Lecture 7- CO2 transport in the blood

Question 1

how is carbon dioxide transported in the blood

Answer 1

• dissolved (CO2 is more soluble than oxygen)
• CO2 reacts chemically with water to form bicarbonate- HCO3-
• As a carbamino-haemoglobin compound

Question 2

total content of O2 in arterial blood

Answer 2

8.9 mmol/L

Question 3

total content of CO2 in arterial blood

Answer 3

21

Lots of CO2 in blood going to tissues- not a waste product.

Question 4

CO2 has a major role in

Answer 4

controlling blood pH (acid-base balance)

Chemical reactant in the major pH buffering system of blood

Question 5

blood pH must be kept within a narrow range

Answer 5

arterial pH 7.35 – pH 7.45; venous 7.31-7.41

Question 6

body has several buffering systems to control blood pH but…..

Answer 6

one using CO2 most important

Question 7

what is a buffer

Answer 7

• Buffers are compounds which are able to bind or release hydrogen ions such that they dampen swings in the pH.

Question 8

outline the bicarbonate buffer system

Answer 8

• CO2 reacts with H20 to form carbonic acid
• carbonic acid dissoaciates to form H+ and HCO3-

Question 9

how to workout CO2 dissolved in arterial blood

Answer 9

[CO2] dissolved= solubility x pCO2

• solubility factor for CO2 at 37 degrees= 0.23 mmol/L/kPa
• pCO2 of CO2 arterial blood= 5.3 kPa
• therefore:
• 5.3 kPa x 0.23 mmol/L/kPa= 1.2 mmol/L dissolved CO2

Question 10

why is [CO2] dissolved higher than [O2] dissolved

Answer 10

more dissolved CO2 even though lower pCO2 than pO2 because of markedly increased solubility

Question 11

carbon dioxide in plasma

Answer 11

• dissolved CO2 reacts with water to form carbonic acid –> H2CO3

Question 12

Carbonic acid very quickly

Answer 12

dissociates to H+ and HCO3-

Question 13

bicarbonate buffer equation is

Answer 13

reversible

Question 14

the direction the bicarbonate buffer favours depends on

Answer 14

• The rate of the reaction depends on the amount of reactants (on the left) and productions (on the right)- “law of mass action”
• Reaction can go on either direction- dependent on conc of chemical

Question 15

which equation can be used to calculate the pH of plasma

Answer 15

Henderson-Hasselbalch

Question 16

Henderson Hasselbalch equation applied to the carbon dioxide-bicarb buffer system

Answer 16

Question 17

plasma pH is dependent on

Answer 17

• on how much [CO2] reacts with water to form H+
• if CO2 is high then the reaction will be favoured towards the right producing lots of H+ and HCO3-

Question 18

higher [HCO3-] in plasma will

Answer 18

push the reaction to the left

in the body the reaction is favoured in this direction because HCO3-> CO2- why our body pH slightly alkaline 7.4

Question 19

• Amount of Co2 dissolved depends directly

Answer 19

• on the partial pressure of CO2
  *

Question 20

If pCO2 rises (and no change in bicarbonate)

Answer 20

plasma pH will fall (become more acidic)

Question 21

If pCO2 falls (and there is no change in bicarbonate)

Answer 21

plasma pH will rise (become more alkaline)

Question 22

what is the determining factor of the amount of CO2 dissolved

Answer 22

• The pCO2 of alveoli’s is the determining factor- determines arterial pCO2
• Alveolar, and hence arterial pCOP2 controlled by altering the rate of breathing

Question 24

how much HCO3- in plasma

Answer 24

25 mmol/L

• cation associated with this is mainly na+ not H+

Question 25

where is most of HCO3- made

Answer 25

the RBC the high [HCO3-] cannot coem from O2 alone in plasma- not enough dissolved CO2 in plasma to create this much HCO3-

Question 26

why is HCO3- production in the RBC much faster than the plasma

Answer 26

* Reaction speeded up by **Carbonic anhydrase** (CA) enzyme present in RBC but not present in plasma

Question 27

why does HCO3- production in the RBC proceed in the froward direction

Answer 27

- produces HCO3- because the products are mopped up in the RBC

Question 28

How are products of HCO3- production mopped up in the RBC (to keep reaction in forward direction)

Answer 28

* HCO3- is transported out of the RBC by the chloride/ HCO3- exchanger * creates a plasma conc of 25 mmol/L HCO3- * H+ is bound to Hb

Question 29

although RBC are the major producers of HCO3-, they do not control the levels. Where does this happen?

