Gas transport (CO2)

Question 1

Which is more soluble in blood, CO2 or oxygen?

Answer 1

CO2

Question 2

Is there more CO2 or O2 in the blood?

Answer 2

Around 3x more CO2 than O2 in blood, either in solution or in chemical combination.

Question 3

How is carbon dioxide transported in the blood?

Answer 3

1) As dissolved CO2 (~10%)
2) As carbamino compounds (~21%)
3) As bicarbonate (HCO3-) (~69%)

Question 4

Define an acid

Answer 4

Any chemical that can donate H+ (proton)

Question 5

Define a base

Answer 5

Any chemical that can accept H+ (proton)

Question 6

What is the difference between a strong acid and a weak acid?

Answer 6

Strong acids e.g. hydrochloric acid, completely dissociate in solution, releasing large amounts of H+

HCl → H+ + Cl-

Weak acids e.g. carbonic acid, only partially dissociate in solution

H2CO3 ↔ H+ + HCO3-

Note that the weak acid is in equilibrium with its conjugate base, forming a buffer pair that can respond to changes in [H+] by reversibly binding H+

Question 7

What is the average pH of blood?

Answer 7

7.4

Question 8

What are some sources of H+ in the body?

Answer 8

Volatile acids
~14 000 mmol H+ generated each day from aerobic metabolism and CO2 production by tissues (H2CO3)

Can leave solution and enter atmosphere (‘volatile’)

Excreted by lungs

Non-volatile (fixed or non-respiratory) acids
~70-100 mmol H+ generated each day from other metabolic processes forming e.g. sulphuric acid

Organic acids such as lactic acid or keto acids may also be formed in certain circumstances

Excreted by kidneys

Question 9

What is the Henderson-Hasselbalch Equation?

Answer 9

pH = pK + log10 ([HCO3-]/[CO2])

or

pH = pK + log10([HCO3-]/(pCO2 x 0.23))

Question 10

What does the Henderson-Hasselbalch equation allow us to do?

Answer 10

Allows us to calculate pH based on measurements of [HCO3-] and [CO2]

pH = pK + log10 ([HCO3-]/[CO2])

Question 11

At what pH do buffer systems work best at?

Answer 11

Usually buffer systems work best at a pH close to their pK.

But pH of blood is 7.4
And pK of this system is 6.1

Question 12

Which physiological mechanisms control bicarbonate concentration and pCO2

Answer 12

[HCO3-] is controlled by renal
If pCO2 is too high, kidneys excrete less HCO3-
[HCO3-]plasma is raised, restoring pH.

pCO2 is controlled by respiratory.
If the body produces acid, H+ reacts with bicarbonate ions to form CO2.
CO2 is breathed out, restoring pH.

Question 13

What enzyme catalyses the reaction between CO2 and H2) and where does this happen?

Answer 13

The reaction between CO2 and H2O is catalysed by the enzyme carbonic anhydrase (dehydratase) present in erythrocytes (red blood cells) but not in plasma

Therefore reaction occurs more rapidly in erythrocytes

The reaction is further promoted as the products of the reaction are removed – H+ is buffered by haemoglobin and HCO3- is transferred to plasma

Question 14

What is the carbonic anhydrase chemical reaction?

Answer 14

CO2 + H2O H+ + HCO3-

Question 15

What buffers the H+ produced by the carbonic anhydrase reaction?

Answer 15

H+ is buffered by haemoglobin - by histidine residues of globing chains

H+ + Hb- HHb

Question 16

What is the chloride or Hamburger shift?

Answer 16

The increase in HCO3- within the erythrocyte leads to HCO3- leaving the cell

Chloride (Cl-) enters the erythrocyte in exchange for HCO3- to aid this process, carried by the Cl-/HCO3- exchanger

Question 17

What effect does deoxygenation have on the buffering ability of haemoglobin?

Answer 17

Buffering ability of haemoglobin is enhanced by deoxygenation

Therefore venous blood can carry more CO2 than can arterial blood

Deoxygenation results in uptake of CO2; oxygenation results in giving up CO2

If O2 is given up without taking up CO2, pHcell will rise

HbO2 + CO2 + H2O HHb + O2 + HCO3-

Question 18

Describe how CO2 is transported as carbamino compounds?

Answer 18

Some CO2 transported as carbamino compounds

Formed as CO2 reacts with protein amino groups (especially haemoglobin)

CO2 + Protein-NH2 ↔ Protein-NHCOOH

Reaction with haemoglobin results in the formation of carbaminohaemoglobin

Resulting change in conformation of haemoglobin, reduces O2 affinity (contributing to Bohr effect)

Question 19

What is the shape of the CO2 dissociation curve within the physiological range and what is the importance of this?

Answer 19

Arterial blood typically has a pCO2 of 5.3kPa

Mixed venous blood has a pCO2 of 6.1kPa

Dissociation curve is almost straight line in physiological range

Therefore CO2 content does not saturate with increasing pCO2 i.e. increases in pCO2 lead to an increase in CO2 content