Lecture 7 - CO2 in blood

Question 1

What does CO2 react with

Answer 1

Water - ECF

Hb - at a different site to O2

Question 2

Purpose of CO2 in blood

Answer 2

Maintains pH of ECF via acid - base balance

Question 3

Arterial blood pH

Answer 3

7.35 - 7.45

Question 4

Dissolved CO2

Answer 4

Reacts with water to form H2CO3 - carbonic acid using carbonic anhydrase
(slow reaction as little carbonic anhydrase in plasma)
Carbonic acid quickly dissociates into H+ and HCO3-

Question 5

What determines pH

Answer 5

Extent of dissociation of H+ in plasma and ECF (acid base balance)

Dependent on:
Amount of CO2 dissolved - rate of breathing
Concentration of HCO3- in blood

Therefore the ratio between CO2 and HCO3-

Question 6

Increase in CO2

Answer 6

More H+ is produced

pH falls - acidosis

Question 7

Increase in HCO3-

Answer 7

The increased HCO3- reacts with H+ to form CO2
CO2 is breathed off
pH increases - alkalosis

Question 8

Henderson - Hasselbach equation

Answer 8

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

pK - constant (6.1 at 37 degrees)
0.23 = CO2 solubility coefficient

Question 9

Calculate pH at body temperature

Answer 9

pH = 6.1 + log (20)
pH= 6.1 + 1.3 = 7.4 

Log 20 as 20x more HCO3- than the amount of CO2 dissolved

Question 10

Ratio of HCO3- to CO2

Answer 10

20:1

Question 11

What determines pCO2 in arteries

Answer 11

Alveolar pCO2 which is controlled by the rate of ventilation

Question 12

Hydrogen carbonate production in RBCs

Answer 12

In red blood cells
CO2 + H20 –> H2CO3- —> H+ + HCO3-

H+ Hb- —> HbH
H+ binds to Hb which are negatively charged which drives the reaction in the forwards direction

HCO3- is transported out of the cell via a chloride bicarbonate exchanger into the plasma where it is dissolved

Reaction speeded up by carbonic anhydrase in RBCs

Question 13

HCO3- plasma concentration

Answer 13

25 mmol/L

Question 14

H+ binding to Hb

Answer 14

Hb has a large capacity to bind H+

Controls amount of HCO3- produced by the erythrocyte

Question 15

What controls HCO3- concentration in the blood?

Answer 15

Kidneys by varying HCO3- excretion

Question 16

Why is pCO2 higher in venous blood?

Answer 16

CO2 produced by metabolically active tissues in mitochondria and dissolved in blood

More HCO3- is produced in RBCs due to T state of Hb therefore can produce CO2

Question 17

Venous blood

Answer 17

In tissues, Hb is less saturated with O2 as unloaded more. (T state)

Therefore it is a better buffer as H+ more readily binds.

Increased drive for forward reaction producing HCO3- so more CO2 is created

Question 18

Venous blood at lungs

Answer 18

1. Higher pO2 therefore Hb have a higher affinity for O2 and load more (R state)
2. Hb gives up extra H+
3. H+ reacts with HCO3- to form CO2
4. CO2 is breathed out

Question 19

Carbamino compounds

Answer 19

CO2 can directly bind to the amine group of globin in Hb

For CO2 transport

More carbamino compounds are formed in tissues due to a higher pCO2 and the unloading of O2 facilitates the binding of CO2 to Hb

This CO2 is given up at the lungs

Question 20

How is CO2 transported

Answer 20

Dissolved CO2
Hydrogen carbonate - reacted form
Carbamino compounds

Question 21

What percentage of CO2 is a waste product

Answer 21

Venous [CO2] - 23.3
Arterial [CO2] - 21.5

23.3 - 21.5 = 1.8 mmol/L

8% = waste

92% used in the pH buffer system