Resp 4

Question 1

What is the solubility of O2 in water and what is the consequence for O2 transport in the plasma?

Answer 1

At pO2 of 13.3kPa O2 solubility = 0.13mmol.l-1

We need 12L a minute at rest

This is contained within 92L of blood

Therefore Transport proteins (Haemoglobin and other pigments) are required

Question 2

Describe oxygen dissociation curves

Answer 2

Shows the amount of oxygen bound to respiratory pigment at a given pO2

O2 bound normally expressed as percentage of amount of O2 bound at saturation

Independent of pigment conc

Question 3

How might we use a dissociation curve to show the amount of O2 bound or released from pigment during a shift in pO2?

Answer 3

Work out the difference in fractional saturations at the two kPa

Multiply this by the amount of O2 bound at saturation

Question 4

Why is the O2 dissociation curve for Haemoglobin sigmoidal?

Answer 4

(T)ense and (R)elaxed states

T when no O2 bound

Shifts to R state as O2 binds (up to 4 molecules)

Leads to initial shallow relationship (T)

Curve steepens rapidly (T to R shift)

Question 5

At what pO2 is:

Haemoglobin saturated?

Virtually unsaturated?

Half saturated?

What can we conclude from this?

Answer 5

Saturated:

>8.5kPa

Virtually unsaturated:

<1kPa

Half:

3.5 - 4kPa

Conclusion:

Highly reversible binding reaction

Question 6

Describe the binding of O2 as a Haemoglobin molecule moves from lungs to tissue

Include typical values for O2 conc

Answer 6

Lungs:

Haemoglobin well saturated at alveolar 13.3kPa

8.8mmol.l-1 of O2

(Hb = 2.2mmol.l-1 – 4 binding sites per molecule = 8.8)

Tissues:

O2 falls to 5kPa

Haemoglobin 65% saturated at this level

Change in binding = 35%

8.8 x 0.35 = 3mmol.l-1

Question 7

How might we maximise O2 unloading from Hb in tissues?

Answer 7

Bohr shift:

H+ and CO2 in tissues (lowering pH) relaxes Hb

This leads to shifting of the dissociation curve to the right, therefore more O2 will be given up at any given pO2

Temperature:

Increase shifts curve to right

Capillary density:

Increased capillary density can increase dissociation of O2

Question 8

Why is the Bohr shift necessary?

Answer 8

Tissue pO2 can only fall so low

Must be high enough to drive O2 into cells

Typically 3kPa is the absolute minimum

Question 9

What are the typical percentages for O2 given up in arterial blood in:

Rest

Exercise

Answer 9

Rest:

27%

Exercise:

Up to 70%

Question 10

What is the major role of CO2 in the body (not including it’s presence as a waste product)?

Answer 10

Involved in Acid base control

Question 11

What is the typical pCO2 in plasma and at what concentration does it dissolve at this partial pressure?

Answer 11

5. 3kPa
6. 2mmol.l-1

Question 12

Describe how CO2 is involved in acid base balance?

Answer 12

When dissolved in plasma CO2 reacts with water

CO2 + H2O <===> H+ + HCO3-

Reversible reaction dependent on Concentrations of HCO3-, H+ and CO2 (Products and reactants)

If CO2 falls, H+ falls and pH rises

If CO2 rises, H+ rises and pH falls

Question 13

What is the predominant source of HCO3- in the plasma and why is this important to acid/base?

Answer 13

Red Cells

Prevents majority of CO2 from reacting

Therefore pH of plasma is alkaline

Question 14

What is the Henderson Hasselbach equation?

What does it show?

Answer 14

pH = pK + Log ( [HCO3-] / (pCO2 x 0.23) )

pK = 6.1

Shows that at normal pH of plasma (7.4) there is 20x more HCO3- than CO2

Question 15

Describe the reactions of CO2 within a red cell

Answer 15

Dissolved CO2 reacts with water, same as in plasma

H+ binds to Haemoglobin, this determines the amount of CO2 that can react

So lots of CO2 reacts as the product is removed

Therefore lots of HCO3- formed

Question 16

Describe the interaction between CO2 reactions in the Red cell and plasma and how they together contribute towards determining pH

Answer 16

HCO3- formed in red cell leaves in exchange for inward chloride movement

This forms the 25mmol.l-1 of HCO3- in the plasma that prevents a majority of plasma CO2 from reacting

Therefore pH of plasma depends on ratio of:

• The reaction of CO2 in red cell
• The reaction of CO2 in plasma

Question 17

What organ controls plasma HCO3- conc?

Answer 17

Kidneys

Question 18

Describe plasma pH buffering

Answer 18

If body produces acid then it reacts with HCO3-

This forms CO2 which can be breathed out

This prevents pH from falling to much

Question 19

Describe the mechanism of buffering when extra CO2 is picked up in the venous blood

Answer 19

Hb dissociates from O2

Therefore more H+ can bind

CO2 in red cells reacts to form more HCO3- + H+

As both pCO2 and [HCO3-] are increasing, change in pH is minimal

Question 20

Describe what happens to the HCO3- and H+ in venous blood as it reaches the lungs

Answer 20

HB picks up O2

So gives up H+

This reacts with HCO3- and produces CO2 to be breathed out

Question 21

What is the significance of Carbamino compounds ro acid/base balance?

Answer 21

Not significant, doesn’t affect acid/base

CO2 binds directly, therefore they only contribute to CO2 transport

Question 22

What are the different forms of carried CO2 in arterial and venous blood?

What proportions of carried CO2 do each represent?

What is the total carried CO2 in each?

Answer 22

Forms:

Dissolved directly in plasma

Dissolved in cells

Carried as HCO3- in cells and plasma

Carried by Carbaminos

Proportions:

Dissolved CO2 = 8%

Carbaminos = 11%

HCO3- = 80%

Totals:

Arterial = 21.5mmol.l-1 of CO2

Venous = 23.5mmol.l-1 of CO2