Answer 29

the kidineys

Question 30

Role of kidney and lungs in CO2: bicarbonate levels

Answer 30

* [HCO3-] normally doesnt change much with changed in pCO2- low levels dissolved CO2 * HCO3- comes from the RBCs (reaction is mostly determined by H+ binding to Hb) * **Kidneys control amount of H CO3- by varying excretion**- can also create more HCO3- * therefore pH is dependent on: * how much CO2 is present (controlled by rate and depth of breathing) * how much HCO3- is present (controlled by kidneys)

Question 31

what does bicarbonate buffer

Answer 31

extra acid * As part of normal metabolism body produces acids * Lactic acids, keto acids, suphuric acid

Question 32

how does bicarbonate act as a buffer for extra acids

Answer 32

* Acids react with HCO3- to produce CO2 * Reaction pushed to the left * CO2 levels increase * removed by increased ventilation- pH changes are minimised (buffered) * More HCO3- needs to be produced to replenish

Question 33

pCO2 is .......... in venous blood

Answer 33

higher - due to veins draining metabolically acitive tissues - more CO2 will be dissolved

Question 34

Properties of Hb important for H+ buffering

Answer 34

* Buffering of H+ by Hb depends on level of oxygenation * There are always H+ ions bound to Hb, but **amount** depends on the **state of the Hb** molecule * If more O2 binds Hb → R-state and **less** H+ ions bind * As at lungs * If less O2 binds Hb → T-state and **more** H+ ions bind – As at tissues

Question 35

H+ buffering **At the tissues to venous blood**

Answer 35

* Less O2 binds to Hb → T-state and **more** H+ ions bind * If Hb binds more H+ in RBCs in venous blood then more HCO3- can be produced as ↓**H+ drives reaction to right - CO2 + H2O** → **H+ + HCO3- IN RBCs** * Therefore more **HCO3-** transported **out of RBCs** into plasma in venous blood * Effectively 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 36

**What happens when venous blood arrives at the lungs**

Answer 36

* Hb picks up O2 and goes into R state ( due to higher pO2)- lower affinity for H+ * This causes Hb to go into the relaxed state and give up the extra H+ it took on at the tissues * H+ reacts with HCO3- to form CO2 * Reaction push to the left * CO2 diffuses out of the blood into the alveoli * CO2 is breathed out

Question 37

**Red blood cells, CO2 and bicarbonate in body tissue and the lungs - summary**

Answer 37

Question 38

**Formation of carbamino compounds** ​

Answer 38

* CO2 can bind directly to Hb- not same site as O2 * Binds directly to **amine groups** on globin of Hb- carbamino compound

Question 39

Binding of molecular CO2 onto Hb is not part of

Answer 39

acid base balance but contributes to CO2 transport

Question 40

More carbamino compounds are formed at

Answer 40

the tissues- both because pCO2 higher, and unloading of O2 from Hb facilitates binding of CO2 to Hb- oxygen poor Hb (**T state Hb binds CO2 better**) ## Footnote **This CO2 is given up at the lungs as Hb becomes oxygen rich- oxygenated Hb unloading CO2= Haldane effect**

Question 41

Haldane effect

Answer 41

CO2 is given up at the lungs as Hb becomes oxygen rich- oxygenated Hb unloading CO2

Question 42

how much CO2 is transported to be eliminated?

Answer 42

* Calculations reveal that at rest only ~8% of CO2 is transported to the lungs to be eliminated- remember more CO2 can be eliminated if there is extra acid that needed to be buffered * The rest of the CO2 in blood is there as part of the pH buffering system

Question 43

% of dissolved CO2 transported to the lungs

Answer 43

10%

Question 44

% of bicarbonate transported to the lungs

Answer 44

60% of CO2 in blood

Question 45

% of dissolved carbamino compounds transported to the lungs

Answer 45

**30%** of dissolved CO2 transported to the lungs