Summary respiration - co2 transport

Respiration - CO2 Transport
Topics covered:

 The three distinct chemical forms in which blood carries CO2, and that
which is normally the most important
 The two-step chemical equation for the formation of bicarbonate from
CO2 and water, and the role of carbonic anhydrase in this.
 The chemical factors which shift this equation towards bicarbonate in
tissue capillaries and towards CO2 in alveolar capillaries.
 Haemoglobin's role as a buffer of H+ ions and the dispersal of
bicarbonate by the chloride shift.
 How carbonic anhydrase inhibitors alter CO2 carriage.
 A carbon dioxide dissociation curve for arterial blood. Why, in contrast
to the Hb/O2 dissociation curve, only CO2 content (not % saturation) is
appropriate for the vertical axis.
 A curve, on the axes from bulletpoint 6, describing the dissociation
curve in typical mixed venous blood.
 A chemical explanation for the shift of the curve between arterial and
venous blood (the Haldane shift).
 The partial pressures of O2, CO2 and N2 in humid air at 37�C, in
arterial blood and in mixed venous blood. An explanation, from the O2
and CO2 dissociation curves, why their totals are not equal.
 Use these values, an explanation of why a bubble of air, trapped in the
tissues, is gradually absorbed.
 The disadvantages which would follow from haemoglobin being free in
solution rather than packaged in cells.
 How other gases are exchanged by the lungs and carried in the blood,
such as N2 and anaesthetic gases.


 How CO2 is transported
 Role of carbonic anhydrase
 Air bubbles in tissues
 Other gases

Carbon Dioxide Transport

CO2 moves from cell to blood and from blood to alveolus by diffusion, it is highly
soluble (24x that of O2) and diffuses rapidly (diffusion coefficient x20 that of O2).
Therefore a carrier molecule is not required.

CO2 is carried by the blood in three forms:
1) In solution (7%)
2) As bicarbonate (70%)
3) Carbamino compounds (23%)

CO2 in Solution

, Arterial blood has a partial pressure of CO2 (PCO2) of 5.3kPa, carrying 27ml l-1 of CO2
Venous blood has a partial pressure of CO2 (PCO2) of 6.0kPa, carrying 31ml l-1 of CO2

Fick’s principle can tell us about CO2 delivery

CO2 delivery = cardiac output x (venous CO2 – arterial CO2)

Using this we can find that delivery of CO2 in solution to the lungs for excretion is
20ml min-1
Resting CO2 output is around 200ml min-1

Therefore, only 10% of CO2 is delivered to the lungs as dissolved gas in the plasma

CO2 as Bicarbonate

The reaction of CO2 to become bicarbonate is as follows—

CO2 + H2O <--------> H2CO3 <----------> H+ + HCO3-
Carbonic anhydrase

Carbonic anhydrase accelerates the reaction by 2000x
H2CO3 = carbonic acid

There is then buffering (prevents changes in conc. and hence pH of blood) of H + ions
by haemoglobin (i.e. Hb binds the H+).
Deoxygenated blood has a high affinity for H+ and by removing protons causes the
reaction to shift to the right, as a result deoxygenated blood can carry more CO 2 than
arterial blood.

There is also transport of HCO3- into the plasma in exchange for Cl- (chloride shift)

Approximately 70% of CO2 is delivered to lungs as bicarbonate

CO2 as Carbamino Compounds

CO2 reacts with the –NH2 groups at the N-terminal of peptide chains.

R-NH2 + CO2 <----------------> RNHCOOH

Haemoglobin is the most abundant protein in blood, so CO 2 binds to it and forms
carbaminohaemoglobin

Approximately 20% of CO2 is delivered to lungs as carbamino compounds