Session 4 Flashcards
1
Q
Carbon dioxide in blood
A
- CO2 is more soluble than oxygen
- CO2 reacts chemically with water
- CO2 also reacts with Hb (but at different site from O2)
Arterial blood
- Has almost 2.5x as much CO2 as O2
- More dissolved and more reacted with water
Carbon dioxide in arterial blood
- Total content of gases (dissolved and reacted)
- Total content CO2 in arterial blood ≈ 21 mmol.l-1
- Total content O2 in arterial blood ≈ 8.9 mmol.l-1
- Why so much CO2 in blood going to the tissues?
- CO2 dissolves in water so can be used to control pH
- [CO2]dissolved = solubility x pCO2 – Solubility factor for CO2 at 37oC = 0.23
- At pCO2 of 5.3 kPa water dissolves 1.2 mmol.l-1 CO2
- Dissolved CO2 reacts with water in plasma and in red blood cells
- CO2 in arterial blood is not there as a waste product
Carbon dioxide in plasma
- Dissolved CO2 reacts with water to form carbonic acid (H2CO3)
- Carbonic acid very quickly dissociates to hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)
- We can effectively ignore the intermediate carbonic acid when we study the reaction
- Dissolved CO2 reacts with water to form hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)
- The reaction is reversible
- The rate of the reaction depends on the reactants and products
2
Q
Control of blood pH (acid-base balance)
A
- CO2 has a major role in controlling blood pH
- Controlling CO2 is more important for pH than for transporting it from tissues to the lungs
- Arterial blood pH must be kept within a narrow range (pH 7.35 – pH 7.45)
- First consider CO2 in arterial blood
3
Q
pH of plasma
A
•Depends on how much CO2 reacts to form H+
– This in turn depends on [CO2] dissolved which pushes the reaction to the right – And on [HCO3-] which pushes the reaction to the left
CO2 + H2O ⇔ H+ + HCO3
[dissolved CO2] 1.2 mmol.l-1
[HCO3-] 25 mmol.l-1
- Depends on dissolved CO2
- And concentration of hydrogen carbonate
4
Q
Dissolved CO2
A
- The amount of CO2 dissolved depends directly on the partial pressure of CO2
- If pCO2 rises plasma pH will fall (becomes more acidic)
- If pCO2 falls plasma pH will rise (becomes more alkaline)
- The pCO2 of alveoli is the determining factor
- This is controlled by altering the rate of breathing
5
Q
Hydrogen carbonate in plasma
A
- Plasma contains 25mmol.l-1 HCO3– The cation associated with this is mostly Na+ not H+
- This high [HCO3-] cannot come from CO2 in plasma
- High [HCO3-] prevents nearly all dissolved CO2 from reacting – pH of plasma is alkaline
6
Q
Henderson-Hasselbalch equation
A
- Provides a way of calculating pH from pCO2 and [HCO3-]
- pH = pK + Log ([HCO3-] /(pCO2 x 0.23))
- pK is a constant; pK = 6.1 at 37oC
- 20 times as much HCO3-as dissolved CO2
- Log 20 = 1.3
- pH = 6.1 + 1.3 = 7.4
CO2 + H2O ⇔ H+ + HCO3
[dissolved CO2] 1.2 mmol.l-1
[HCO3-] 25 mmol.l-1
7
Q
pH of arterial blood
A
- Ratio of [HCO3-] and pCO2 determine pH
- pCO2 determined by alveolar pCO2 – Rate of ventilation
8
Q
Hydrogen carbonate production in red blood cells
A
- Reaction is speeded up by the enzyme carbonic anhydrase (CA) in RBCs
- The reaction proceeds in the forward direction because the products are mopped up in the RBC
- H+ ions bind to the negatively charged Hb inside RBCs
- Chloride-bicarbonate exchanger transports HCO3- out of RBCs
- Creates a plasma concentration of 25mmol.l-1 HCO3
slide 14 lec 1
9
Q
Binding of H+ to haemoglobin
A
- Haemoglobin has a large capacity for binding H+ ions
- The amount of HCO3- that erythrocytes produce depends on the binding of H+ to haemoglobin
- Erythrocytes produce HCO3- but they don’t control concentration of HCO3- in plasma
10
Q
Plasma hydrogen carbonate
A
- [HCO3-] doesn’t change much with pCO2
- HCO3- comes from the RBCs – The reaction is mostly determined by H+ binding to Hb
11
Q
Role of kidney in controlling [HCO3-]
A
- Kidney controls amount of HCO3- by varying excretion
- Therefore pH is dependent on how much CO2 is present (controlled by rate of breathing)
- and how much bicarbonate is present (controlled by kidneys)
- pH = pK + Log ([HCO3-] /(pCO2 x 0.23))
12
Q
Hydrogen carbonate buffers extra acid
A
- The body produces acids – Lactic acid, keto acids, sulphuric acid
- Acids react with HCO3- to produce CO2
- Therefore [HCO3-] goes down
- The CO2 produced is removed by breathing and pH changes are minimised (buffered)
13
Q
Arterial and venous pCO2
A
- Arterial pCO2 is determined by alveolar pCO2
- This determines how much CO2 is dissolved
- And therefore affects pH
- pCO2 is higher in venous blood – Comes from metabolically active tissues
- This means a bit more CO2 will dissolve
- But need to consider how venous blood can transport and give up the correct amount of CO2 to the lungs
14
Q
Properties of haemoglobin are important for CO2 transport
A
- Buffering of H+ by Hb depends on level of oxygenation – Always H+ ions bound to Hb, but amount depends on the state of the Hb molecule
- If more O2 binds Hb → R-state and lessH+ ions bind – As at lungs
- If less O2 binds Hb → T-state and more H+ ions bind – As at tissues
At the tissues
- Less O2 binds to Hb → T-state and more H+ ions 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
- Hb has lost O2 and so binds more H+
- This allows more HCO3- to form
- HCO3- is exported to the plasma
15
Q
Extra CO2 in venous blood
A
- [Dissolved CO2] increases a little
- Much more converted to HCO3- due to the increased capacity of Hb for H+
– ↑ [HCO3-]
- There is only a very small change in plasma pH because both [HCO3-] and pCO2 have increased
- pH = pK + Log ([HCO3-] /(pCO2 x 0.23))
16
Q
What happens when venous blood arrives at the lungs
A
- Hb picks up O2 and goes into R-state
- This causes Hb to give up the extra H+ it took on at the tissues
- H+ reacts with HCO3- to form CO2
- CO2 is breathed out
