The carriage of respiratory gases Slide 0

The carriage of respiratory gases Slide 0

The carriage of respiratory

gases

The carriage of respiratory

gases

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Basics 1. The composition of air

(%) N2 O2 CO2

Atmosphere 79 21 0.03Alveoli 79 14 6

Exhaled air is therefore a mixture of these values, typically 79 16 4but varies according to physiological state.


Basics 2. Atmospheric and partial pressures

Partial Pressure is the pressure exerted by one gas in a mixture of gases.

Atmospheric pressure (at sea level) is about 100 kPa.

One fifth of the air is oxygen. So one fifth of atmospheric pressure is due to oxygen.

Therefore the PARTIAL PRESSURE OF OXYGEN is about 20 kPa.

(Partial pressure of oxygen can be written as ppO2)


Basics 3. Partial pressures of oxygen

Some key values for ppO2 are:

Atmospheric 20 kPaAlveolar 13 kPa

And a typical value for blood flowingthrough tissues of the body: 5kPa


4. Oxygen transport

Oxygen is picked up by the blood in the lungs and transported in the circulation.

As the blood passes through the capillary beds of the tissues, a proportion of the oxygen is lost from the blood to the tissues.

Oxygen is hardly soluble in plasma; it is carried by HAEMOGLOBIN in the red blood cells.

Thus haemoglobin loads oxygen in the lung capillaries and unloads it in the tissue capillaries.


5. Oxygen loading

So to function efficiently, haemoglobin must attract oxygen under certain conditions, but lose it under other conditions.

Loading/unloading is a reversible reaction, in which one haemoglobin (Hb) can attract up to 4 oxygen molecules:

Hb + 4O2 HbO8loading

unloading


6. A reminder of the structure of a haemoglobin molecule

Haemoglobin is a conjugated quaternary protein comprising:

• two alpha-globulins• two beta-globulins• four haem groups, each with an iron atom at its core

It is the iron atoms which bind oxygen.


7. Haemoglobin structure


8. Loading and saturation

Hb loads/unloads one O2 at a time, so can exist as Hb, HbO2, HbO4, HbO6, or HbO8.

There are billions of Hb molecules in the blood. The term %saturation refers to the overall degree of loading of Hb with oxygen.


9. So what factors affect the loading/unloading equilibrium?

Hb + 4O2 HbO8 loading

unloading

• oxygen levels, i.e. ppO2

• carbon dioxide levels, i.e. ppCO2

• blood pH

• temperature


10. Loading


unloading

•Loading occurs at the alveolar exchange surface, where:

•ppO2 is

•ppCO2 is

•blood pH is therefore

•and temperature is

relatively high (13 kPa)

relatively low

relatively high

relatively low

WHY?


11. Unloading


unloading

•Unloading occurs in the tissue capillaries, where:

•ppO2 is

•ppCO2 is

•blood pH is therefore

•and temperature is

relatively low (5 kPa)

relatively high

relatively lower

relatively higher

WHY?


12. Saturation and oxygen dissociation

Hb + 4O2 HbO8 In fact, blood passing through the tissues never totally unloads all its oxygen. Here, blood oxygen saturation may fall to around 50%, in contrast to 100% saturation in the lung capillaries.

OXYGEN DISSOCIATION CURVES are graphs which show the relationship between the ppO2 and the degree of saturation.

loading

unloading


13. Oxygen dissociation curve

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partial pressure of oxygen (kPa)

What happens in the lungs and in the tissues?

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O2

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14. Oxygen dissociation curve explained

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In the upper part of the graph Hb is loaded with oxygen.

In the lower part Hb is unloading its oxygen.

lungs

tissues

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O2

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The carriage of respiratory gases Slide 14a

14a. Effect of changes in ppO2

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What happens to %saturation when there is a small change in ppO2

a) in the lungs?b) in the tissues?

See how actively respiring tissues promote more O2 unloading.

lungstissues

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15. Oxygen dissociation curve

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How do actively respiring tissues affect other factors:•ppCO2

•pH•temp?

