5. Carbon Dioxide In The Blood

Question 1

Why does the arterial blood contain 2.5 times the amount of carbon dioxide than oxygen?

Answer 1

Has a major role in controlling blood pH.

Question 2

What is the normal range for blood pH?

Answer 2

7.35-7.45

Question 3

What does carbon dioxide react with water for form?

Answer 3

CO2 + H2O H+ + HCO3-

Question 4

What does the pH of plasma depend on?

Answer 4

How much CO2 reacts to form H+.
High [CO2]dissolved pushes reaction to the right.
High [HCO3-] pushes the reaction to the left.

Question 5

What is the determining factor for the pH of the blood?

How is this controlled?

Answer 5

pCO2 of the alveoli.

Controlled by controlling the rate of breathing.

Question 6

What happens to the plasma pH is the pCO2 rises?

Answer 6

pH falls (becomes more acidic).

Question 7

What happens to the plasma pH if the pCO2 falls?

Answer 7

pH rises (becomes more alkaline).

Question 8

What prevents nearly all the dissolved CO2 in the plasma from reacting?

Answer 8

The high [HCO3-].

Question 9

What cation is mostly associated with HCO3-?

Answer 9

Na+

Question 10

What determines the blood pH?

Answer 10

The ratio of [HCO3-] and pCO2.

Question 11

What enzyme speeds up the reaction of carbon dioxide and water in RBCs?

Answer 11

Carbonic anhydrase.

Question 12

What happens to the H+ and HCO3- in a RBC?

Answer 12

H+ ions bind to the negatively charged Hb inside the RBCs.

Chloride-bicarbonate exchangers transport HCO3- out of the RBCs, to create the plasma concentration of 25mmol.l^-1.

Question 13

What does the amount of HCO3- that erythrocytes produce depend on?

Answer 13

The binding of H+ to haemoglobin.

Question 14

How do the kidneys control [HCO3-]?

Answer 14

By varying HCO3- excretion.

Question 15

What does hydrogen carbonate buffer? And how?

Answer 15

Extra acids produced by the body eg lactic acids, keto acids, sulphuric acid.
Acids react with HCO3- to produce CO2, so [HCO3-] goes down. CO2 produced is removed by breathing and pH changes are minimised.

Question 16

Arterial pCO2 is determined by alveolar pCO2. How does this affect pH?

Answer 16

Alveolar pCO2 determines how much CO2 is dissolved in arterial blood. Reacts to form HCO3-, which then reacts with acids produced by the body to produce CO2 again. Therefore pCO2 affects the pH of the blood.

Question 17

What does buffering of H+ by Hb depend on?

Answer 17

Level of oxygenation, in the R-state less H+ ions bind to Hb, and in the T-state more H+ ions bind.

Question 18

How is the extra CO2 in the venous system transported?

Answer 18

At the tissues, less O2 binds to Hb, so haemoglobin moves towards the T-state, more H+ ions bind to Hb. This means that more HCO3- can be produced, and so more CO2 is present in the venous system in both the dissolved and reacted form.

Question 19

If [CO2] increases in the plasma of venous blood, then why doesn’t the pH change?

Answer 19

More CO2 dissolved, but also more HCO3- as more CO2 reacts in RBC to form more H+ ions bound with Hb. Therefore more HCO3- is produced and transported out the of the RBC by the chloride bicarbonate exchanger, and plasma [HCO3-] increases.
As they have both increased, the ratio of plasma [CO2] to plasma [HCO3-] has stayed the same and pH doesn’t change.

Question 20

What happens when venous blood arrives at the lungs so that CO2 can be breathed out?

Answer 20

Hb picks up O2, and so goes into the R-state. Therefore Hb gives up the extra H+ it took on at the tissues. H+ then reacts with HCO3- to form CO2, and CO2 is breathed out.

Question 21

CO2 can bind directly to proteins. Where on Hb does it bind?

Answer 21

To amine groups on the globin of Hb.

Question 22

Is binding of molecular CO2 onto Hb part of the acid base balance?

Answer 22

Not part of the acid base balance, but contributes to CO2 transport.

Question 23

Why are more carbamino compounds formed at the tissues?

Answer 23

pCO2 is higher and unloading of O2 facilitates binding of CO2 to Hb.

Question 24

In what 3 forms is CO2 transported in the blood, in order of most to least?

Answer 24

As hydrogen carbonate.
As carbamino compounds.
As dissolved CO2.

Question 25

What is the normal range for bicarbonate concentration in the blood?

Answer 25

22-26 mmol/L.

Question 26

What is the normal range for PaCO2 in the blood?

Answer 26

4.7-6.0 kPa.