Week 4 CO2 in blood

Question 1

what is the distribution of CO2 and O2 like in arterial blood?

Answer 1

2.5x as much CO2 as O2
CO2= ~21mmol.l-1
O2= ~8.9mmol.L-1

Question 2

what is the major importance of controlling Co2?

Answer 2

to control blood pH- acid-base balance
-must be kept between 7.35-7.45
more important than transporting it from the tissues to lungs

Question 3

what is the solubility factor of CO2?

Answer 3

0.23 compared to oxygen 0.01

Question 4

how does CO2 mainly differ from O2?

Answer 4

Co2 much more soluble

Question 5

how much CO2 is dissolved at an arterial blood pCO2 of 5.3kPa and what does this dissolved CO2 react with?

Answer 5

1.2mmol.L of CO2 dissolved

reacts with water in plasma and RBCs

Question 6

what affect does dissolved CO2 have in the plasma- what reaction and what controls the rate?

Answer 6

reacts with H2O = carbonic acid = H+ + HCO-3 (hydrogen carbonate)
REVERSIBLE REACTION - rate depends on amount of product and reactants

Question 7

what affect does CO2 in plasma have on the pH of plasma?

Answer 7

depending on how much CO2 reacts to form H+ which in turn

• depends on how much CO2 is dissolved- more pushes reaction to R
• also depends on how much HCO3- produced which pushes reaction to L

Question 8

what is the amount of dissolved CO2 dependant on and what affect do changes have?

Answer 8

pCO2 - controlled by breathing

• if pCO2 rises more will be dissolved so more conversion to H+ so lower pH- acidic
• if pCO2 falls less dissolved so less H produces and pH will rise- alkaline

Question 9

how much HCO3- (hydrogen carbonate) is present in the plasma and what cation is mostly associated with it?

Answer 9

25mmol.L-1

Na+

Question 10

what does a plasma conc. of 25mmol.L-1 of HCO3- have on the reaction of CO2?

Answer 10

has conc is so high prevents nearly all dissolved CO2 from reacting- pH in plasma is alkaline

Question 11

what is the henderson- hasselbalch equation?

Answer 11

pH= pK + log ([HCO3-] / (pCO x0.23(solubility factor))
pK is a constant= 6.1 @ 37degrees
20x as much HCO3- as dissolved CO2 so log 20= 1.3
pH= 6.1 + 1.3 = 7.4

Question 12

how is the high levels of HCO3- established?

Answer 12

hydrogen carbonate is produced in RBCs- reaction is sped up by enzyme carbonic anhydrase (CA) present in RBCs

• the products are removed- H is picked up by Hb and HCO3- is exchanged for chloride by the chloride/bicarbonate exchanger
• this ensures the reaction continues

Question 13

what is the production of HCO3- in the RBCs mostly dependant on?

Answer 13

not the pCO2

is dependant on H binding to Hb

Question 14

what role does the kidneys have in relation to HCO3- conc and how does this relate to pH?

Answer 14

kidney control excretion of HCO3-
therefore pH is dependant on pCO2 (breathing) and how much HCO3- present (controlled by kidneys)
pH = PK + LOG ([HCO3-]-KIDNEYS / (PCO2 (LUNGS) X 0.23)

Question 15

how does HCO3- buffer extra acid?

Answer 15

acid produced by the body- lactic, keto- reacts with HCO3- to produce CO2
= reduced [HCO3-] and the CO2 produced is breathed out minimising ph changes

Question 16

how does the state of the Hb molecule affect the binding of H to Hb and therefore the transport of CO2/

Answer 16

if more O2 binds Hb- Relaxed state and less H binds = LUNGS
if less O2 binds- tense state- more H binds= TISSUES
- if Hb binds more H in RBCs then more HCO3- produced - more CO2 in plasma in both REACTED and DISSOLVED form

Question 17

what is the amount of binding of H to Hb in venous blood generally?

Answer 17

as less O2 more ability to bind H to Hb

Question 18

what happens when venous blood arrives at the lungs?

Answer 18

Hb goes into R state and picks up O2 so gives up H+ it took up in the tissues = H+ reacts with HCO3- to form CO2 which is breathed out

Question 19

what are carbamino compounds and where are they formed?

Answer 19

CO2 can bind directly to proteins- amine groups on globin of Hb- contributes to transport of CO2
- more formed in tissues as pCO2 higher and unloading of O2 helps binding of CO2

Question 20

what are the 3 transported forms of CO2 and what proportion of CO2 is transported by each?

Answer 20

dissolved CO2 - 10%
as hydrogen carbonate (HCO3-) - 60%
carbamino compounds- 30%

Question 21

what is the total transported CO2 in arterial and venous blood?

Answer 21

arterial 21.5mmol.L-1
venous 23.3mmol.L-1
difference is only 1.8mmol.L-1 so only 8% is transported- rest there as part of ph buffer