6. Carbon dioxide in the blood

Question 1

What is the normal pH of arterial blood?

Answer 1

7.35-7.45

Question 2

How is amount CO2 dissolved in plasma calculated?

Answer 2

Solubility x pCO2

Question 3

What is the solubility if CO2?

Answer 3

0.23 at 37 degrees

Question 4

why is there a high co2 conc in arterial blood?

Answer 4

• CO2 has a major role in controlling blood pH

* Chemical reactant in the major pH buffering system of blood

Question 5

What does dissolved CO2 react with to form?

Answer 5

Reacts with water to form Carbonic acid which very quickly dissociates into H+ and HCO3-

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

Question 6

Why is there more dissolved CO2 even though lower pCO2 c/w pO2 ?

Answer 6

because of markedly increased solubility c/w oxygen

Question 7

WHat does pH of plasma depend on?

Answer 7

Depends on how much CO2 reacts to form H+
- which depends on dissolved [CO2] and [HCO3-]

And the ratio of [HCO3-]:[CO2]

Question 8

What does the amount of CO2 dissolved depend on?

Answer 8

Partial pressure of CO2

Question 9

What happens to pH if arterial pCO2 increases?

Answer 9

Plasma pH will fall, more H+ ions produced

Question 10

What happens to pH if arterial pCO2 decreases?

Answer 10

Plasma pH will rise, H+ ions react with HCO3- (to form CO2)

Question 11

what determines arterial pCO2?

Answer 11

The pCO2 of alveoli

Question 12

how is Alveolar, and hence arterial pCO2 controlled

Answer 12

by altering the rate of breathing

Question 13

What is the Henderson-Hasselbalch equation?

Answer 13

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

Question 14

What is the normal amount of bicarbonate dissolved in arterial plasma?

Answer 14

25 mmol/L

Question 15

How is high [HCO3-] established?

Answer 15

CO2 in RBCs react with water rapidly forming H+ and HCO3-. H+ are bound to haemoglobin which has a large buffering capacity. Reaction is therefore drawn towards production of HCO3-.

Question 16

Why is the reaction of CO2 with water in RBCs rapid?

Answer 16

• Reaction is speeded up by the enzyme carbonic anhydrase (CA) enzyme present in RBCs but not present in plasma

Question 17

How is bicarbonate transported out of RBC?

Answer 17

The HCO3- is transported out of the RBC chloride:bicarbonate exchanger

Question 18

What happens to the H+ produced?

Answer 18

the H+ is bound to haemoglobin

Question 19

What does the amount of HCO3- produced in RBCs depend on?

Answer 19

Depends primarily upon the buffering effects of haemoglobin (binding of H+ top Hb)

Question 20

What controls concentration of HCO3- in the blood?

Answer 20

although RBC makes the HCO3- it does not CONTROL

HCO3- concentration - HCO3- concentration controlled by KIDNEY - varying excretion– also can create more HCO3-

Question 21

what is pH dependent on?

Answer 21

– How much CO2 is present (controlled by rate and depth of breathing)
– And how much HCO3
- is present (controlled by kidneys)

Question 22

describe how Bicarbonate buffers extra acid?

Answer 22

• As part of normal metabolism body produces acids
– Lactic acid, keto acids, sulphuric acid
• Acids react with HCO3- to produce CO2
– Reaction pushed to the left CO2 + H2O ← H+ + HCO3-
– CO2 levels increase
– The extra CO2 produced is removed by breathing and pH changes are minimised (buffered)
– More HCO3 needs to be produced to replenish

Question 23

What is the value of venous pCO2, why is it higher than arterial?

Answer 23

6.0kPa, comes from metabolically active tissues

Question 24

What determines the amount of H+ bound to Hb?

Answer 24

The state of the Hb:
-– If more O2 binds Hb → R-state and less H+ ions bind (HbO2, at lungs)
If less O2 binds Hb → T-state and more H+ ions bind (Hb, as tissues)

Question 25

Why is [HCO3-] in venous blood higher?

Answer 25

Hb at tissues in T state (no oxygen bound) therefore more H+ ions bound to Hb. More conversion of CO2 to HCO3- in RBCs as ↓H+ drives reaction to right -
CO2 + H2O → H+ + HCO3

• more HCO3- transported out of RBCs into plasma in venous blood
• increasing amount transported CO2 in blood – in form of bicarbonate (HCO3- )
• Also increased dissolved CO2 in blood – down partial pressure gradient from tissue to blood

Question 26

What happens to HbH when it arrives at the lungs?

Answer 26

Haemoblogin picks up oxygen and goes into R state, H+ dissociate from Hb. H+ reacts with HCO3- to form CO2 which is breathed out

Question 27

What are carbamino compounds?

Answer 27

• CO2 can bind directly to haemoglobin – not same site as O2

* Binds directly to amine groups on globin of Hb – carbamino compound

Question 28

What is the function of carbamino compounds?

Answer 28

Contributes to CO2 transport (it does not contribute to acid base balance)
• This CO2 is given up at the lungs as Hb becomes oxygen rich – oxygenated Hb unloading
CO2 - Haldane Effect

Question 29

Why are more carbamino compounds formed at tissues?

Answer 29

Higher pCO2, unloading of O2 facilitates binding of CO2

Question 30

What are the 3 forms CO2 is transported?

Answer 30

Dissolved CO2, bicarbonate, carbamino compunds

Question 31

How is transported CO2 calculated?

Answer 31

Determined by difference between the venous and arterial CO2 content.

Question 32

What are the proportion of the different forms of CO2 in transported CO2?

Answer 32

60% hydrogen bicarbonate, 30% carbamino compunds, 10% dissolved CO2 (different from workbook, query)