Carbon Dioxide Transport Flashcards
How is Carbon Dioxide (CO2) formed and what happens to it?
Formed in tissues as a waste product + needs transporting in blood to lungs to be exhaled
Is CO2 more or less soluble than O2? What effect does this have on their blood concentrations?
CO2 is more soluble than O2 which affects transport + diffusion -> reacts chemically with H2O so there is 3x more CO2 in blood than O2 either in solution or chemical combination so there is large total amounts of CO2 in blood
What are the 3 ways in which CO2 is transported in the blood?
- As dissolved CO2 (10%)
- Carbamino compounds (21%)
- Bicarbonate (69%)
What are the main roles of CO2?
- Body fluids pH
- Breathing i.e. respiratory rate
At 25% along the length of the capillary bed in a normal healthy individual, what happens?
Arterial blood comes into equilibrium with alveolar air i.e. the pO2 + pCO2 in the alveolus are the same as in the arterial blood adjacent to it
How is CO2 dissolved?
When arterial blood equilibrates with alveolar air, pCO2 is 5.3 kPa
The amount dissolved is proportional to gas tension (Henry’s Law)
Dissolved CO2 reacts with H2O in plasma + RBCs H2CO3 H+ + HCO3-
Why does the CO2 + H2O equation not proceed rapidly to the right?
CO2 + H2O H2CO3 H+ + HCO3-
There is high [HCO3-] due to this reaction but also, from kidneys production too pushing reaction to the left
Also, H2CO3 is a weak acid present in negligible amounts
Define the terms acid and base.
Acid: any chemical that can donate H+ (proton) e.g. HCL -> H+ + Cl-
Base: any chemical that can accept H+ e.g. NaOH -> Na+ + OH- allowing OH- + H+ -> H2O
Both can be strong or weak
What is the difference between strong acids and weak acids?
Strong: completely dissociate in H2O releasing large amounts of H+ e.g. HCl -> H+ + Cl-
Weak acids: incompletely dissociate in water + reaches equilibrium with its conjugate base forming a buffer pair that responds to changes in [H+] by reversibly binding H+ e.g. H2CO3 H+ + HCO3-
How do you measure acidity?
By measuring [H+] (mol/L) in solution - in chemistry, wide range of [H+] encountered so take negative logarithm to base 10 [H+] = pH (makes numbers + scale more manageable in range 1-14)
What is the average pH range of blood? What is this in terms of [H+]?
7.36 - 7.44 (~7.4)
44 nanomoles/litre
Survival for short periods is possible at pH values ranging from __ - __.
- 8
8. 0
What is the relationship between [H+] and pH?
Inverse relationship where 1 pH unit change is equivalent to a 10-fold change in [H+]
What are the sources of H+ in the body?
Volatile acids (more easily vaporised): aerobic metabolism + CO2 production by tissues (H2CO3), can leave solution + enter atmosphere -> excreted by lungs
Non-volatile acids (fixed/non-respiratory): Other metabolic processes forming e.g. sulphuric acid + also, lactic acid + keto acids sometimes -> excreted by kidneys
Non-volatile < volatile
How does [CO2] + [HCO3-] affect plasma pH?
CO2 + H20 H+ + HCO3-
[H+] and thus pH determined by [CO2] because reaction is pushed to right decreasing pH due to increased [H+]
If there is high [HCO3-], the reaction is pushed to the left so there is a higher pH due to higher [HCO3-]
What is the Henderson-Hasselbalch equation?
pH = pK + log10[HCO3-]/[CO2]
pK = 6.1 -> constant indicating ratio of dissociated + undissociated weak acids so gives indication of extent of buffering at a given pH
What is the main aim of the Henderson-Hasselbalch equation?
Allows the calculation of pH based on measurements of [HCO3-] + [CO2] (ratio is of importance)
Why might you need a conversion factor in the Henderson-Hasselbalch equation?
As [HCO3-] + [CO2] should not be in mM, they should be expressed in kPa
When do buffer systems work best?
At a pH close to their pK
What is a physiological buffer system?
Where different physiological mechanisms control the concentrations e.g. HCO3 is controlled by kidneys + CO2 is controlled by lungs - sensors in body will detect their levels + modify them appropriately -> affects Henderson-Hasselbalch equation/ratio + pH of arterial blood
Explain the mechanism of physiological buffering for HCO3- + CO2.
Respiratory: if body produces acid (H+), H+ reacts with HCO3- to form CO2 which is breathed out restoring pH
Renal: if pCO2 is too high, kidneys excrete less HCO3- so [HCO3-] is raised restoring pH
What is the reaction of CO2 in red blood cells?
