8.4. TRANSPORT OF OXYGEN AND CARBON DIOXIDE IN THE BLOOD

Question 1

what is the main function of erythrocytes?

Answer 1

• transporting oxygen

Question 2

how does having a biconcave shape help erythrocytes to function?

Answer 2

• has a large surface area than a simple disc structure or sphere, increasing the surface area available for diffusion of gases
• also helps them to pass through narrow capillaries
• in adults, erythrocytes are formed continuously in the red bone marrow, by the time mature erythrocytes enter the circulation they have lost their nuclei, which maximises the amount of haemoglobin that fits into the cells (also limits their life)

Question 3

what do erythrocytes contain?

Answer 3

• haemoglobin, red pigment that carries oxygen and also gives them their colour

Question 4

what is haemoglobin?

Answer 4

• large globular conjugated protein made up of four peptide chains, each with an iron-containing haem prosthetic group
• each haemoglobin molecule can bind to four oxygen molecules (forming oxyhaemoglobin), this reaction is reversible

Question 5

what is the symbol formula for haemoglobin + oxygen -> oxyhaemoglobin

Answer 5

Hb + 4O2 -> Hb (O2)4

Question 6

why is there a steep concentration gradient in between the inside of the erythrocytes and the air in the alveoli?

Answer 6

• as when the erythrocytes enter the capillaries in the lungs, the oxygen levels in the cells are relatively low
• oxygen moves into the erythrocytes and binds with the haemoglobin

Question 7

what happens when one oxygen molecule binds to a haem group in the lungs?

Answer 7

• the haemoglobin molecule changes shape, making it easier for the next oxygen molecules to bind (positive cooperativity)

Question 8

what happens when oxygen is bound to the haemoglobin?

Answer 8

the free oxygen concentration in the erythrocytes stays low, so steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen

Question 9

what happens when the blood reaches the body tissues?

Answer 9

• situation is reversed
• concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes
• as a result, oxygen moves out of the erythrocytes down a concentration gradient
• once the first oxygen molecule is released by the haemoglobin, the molecule again changes shape and it becomes easier to remove the remaining oxygen molecules

Question 10

what is an oxygen dissociation curve?

Answer 10

• an important tool for understanding how blood carries and releases oxygen
• percentage saturation is plotted against the partial pressure of oxygen (pO2)
• show the affinity of haemoglobin for oxygen

Question 11

why does a very small change in the partial pressure of oxygen in the surroundings make a small significant difference to the saturation of the haemoglobin with oxygen?

Answer 11

• because once the first molecule becomes attached, the change in the shape of the haemoglobin molecule means other oxygen molecules are added rapidly

Question 12

why does the curve level out at the highest partial pressure of oxygen?

Answer 12

• because all the haem groups are bound to oxygen and so the haemoglobin is saturated and cannot take up any more

Question 13

what does a relatively small drop in oxygen levels in the respiring tissues lead to?

Answer 13

• means oxygen is released rapidly from the haemoglobin to diffuse into the cells
• effect is enhanced by the relatively low pH in the tissues compared with the lungs

Question 14

what happens when you are not very active?

Answer 14

• only about 25% of the oxygen carried in your erythrocytes is released into the body cells
• the rest acts as a reservoir for when the demands of the body increase suddenly

Question 15

what is the Bohr effect?

Answer 15

• as the partial pressure of carbon dioxide rises, haemoglobin gives up oxygen more easily

Question 16

why is the Bohr effect important in the body?

Answer 16

because as a result:
- in active tissues with a high partial pressure of carbon dioxide, haemoglobin gives up its oxygen more readily
- in the lungs where the proportion of carbon dioxide in the air is relatively low, oxygen binds to the haemoglobin molecules easily

Question 17

how does a foetus developing in the uterus rely on its mother to supply it with oxygen?

Answer 17

• oxygenated blood from the mother runs close to the deoxygenated foetal blood in the placenta
• if the blood of the foetus had the same affinity for oxygen as the blood of the mother then little or no oxygen would be transferred to the blood of the foetus
• however, foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin at each point along the dissociation curve
• so it removes oxygen from the maternal blood as they move past each other

Question 18

how is carbon dioxide transported from the tissues to the lungs?

Answer 18

• about 5% is carried dissolved in the plasma
• 10-20% is combined with the amino groups in the polypeptide chains of haemoglobin to form a compound called oxyhaemoglobin
• 75-85% is converted into hydrogen carbonate ions (HCO3-) in the cytoplasm of the red blood cells

Question 19

how are hydrogen carbonate ions formed?

Answer 19

• carbon dioxide reacts slowly with water to form carbonic acid
• carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions

Question 20

how is the formation and dissociation of carbonic acid sped up in the cytoplasm of red blood cells?

Answer 20

• by carbonic anhydrase
• catalyses the reversible reaction between carbon dioxide and water to form carbonic acid and then dissociates it to form hydrogen carbonate ions and hydrogen ions

Question 21

what is the chloride shift?

Answer 21

• negatively charged hydrogen ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient
• negatively charged chloride ions move into the erythrocytes, which maintains the electrical balance of the cell

Question 22

why do erythrocytes remove carbon dioxide and convert it to hydrogen carbonate ions?

Answer 22

• to maintain a steep concentration gradient for carbon dioxide to diffuse from the respiring tissues into the erythrocytes

Question 23

how is haemoglobinic acid formed?

Answer 23

• haemoglobin acts as a buffer and prevents changes in the pH by accepting free hydrogen ions in a reversible reaction (to form haemoglobinic acid)