Blood Gas Carriage

Question 0

What is the typical partial pressure of oxygen in the lungs?

Answer 0

13.3kPa

Allows for 100% saturation of haemoglobin

Question 1

What is the solubility of oxygen in blood plasma? Under what conditions?

Answer 1

At a partial pressure of 13.3kPa and 37*C

Plasma contains 12mmol of oxygen/litre

Question 2

How many molecules of oxygen does each haemoglobin bind?

Answer 2

4

Question 3

In which direction does increased pH and deceased temperature move the curve for haemoglobin saturation?

Answer 3

To the left - increases haemoglobin’s affinity for oxygen

Question 4

What determines the rate of gas exchange in the lungs?

Answer 4

Surface area of capillaries
Resistance to diffusion
Gradient of partial pressure

Question 5

What is diffusion capacity?

Answer 5

Resistance to diffusion in the lungs

Question 6

What is used to measure diffusion capacity?

Answer 6

Carbon monoxide transfer factor (TLCO)

Question 7

How is carbon monoxide transfer factor done?

Answer 7

Take a vital capacity breath of a gas mixture containing air, 14% helium and 0.1% carbon monoxide
Hold breath for a few seconds then exhale
Discard first 750ml, measure helium and carbon monoxide in next litre
Assumed all carbon monoxide in blood will bind to haemoglobin
Amount of CO transferred from alveoli to blood is an estimate of diffusion capacity of the lung

Question 8

Why use helium in a TLCO?

Answer 8

Helium allows correction for the diluting effects of air in the lungs (residual volume) when the measurement begins
Because helium is insoluble in blood

Question 9

Reactions of carbon dioxide in blood?

Answer 9

Dissolves
Reacts with water CO2 + H20 -> HCO3 + H+
Binds directly to protein part of haemoglobin to form carbamino compounds

Question 10

What determines the pH of plasma?

Answer 10

The extent of dissociation of CO2 to form H+ and HCO3-

So the ratio of HCO3 concentration to pCO2

Question 11

At pCO2 of 5.3kPa, how much carbon dioxide is dissolved in the blood?

Answer 11

1.2mmol

Question 12

If HCO3 concentration increases, what happens to the pH?

Answer 12

It increases

Question 13

Equation for working out the pH? (Henderson-Hasselbach equation)

Answer 13

pH = pKa + log{ [HCO3] / (pCO2 x 0.23) }
pKa = 6.1
(0.23 = carbon dioxide solubility constant)

Question 14

How does the reaction of CO2 in the red blood cell affect plasma pH?

Answer 14

CO2 diffuses into the cell
CO2 + H2O  H+ + HCO3-
H+ reacts with Hb
HCO3- leaves cell (in exchange for Cl-)
This is what forms the 25mmol of HCO3- in plasma

Question 15

What do carbamino compounds do?

Answer 15

Contribute transport of carbon dioxide by haemoglobin

Question 16

Does arterial or venous blood have more carbamino compounds?

Answer 16

Venous blood

There is a high pp of carbon dioxide

Question 17

Normal carbon dioxide content in arterial and venous blood?

Answer 17

Arterial - 21.5 mmol/L

Venous - 23.5 mmol/L

Question 18

What affects how much H+ binds to haemoglobin? What effect does this have?

Answer 18

Less oxygen means more H+ can bind

This forms more HCO3-, increasing oH

Question 19

Why does venous blood pH not decease much, even though tissues are respiring and producing carbon dioxide?

Answer 19

Hb has lost oxygen, so binds more H+ so more HCO3- formed

Therefore both pCO2 and HCO3- increase

Question 20

What happens to the Hb bound to haemoglobin when it reaches the lungs?

Answer 20

Replaced by oxygen
H+ released and reacts with HCO3- -> carbon dioxide
Breathed out

Question 21

What is the equation to work out amount of carbon dioxide transported from tissues to lungs?

Answer 21

Venous blood CO2 - arterial blood CO2 = amount transported from tissue to lungs

Question 22

What are the proportions of carbon dioxide travelling in its different forms?

Answer 22

80% as HCO3-
11% as carbamino compounds
8% as dissolved CO2