Module 2Exchange and transport

1.2.12 Carriage of carbon dioxide

Learning Objectives Success Criteria

• To understand how haemoglobin carries oxygen

• Describe and explain the significance of the dissociation curves of adult oxyhaemoglobin at different carbon dioxide levels

• List the ways carbon dioxide can be transported by the blood(Grade E - D)

• Describe the role of haemoglobin in carrying carbon dioxide(Grade C –B)

• Describe and explain how haemoglobin can supply more oxygen to actively respiring tissues than to those that have a lower level of respiration(Grade B – A)

How is carbon dioxide transported?

• Carbon dioxide is released from respiring tissues and transported to the lungs in 3 different ways

1)85% transported in the form of Hydrogencarbonate - HCO3

− which will eventually diffuse back into the plasma

2)10% combined directly with haemoglobin to form carbaminohaemoglobin

3)5% dissolved directly into plasma•List the ways carbon dioxide can

be transported by the blood(Grade E - D)

Explanation of Bohr EffectCarbon dioxide diffuses into the blood combining with

water forming carbonic acid, catalysed by enzyme carbonic anhydrase. Carbonic acid dissociates to release H+ and HCO3

−

• CO2 + H2O H2CO3 H+ + HCO3−

carbonic acid hydrogen ions + hydrogen carbonate ions

HCO3− diffuses out of RBC s and into plasma. The charge

inside the RBC is maintained by the movement of chloride ions from the plasma into the RBC (chloride shift)

H+ are taken up by haemoglobin to produce HHb, to maintain pH

• Hb + H+ Haemoglobinic acid– H+ cause acidity that promotes oxygen release

Effects of Carbon Dioxide ConcentrationHow do you think CO2 will affect the affinity of

haemoglobin for oxygen?At a high CO2 concentration: Haemoglobin has a

reduced affinity for oxygenAt a low CO2 concentration: Haemoglobin has a

higher affinity for oxygen

CO2+ H2O H+ + HCO-3

Bohr Effect

Effects of Carbon Dioxide ConcentrationHb gives up its oxygen more

readily at higher partial pressures of carbon dioxide.

1. Cells respire producing CO2 this raises p CO2

2. This increases the rate of oxygen unloading (dissociation curve shifts down)The oxygen is lower for a given pO2 meaning more oxygen is released

The Bohr Effect – graph shifts to the right

TASK – Describe and explain the significance of the dissociation curves of adult oxyhaemoglobin at different carbon dioxide levels

•Describe and explain how haemoglobin can supply more oxygen to actively respiring tissues than to those that have a lower level of respiration(Grade B – A)

AnswersQuestions:1) How does this explain behaviour of haemoglobin in the lungs?2) How does this explain behaviour of haemoglobin in the muscles?3) Why does haemoglobin have a reduced affinity for oxygen at higher

CO2 concentrations?

1) In the lungs the level of carbon dioxide is low because it diffuses across the surface and is breathed out. So, haemoglobin has a higher affinity for oxygen (there is also a higher concentration of oxygen in the lungs). This means oxygen is readily loaded by haemoglobin.

2) In the muscles the level of carbon dioxide is high because it is produced by respiration. So, haemoglobin has a lower affinity for oxygen (there is also a lower concentration of oxygen in the muscles). This means oxygen is readily unloaded by haemoglobin.

3) Dissolved CO2 is acidic and the low pH causes haemoglobin to change shape, making it readily lose its oxygen when there is a high concentration of CO2.

CO2+ H2O H+ + HCO-3

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