners number?
1.3 - 1.39 ml/g of Hb
refering to O2 carriage capacity
What are the corresponding O2 saturations under standard conditions for the below PO2
- 0
- 10
- 40
- 60
- 100
- 150
Saturations of
0%
20%
75%
90%
97.5%
99.8%
WHy do we use the P50 as the reference point for oxygen affinity?
It is the steepest part of the curve and therefore the most sensitive to change
What is 2,3 DPG
2,3 diphosphoglycerate
Where does 2,3 DPG bind
Beta chains of adult haemoglobin - more avidly to deoxyhaemoglobin
How does a foetal oxygen haemoglobin dissociation curve differ to a maternal one?
Foetal both left shifted but also with greater total oxygen content due to increased Hb concentration
P50 of myoglobin
2.75mmHg
How is myoglobin structure different to Hb
Single globin chain
Why does the shape of the oxyhaemoglobin dissociation curve matter
Flat uppe rpart mean sO2 content of blood doesnt markedly fall even with some lung impairment
Oxygen offloading in tissues is facilitated where increased requirement as well as maintaining a diffusion gradient from capillary to cell
Define saturations
Actual oxygen content of haemoglobin / maximum oxygen content of haemoglobin
How is the oxyhaemoglobin dissociation curve affected by anaemia
The curve would not change if saturations versus PO2 is graphed
However if O2 content vs PO2 was graphed there would be a proportional decrease bu the shape would remain the same –> in chronic anaemia there is increased 2,3 DPG which will right shift the curve
What is the effect of carboxyhaemoglobin on the O2 dissociation curve?
Curve shifted left for saturation vs PO2
O2 content reduced for O2 content vs partial pressure
% of CO2 carriage via different mechanisms
5% dissolved
90% bicarbonate
5% carbamino compounds
% of CO2 carriage contributing to AV difference
10% dissolved CO2
60% bicarbonate
30% carbamino
Where does CO2 react with to form carbamino compounds?
terminal amino groups of proteins
The amino groups in the side chains ofa rginine and lysine
What would PCO2 be in the veinous system in the absence of the Haldane effect
55mmHg
How does deoxyhaemoglobin compare with oxyHb in forming carbamino compounds
3.5x more effective
What 6 differences are there between the base adn apex of the lung in a healthy uprgiht adult?
Alveoli at the top of the lung
1. Larger at end expiration
2. Lower ventilation
3. Lower perfusion
4. Higher V/Q ratio 3 vs 0.6
V/Q difference results in different PO2 and PCO2
- PO2 132mmHg vs 89mmHg
- PCO2 28mmHg vs 42mmHg
pH therefore 7.51 vs 7.39
How different are the pulmonary artery pressures betweent he top and bottom of an upright lung if healthy
30mmhG
Functions of a red cell
Oxygen transport
CO2 transport
Acid base buffering
Package for Hb
Why is it important for Hb to be inside cells?
Protects Hb from glomerular filtration
Avoids increase in plasma oncotic pressure
Provides environemnt which raises its P50 to 26.6 due to 2,3 DPG
Maintains Hb as a stable tetramer
Same location as carbonic anhydrase
Methaemoglobin reductase
What % of bicarnonate leaves the cell in the Hamburger effect
70%
What is a carbamino compound and how is it formed
5% of CO2 carriage
30% of the increased CO2 carriage in veinous blood
CO2 reaction with the terminal amino groups of proteins
Net production of H+ as a result as low pKa of carbamic acid
De-oxyhaemoglobin 3.5x more capable of forming these than oxyhaemoglobin accounting for 70% of the Haldane effect
