4.1 Carbon Dioxide in the Blood

Question 1

How much more soluble is carbon dioxide than oxygen?

Answer 1

X20

Question 2

What is the ratio of carbon dioxide to oxygen in the arterial blood?

Answer 2

2.5x more carbon dioxide

There is more dissolved and more reacted with water

Question 3

What is the total content of CO2 in arterial blood?

Answer 3

21mmol/L

Question 4

What is the total content of O2 in arterial blood?

Answer 4

8. 9mmol/L

8. 8 bound to Hb, 0.1 dissolved

Question 5

What is one of the major roles of CO2 in arterial blood?

Answer 5

Controlling blood pH

Narrow range pH 7.35-7.45

Question 6

What happens to carbon dioxide in the plasma?

Answer 6

Dissolved CO2 reacts with water to form hydrogen ions and hydrogen carbonate ions
H2CO3 is a short lived intermediate

Reversible reaction

Question 7

What does the amount of dissolved CO2 depend on?

Answer 7

The partial pressure of CO2

Question 8

What effect does pCO2 have on pH?

What determines this?

Answer 8

Increased pCO2, Decreased pH

Decreased pCO2, Increased pH

pCO2 of alveoli is their determining factor

Question 9

How is pCO2 of alveoli controlled?

Answer 9

By controlling the rate of breathing

Question 10

What is the normal plasma concentration of HCO3-?

Answer 10

25mmol/L

Question 11

How do you calculate pH from pCO2 and [HCO3-] using the Henderson-Hasselbalch equation?

Answer 11

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

Question 12

Using Henderson-Hasselbalch equation explain why pH is 7.4?

Answer 12

pK is a constant = 6.1
20 times more HCO3- than dissolved CO2
Log 20 = 1.3

pH = 6.1 + 1.3 = 7.4

Question 13

What determines the pH of arterial blood?

Answer 13

RATIO of [HCO3-] and pCO2

Question 14

How is pCO2 determined?

Answer 14

By alveolar pCO2

= rate of ventilation

Question 15

What enzyme speeds up the production of hydrogen carbonate in RBCs?

Answer 15

Carbonic anhydrase

Question 16

In what direction does the reaction producing hydrogen carbonate proceed and why?

Answer 16

Forwards

Because the products are mopped up in the RBC

Question 17

How are the products of hydrogen carbonate production removed?

Answer 17

H+ binds to negatively charged Hb in RBCs

HCO3- transported to plasma through chloride bicarbonate exchanger
Creates a plasma concentration of 25mmol/L

Question 18

What is the reaction for the production of hydrogen carbonate?

Answer 18

CO2 + H2O H+ + HCO3-

Question 19

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

Answer 19

The binding of H+ to Hb

Question 20

Where does control of concentration of HCO3- in plasma occur?

Answer 20

Kidney

Question 21

How does the kidney control the amount of HCO3-?

Answer 21

By varying excretion

Question 22

Name some acids produced by the body

Answer 22

Lactic acid, keto acids, sulphuric acid

Question 23

What is a role of HCO3- and how does it do it?

Answer 23

Buffers extra acid

Acids react with HCO3- to produce CO2
[HCO3-] decreases
CO2 produced is removed by breathing
So pH changes are minimised

Question 24

Why is pCO2 higher in venous blood and what happens as a result?

Answer 24

Because it comes from metabolically active tissues

More CO2 will dissolve

Question 25

What does buffering of H+ by Hb depend on?

What is the relevance of this for H+ binding?

Answer 25

Level of oxygenation

At lungs, more O2 binds Hb -> R state so less H+ ions bind

At tissues, less O2 binds Hb -> T state so more H+ ions bind

Question 26

How is oxygen binding affecting H+ binding to Hb useful for CO2 transport?

Answer 26

At the tissues, less O2 binds to Hb -> T state so more H+ can bind
If Hb binds more H+ in RBCs then more HCO3- can be produced
Therefore more CO2 is present in plasma in venous system
Both in dissolved and reacted form

Question 27

What is the consequence of there being extra CO2 in venous blood on pH?

Answer 27

The amount of dissolved CO2 increases a bit
Much more is converted to HCO3- due to the increased capacity of Hb for H+

Therefore only a small change in pH
Both [HCO3-] and pCO2 increased
Ratio same 20:1

Question 28

What happens when venous blood arrives at the lungs?

Answer 28

Hb picks up O2 -> Enters R state
Hb gives up the extra H+
H+ reacts with HCO3- to form CO2
CO2 breathed out

Question 29

How are carbamino compounds formed?

Answer 29

CO2 can bind directly to proteins

Binds directly to amine groups on globin of Hb

Question 30

What does the formation of carbamino compounds contribute to?

Answer 30

CO2 transport

Not acid base balance

Question 31

Where are carbamino compounds formed and why?

Answer 31

At the tissues

Because PCO2 is higher and unloading of O2 facilitates binding of CO2 to Hb

The CO2 is given up at the lungs

Question 32

What are the 3 transported forms of CO2?

Answer 32

10% Dissolved CO2
60% As hydrogen carbonate
30% As carbamino compounds

Question 33

How do you work out the amount of transported carbon dioxide?

Answer 33

Transported = Total in venous blood - Total in arterial blood

= 23.3 - 21.5
= 1.8 mmol/L
8%