Lecture 4.2: Carbon Dioxide in Blood

Question 1

What is more soluble, CO2 or O2?

Answer 1

CO2 is more soluble than O2

Question 2

How much more CO2 is there in arterial blood compared to O2?

Answer 2

There is almost three times as much CO2 in arterial blood as there is oxygen

Question 3

What is CO2’s most important role in the blood?

Answer 3

• CO2 is a major part of the system controlling pH of blood
• Much more important process than its transport from tissues to lungs

Question 4

At a pCO2 of 5.3kPa the dissolved CO2 concentration is….?

Answer 4

• 1.2 mmol/L

Question 5

Dissolution of CO2 in Water Equation

Answer 5

(CO2) + (H2O) <———> (H+) + (HCO3-)

Question 6

What does the pH of plasma depend on?

Answer 6

• Depends on how much CO2 reacts to form
H+
• Which depends on dissolved CO2

Question 7

What happens to pH if pCO2 rises?

Answer 7

• pH will fall

Question 8

What happens to pH if pCO2 falls?

Answer 8

• pH will rise

Question 9

What happens to pH if [HCO3-] rises?

Answer 9

• pH will rise

Question 10

How many mmol/L of Hydrogen Carbonate is there in plasma?

Answer 10

25mmol/L

Question 11

What is the role of Hydrogen Carbonate in the plasma?

Answer 11

• Not from CO2 in plasma (sodium hydrogen carbonate)
• Stops nearly all dissolved CO2 from reacting
• So pH is alkaline

Question 12

What is the ratio of [HCO3-] to dissolved CO2 in plasma?

Answer 12

20:1

Question 13

What is the Henderson-Hasselbalch Equation?

Answer 13

pH= pK + log ([HCO3-]/(pCO2x 0.23))

Question 14

What organ controls the Hydrogen Carbonate concentration in plasma?

Answer 14

The Kidney’s

Question 15

How much more HCO3- does there have to be than CO2 in order for plasma pH to be 7.4?

Answer 15

• 20 times more
• It is the ratio of HCO3– to pCO2 which matters, not absolute values

Question 16

Buffering Mechanism in response to Acid in Plasma

Answer 16

If the body produces acid this reacts with hydrogen carbonate to form CO2 which is breathed out stops pH changing too much

Question 17

What has higher pCO2: venous or arteriole blood?

Answer 17

Venous Blood

Question 18

If there is high pCO2 in venous blood, why is the pH not low and why do veins not dissolve?

Answer 18

• Buffering of H+ by Hb
• Hb has lost oxygen, so binds more H+ which forms more HCO3-
• Which is exported to plasma from RBC
• Both pCO2 and [HCO3-] increase pH does not change much
• [HCO3-]/pCO2= constant

Question 19

What happens when venous blood reaches the lungs?

Answer 19

• Hb picks up oxygen
• So gives up H+
• Reacts with hydrogen carbonate
• To form CO2 which is breathed out

Question 20

How many mmol/L of CO2 in arterial blood?

Answer 20

Contains 21.5 mmol CO2 per litre

Question 21

How many mmol/L of CO2 in venous blood?

Answer 21

Contains 23.5 mmol CO2 per litre

Question 22

What percentage of CO2 is transported as Hydrogen Carbonate?

Answer 22

c.80%

Question 23

What percentage of CO2 is transported as Carbamino Compounds?

Answer 23

c.11%

Question 24

What percentage of CO2 is transported as Dissolved CO2?

Answer 24

c.8%