8.4 Transport Of Oxygen And Carbon Dioxide In The Blood

Question 1

Erythrocytes adaptations

Answer 1

Biconcave shape - large SA:V ratio, faster diffusion of substances
Thin walls - short diffusion distance
No nucleus - more space for haemoglobin
Contain haemoglobin- binds reversibly to oxygen

Question 2

Haemoglobin

Answer 2

Large globular conjugated protein
Made of 4 peptide chains - 2 alpha, 2 beta subunits
Contains haem prosthetic group

Question 3

Oxygen entering erythrocytes at the lungs

Answer 3

There is a steep concentration gradient between the erythrocytes and the air in the alveoli
Oxygen moves into the erythrocytes and binds with the haemoglobin
As soon as one oxygen molecule binds to a haem group, the molecule changes shape, so it’s easier for the next oxygen molecules to bind - positive cooperativity
As oxygen is bound to the haemoglobin, free oxygen concentration in the erythrocyte stays low, so steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen

Question 4

Oxygen entering body cells

Answer 4

Concentration of O2 in body cells is lower than the erythrocytes
Oxygen moves out the erythrocytes down a concentration gradient
Once the first oxygen molecule is released by the haemoglobin, it’s easier to remove the remaining oxygen molecules

Question 5

What is shown on an oxygen dissociation curve?

Answer 5

Percentage saturation haemoglobin in the blood
Partial pressure of oxygen (pO2)
It shows the affinity of haemoglobin for oxygen

Question 6

What happens if there is a small change in the partial pressure of oxygen in the surroundings?

Answer 6

It makes a significant difference to the saturation of the haemoglobin with oxygen, because once the first molecule becomes attached, the shape of the haemoglobin molecule means other oxygen molecules are added rapidly

Question 7

What happens at higher partial pressures of CO2?

Answer 7

Haemoglobin gives up oxygen more easily

Question 8

Bohr effect

Answer 8

In active tissues with a high partial pressure of CO2, haemoglobin gives up oxygen more readily
In lungs, where proportion of CO2 is relatively low, oxygen binds to the haemoglobin molecules easily

Question 9

Fetal haemoglobin

Answer 9

This has a higher affinity for oxygen than adult haemoglobin

Question 10

Why does fetal haemoglobin have a higher affinity for oxygen than adult haemoglobin?

Answer 10

Oxygenated blood from the mother runs close to the deoxygenated fetal blood in the placenta
If fetal haemoglobin didn’t have higher affinity, little or no O2 would be transferred to the blood of of the foetus

Question 11

How is CO2 transported from the tissues to the lungs

Answer 11

5% carried dissolved in plasma
10-20% combined with amino acid groups in polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin
75-85% is converted into hydrogen carbonate ions in cytoplasm of erythrocytes

Question 12

CO2 reactions

Answer 12

CO2 + H2O >< H2CO3 >< H+ + HCO3-

Question 13

Enzyme that speeds up reaction of CO2 and H2O

Answer 13

Carbonic anhydrase

Question 14

What is the chloride shift?

Answer 14

Negatively charged hydrogen carbonate ions move out of erythrocytes into the plasma by diffusion down a concentration gradient and negatively charged chloride ions move into erythrocytes to maintain electrical balance of the cell

Question 15

What happens to CO2 at the lungs?

Answer 15

Carbonic anhydrase catalyses reverse reaction, breaking down carbonic acid into carbon dioxide and water
Hydrogen carbonate ions diffuse back into the erythrocytes and react with hydrogen ions to form more carbonic acid
This breaks down to form more carbon dioxide, which diffuses out of the blood into the lungs
Chloride ions diffuse out of RBCs into the plasma down an electrochemical gradient

Question 16

Role of haemoglobin in CO2 offloading

Answer 16

Acts as a buffer and prevents change in pH

Accepts free H+ions in a reversible reaction to form haemoglobinic acid