CO2 transport in blood:

1. Dissolved approx 7%

2. Combined with Hemoglobin 10–20%

3. As bicarbonate 83%

CO2

red cell

CO2 + Hb carbamino-hemoglobin

CO2 + R—NH

HR—N

H

COOH

Note:

• not the same combining site as O2

• reaction is in deoxygenated Hb

• reaction is relatively slow quantitatively not as important as the next slide

CO2

red cell

CO2 + H2O H2CO3

HCO– + H+

HHb H+ + Hb–plasma

H2OCl–

carbonic acid

carbonic anhydrase

HCO3–

ie Combination of CO2 + H2O produces a weak acid – buffered by Hb

Effect of O2 on CO2 transport: Deoxygenated Hb is a better buffer than HbO2

deoxygenated Hb has a greater carrying capacity for CO2 (Haldane effect)

Haldane Effect

% C

O2

in b

loo

d

(ml /

10

0 m

l blo

od

)

35 40 45 50

50

55

45

B Lung capillaries

PO2 = 100 mmHg

PCO2 (mmHg)

A

PO 2 = 40 mmHg

Tissue capillaries

Carrying capacity for CO2 is low when PO2 is high

= Lungs ~easier unloading of CO2

Carrying capacity for CO2 is high when PO2 is low

= Tissues ~easier loading of CO2

1. CO2 carrying capacity >> O2 carrying capacity

2. CO2 carrying capacity almost linearly with PCO2 in physiological range.

Buffers: HA H+ + A–

Law of mass action:[H+] [A–]

[HA]= K (2)

now pH = negative log of [H+]rearranging (2)

pH = pK + log [A–]

[HA]

Henderson-Hasselbach equation

[H+] = 0.00004 mmol/L pH = 7.4range

7.0 — 7.7

R

Buffers (biological):

Proteins:

1. RCOOH RCOO + H+ ~large conc

RNH3 + RNH2 + H+

Collectively

Protein Protein + H+

2. pK 7.4 Hb (histidine) - 36 per molecule

NH+

HC

H N

H C C

R

N

HC

H N

H C C

+ H+

• Deoxygenated Hb is a better buffer than HbO2

Phosphate:

H2PO4 H+ + HPO4

2

pH = pK + log [A–]

[HA]

pK 6.8

H2CO3 H+ + HCO3–

pH = pK1 + log [HCO3

–]

[H2CO3]

but CO2 + H2O H2CO3

so [H2CO3] is proportional to [CO2]

pH = pK + log [HCO3

–]

[H2CO3]

= pK1 + log [HCO3

–]

0.03 x PCO2

CO2 at 37C dissolves at 0.03 mmol/L/mmHg pK1= 6.1

• [HCO3–] regulated by kidneys cf

• PCO2 regulated by lungs.

Isohydric principle:

• all buffer systems are in equilibrium with one another: e.g.

pH = pK1 + log [A1

–]

[HA1] = pK2 + log

[A2–]

[HA2] = etc

pH = a constant +kidneys

lungs

Respiratory disturbances

• may cause changes in pH

e.g. ventilation PCO2 and pH

respiratory acidosis

HCO3– retention by kidney

• tends to return pH to near normal

Renal compensation takes days to occur

Hyperventilation

PCO2 cerebral vaso-constriction

lightheaded / dizziness

+ Alkalosis

Ca2+ + albumin Ca—Alb

and Ca2+ spontaneous firing nerves

so pH Ca2+

pins and needles

parasthesiae

Respiratory system may also compensate for other problems of acid base balance:

Metabolic acid eg lactic acid, ketones

or losses HCO3–

from severe vomiting of intestinal contents

severe diarrhoea

• injection of H+ pH ventilation

CO2tends to return pH to

near normal

Summary

• CO2 transport and acid base balance

• CO2 in blood

dissolved

carbamino—Hb

bicarbonate

Haldane effect CO2 carried if PO2 is low• biological buffers• respiratory disturbances of acid-base balance• respiratory compensation for acid-base

disturbances