Carriage of CO2

LO: How is CO2 transported in the blood and how does this affect the % saturation of Hb with O2?

STARTERSTARTER 1. PPQs about fetal Hb v adult Hb.

2. Discuss with partner about how you think CO2 is transported.

LEVEL

ALL: Recall the 3 ways CO2 is transported in the blood.

C-D

MOST:

Describe the role of Hb in carrying CO2.

B-C

SOME:

Describe and explain he significance of the dissociation curves of adult HbO at different CO2 levels.

A-B

Question of the lesson!

How is CO2 carried around in the blood?

LiteracyToday’s main keyword is…

Carbonic anhydrase

A sentence with this word in it would be…

CO2 Transport• 5% dissolved directly in plasma (inefficent – diffusion from

tissue to tissue down gradient).• 10% is combined directly to Hb (carbaminohaemoglobin).

It does not interfere with O2 transport as CO2 is carried by the globin protein.

• 85% is transported in the form of hydrogencarbonate ions (HCO3

-)

Tissues Capillary

Carriage of CO2• CO2 diffuses into RBC, combines with H2O to form

_______ acid (catalysed by ________ __________ – H2CO3).

• Carbonic Acid Dissociates to release ________ ion (H+) and ______________________ ions (HCO3

-)

• HCO3- ions diffuse out of _____ into plasma.

• _________ shift occurs (Cl- ions rush into RBC – why?)• H+ ions cause inc/dec pH from approx 7.6 to 7.2.• H+ ions taken up by Hb to make haemoglobinic acid

(HHb), which prevents the pH decreasing further (buffer).

CO2 H2O

H2CO3-

H+ HCO3-

Cl-

HHb

Carbonic anhydraseCarbonic

hydrogen

Hydrogen carbonate

RBC

Chloride

Releasing O2• H+ ions compete for space on the

Hb – what are they competing with?

• When pCO2 is high, the H+ ions will displace O2 causing it to release – is this a bad thing?

• If more CO2 is produced, a greater release of O2 will occur – useful in tissues that have been respiring a lot (e.g?)

Questions1. List the ways in which CO2 is transported in the blood

2. Describe how carbon dioxide is converted to hydrogen carbonate ions

3. Explain the need for a chloride shift4. Explain how the presence of CO2 can reduce the

affinity of Hb for O2.

5. Describe how Hb can supply more oxygen to actively respiring tissues than to those with a lower respiration level.

Examiner tips

When asked how carbon dioxide is transported, remember that there are three ways. Also remember that the hydrogencarbonate ions difuse back into the plasma – so most carbon is carried in the plasma, not in the red blood cells.

Poster work• Produce an A3 poster to show how Oxygen and

carbon dioxide are transported by the blood.• Explain how the blood carries oxygen and carbon

dioxide using diagrams to aid you.• Include Graph sketches to show the % saturation of

oxygen for Carbon dioxide and oxygen carriage.• Explain each graph clearly.• Bullet point key facts given to aid revision of a

difficult topic.• Colourful work will aid your memory of this topic

too.