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