CO2 + H2O combine in a reaction catalysed by carbonic anhydrase (dehydratase) in RBCs but not plasma so reaction occurs quicker in RBCs
Reaction further promoted as products removed -> H+ buffered by Hb by histidine residues of globin + HCO3- leaves cell + transferred to plasma
Where is carbonic anhydrase/dehydratase found?
Found widely in tissues where HCO3- or H+ production is coupled to transport e.g. salivary gland, stomach, pancreas, renal tubular epithelium, choroid plexus + ciliary body
What reaction does carbonic anhydrase/dehydratase catalyse?
CO2 + H2O -> H+ + HCO3-
Both directions of this reaction
What is the chloride/Hamburger shift? Why does it exist?
Cl- enters the RBC in exchange for HCO3- to aid this process carried by the Cl- - HCO3- exchanger
This occurs because [HCO3-] increases in RBC leading to HCO3- leaving the cell + going into plasma which would make RBC become more acidic so Cl- comes into balance out the charge
What is the end result of the CO2 reactions going on within the RBC?
Majority of CO2 is transported in blood as HCO3-
What affects the buffering ability of haemoglobin in the RBC? Why is this useful?
Enhanced by deoxygenation so venous blood can carry more H+ than arterial blood = deoxygenation -> H+ uptake & oxygenation -> H+ release
This is useful as there is less O2 + more CO2 in tissues so more H+ whereas in lungs, there is more O2 + less CO2 so less H+
What would happen is O2 is given up the RBC without taking up CO2?
pH of the cell will rise as deoxygenated Hb will mop up more H+ meaning there is less [H+] in RBC making the pH become more basic
What do the Bohr shift and the Haldane effect both together facilitate?
Bohr effect: take up of CO2 reduces O2 affinity of Hb so allows oxygen release at tissues
Haldane effect: giving up O2 increases CO2 carriage by blood so allows CO2 uptake from tissues
= facilitates gas transfer required at tissue level
What is the Haldane effect?
Increased O2 binding to Hb reduces affinity for binding CO2 + H+ by modifying quaternary structure of Hb into the relaxed form
Shifts CO2 dissociation curve to right in lung capillaries increasing amount of CO2 offloaded by Hb + exhaled
What is the Bohr effect?
In acidic conditions, O2 dissociation curve shifts to right indicates for a given PO2, Hb binds less O2 whereas in alkaline conditions the curve shits left indicating that Hb binds more O2 at a given pO2
What 4 factors shifts the O2 dissociation right?
- Increased H+ ions
- Increased CO2
- Increased temperature
- Increases BPG/DPG (by-products of anaerobic metabolism)
All of which occur in capillary beds at tissue level because want O2 to offload
Why does the pH of blood change between arterial and venous blood?
pCO2 raises slightly in venous blood but ratio between [HCO3-] + [CO2] remains close to 20 so stays relatively constant so plasma pH changes slightly going from 7.4 in arterial blood to 7.38 in venous blood
How is CO2 transported as carbamino compounds?
Formed as CO2 reacts with protein amino groups especially Hb
CO2 + protein-NH2 protein-NHCOOH
Reaction with Hb results in formation of carbaminoHb -> change in Hb conformation reducing O2 affinity contributing to Bohr effect
Explain CO2 dissociation curves.
CO2 content (y-axis) plotted against pCO2 (x-axis)
Amount of dissolved CO2 increases with increasing pCO2 BUT CO2 is also present in chemical combination with water as HCO3 + Hb as carbamino CO2
What will the CO2 dissociation curve show for mixed venous blood?
Mixed venous blood Hb typically 70% saturated with O2
Deoxygenation of Hb leads to increase in CO2 in the form of HCO3- (Haldane effect) at any value of pCO2 -> CO2 content in mixed venous blood will increase
What does the CO2 dissociation curve show for pCO2 in the physiological range?
Arterial blood pCO2 = 5.3 kPa
Mixed venous blood pCO2 = 6.1 kPa
Dissociation curve almost straight line in physiological range so CO2 content does not saturate with increasing pCO2 i.e. increased pCO2 increases CO2 content
Why would you want to shift the O2 dissociation curve to the right in capillaries at tissue level?
You would want to decrease Hb affinity for O2 so more O2 is offloaded into tissues to perfuse them
CO2 affinity of Hb will increase so you will want to pick up waste by-products of respiration i.e. CO2 + carry them back to lungs to be excreted