And how do these affect loading / unloading?

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O2

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16. REMEMBER:

•Unloading occurs in the tissue capillaries, where:

•ppO2 is relatively low (5 kPa)

•ppCO2 is relatively high

•blood pH is therefore relatively lower

•and temperature is relatively higher


17. Shift to the right - the Bohr effect of higher ppCO2

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Increased ppCO2, lower pH and increased tempall have the effect of pushing the curve to the right.

See how this decreases Hb’s affinity for O2, so more is unloaded.

higher CO2normal CO2

Higher affinity

lower affinitySat

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18. Shift to the left

What are the consequences of a shift to the left?

Is Hb’s affinity for oxygen more or less?

Under what circumstances might this graph apply?

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19. Questions

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If the black curve is ‘normal’, which curve represents the effect of elevated levels of carbon dioxide?

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20. Questions

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If the black curve is human haemoglobin, which curve represents bird Hb and which lugworm Hb?

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21. Questions

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If the black curve is human maternal haemoglobin, which curve represents foetal Hb?

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22. The carriage of CO2

CO2 is rather more soluble than O2. It is transported from tissues to lungs in three ways:

• dissolved directly in plasma (about 5%)• combined with the polypeptides of haemoglobin (forming carbamino-haemoglobin) (about 10%)• as HCO3- in the plasma, following a series of reactions within the red blood cells

These reactions will explain why there is a direct relationship between the level of CO2 and the degree unloading of O2 by haemoglobin:


23. Reactions inside the RBC - CO2 reacts with water

CO2 CO2

CO2 from respiring tissues enters the red blood cell and combines with water, forming carbonic acid. The reaction is accelerated by CARBONIC ANHYDRASE.

carbonicanhydrase

H2CO3+ H2O


24. Reactions inside the RBC - dissociation of carbonic acid

CO2 CO2 + H2O H2CO3

Carbonic acid dissociates and HCO3- is transported out of the

RBC, in exchange for Cl-. (This is the CHLORIDE SHIFT)

carbonicanhydrase

HCO3-H+

Cl-


25. Reactions inside the RBC - unloading of oxygen

CO2 CO2 + H2O H2CO3

H+ displaces O2 from haemoglobin, forming HHb - reduced haemoglobin. The O2 is liberated to the tissues.

carbonicanhydrase

HCO3-H+

Cl-

HbO2

HHb

O2


26. Reactions inside the RBC - explanation of the Bohr effect

CO2 CO2 + H2O H2CO3

The more CO2 , the greater the displacement of O2 from Hb. CO2 reduces the affinity of Hb for O2 . This explains the Bohr effect.

carbonicanhydrase

HCO3-H+

Cl-

HbO2

HHb

O2


27. Reactions inside the RBC - reversal in the lungs.

CO2 CO2 + H2O H2CO3

All these reactions are reversible. In the lung capillaries ppO2 is higher and ppCO2 is lower and so this happens:

carbonicanhydrase

HCO3-H+

Cl-

HbO2

HHb

O2


28. Reactions inside the RBC - release of CO2.

CO2 CO2 + H2O

In the lung capillaries ppO2 is higher and ppCO2 is lower, so now O2 binds to Hb and this results in the release of CO2

carbonicanhydrase

HCO3-H+

Cl-

HbO2

HHb

O2

H2CO3

Don’t worry - you don’t have to learn these reactions!


29. Summary

• Most oxygen is transported bound to haemoglobin, as

oxyhaemoglobin• The oxygen saturation of haemoglobin is affected by:

• ppO2

• ppCO2

• pH• temperature

• CO2 is transported in three ways:• dissolved in plasma• bound to haemoglobin as carbamino-haemoglobin• converted to hydrogencarbonate ions in the red cells

• High levels of CO2 facilitate O2 unloading from haemoglobin

through the formation of hydrogen ions. The effect of increased

CO2 / decreased pH on O2 unloading is called the Bohr effect.• High levels of O2 facilitate CO2 unloading from the blood.

• • • end • • •

The carriage of respiratory gases Slide 0

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