Carbon Dioxide Transport

Question 1

How is Carbon Dioxide (CO2) formed and what happens to it?

Answer 1

Formed in tissues as a waste product + needs transporting in blood to lungs to be exhaled

Question 2

Is CO2 more or less soluble than O2? What effect does this have on their blood concentrations?

Answer 2

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

Question 3

What are the 3 ways in which CO2 is transported in the blood?

Answer 3

1. As dissolved CO2 (10%)
2. Carbamino compounds (21%)
3. Bicarbonate (69%)

Question 4

What are the main roles of CO2?

Answer 4

• Body fluids pH

- Breathing i.e. respiratory rate

Question 5

At 25% along the length of the capillary bed in a normal healthy individual, what happens?

Answer 5

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

Question 6

How is CO2 dissolved?

Answer 6

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-

Question 7

Why does the CO2 + H2O equation not proceed rapidly to the right?

Answer 7

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

Question 8

Define the terms acid and base.

Answer 8

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

Question 9

What is the difference between strong acids and weak acids?

Answer 9

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-

Question 10

How do you measure acidity?

Answer 10

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)

Question 11

What is the average pH range of blood? What is this in terms of [H+]?

Answer 11

7.36 - 7.44 (~7.4)

44 nanomoles/litre

Question 12

Survival for short periods is possible at pH values ranging from __ - __.

Answer 12

6. 8

8. 0

Question 13

What is the relationship between [H+] and pH?

Answer 13

Inverse relationship where 1 pH unit change is equivalent to a 10-fold change in [H+]

Question 14

What are the sources of H+ in the body?

Answer 14

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

Question 15

How does [CO2] + [HCO3-] affect plasma pH?

Answer 15

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-]

Question 16

What is the Henderson-Hasselbalch equation?

Answer 16

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

Question 17

What is the main aim of the Henderson-Hasselbalch equation?

Answer 17

Allows the calculation of pH based on measurements of [HCO3-] + [CO2] (ratio is of importance)

Question 18

Why might you need a conversion factor in the Henderson-Hasselbalch equation?

Answer 18

As [HCO3-] + [CO2] should not be in mM, they should be expressed in kPa

Question 19

When do buffer systems work best?

Answer 19

At a pH close to their pK

Question 20

What is a physiological buffer system?

Answer 20

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

Question 21

Explain the mechanism of physiological buffering for HCO3- + CO2.

Answer 21

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

Question 22

What is the reaction of CO2 in red blood cells?

Answer 22

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

Question 23

Where is carbonic anhydrase/dehydratase found?

Answer 23

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

Question 24

What reaction does carbonic anhydrase/dehydratase catalyse?

Answer 24

CO2 + H2O -> H+ + HCO3-

Both directions of this reaction

Question 25

What is the chloride/Hamburger shift? Why does it exist?

Answer 25

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

Question 26

What is the end result of the CO2 reactions going on within the RBC?

Answer 26

Majority of CO2 is transported in blood as HCO3-

Question 27

What affects the buffering ability of haemoglobin in the RBC? Why is this useful?

Answer 27

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+

Question 28

What would happen is O2 is given up the RBC without taking up CO2?

Answer 28

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

Question 29

What do the Bohr shift and the Haldane effect both together facilitate?

Answer 29

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

Question 30

What is the Haldane effect?

Answer 30

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

Question 31

What is the Bohr effect?

Answer 31

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

Question 32

What 4 factors shifts the O2 dissociation right?

Answer 32

1. Increased H+ ions 2. Increased CO2 3. Increased temperature 4. Increases BPG/DPG (by-products of anaerobic metabolism) All of which occur in capillary beds at tissue level because want O2 to offload

Question 33

Why does the pH of blood change between arterial and venous blood?

Answer 33

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

Question 34

How is CO2 transported as carbamino compounds?

Answer 34

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

Question 35

Explain CO2 dissociation curves.

Answer 35

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

Question 36

What will the CO2 dissociation curve show for mixed venous blood?

Answer 36

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

Question 37

What does the CO2 dissociation curve show for pCO2 in the physiological range?

Answer 37

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

Question 38

Why would you want to shift the O2 dissociation curve to the right in capillaries at tissue level?

Answer 38

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