8.4 - Transport of O and CO2 in the blood

Question 1

Transporting oxygen

Answer 1

• The erythrocytes are very specialised, with a number of adaptations to their main function of transporting oxygen
• Erythrocytes have a biconcave shape. This shape has a larger surface area than a simple disc structure or a sphere, increasing the surface are available for diffusion of gases.
• It also helps them to pass continuously in the red bone barrow. By the time mature erythrocytes enter the circulation they have lost their nuclei, which maximises the amount of haemoglobin that fits into the cells.

Question 2

Carrying oxygen

Answer 2

• When the erythrocytes enter the capillaries in the lungs, the oxygen levels in the cells are relatively low.
• This makes a steep concentration gradient between the inside of the erythrocytes and the air in the alveoli.
• Oxygen moves into the erythrocytes and binds with the haemoglobin.
• The arrangement of the haemoglobin molecule means that as soon as one oxygen molecule binds to a haem group, the molecule changes shape , making it easier for the next oxygen molecules to bind.
• This is known as positive cooperativity as the oxygen is bound to the haemoglobin, the free oxygen concentration in the erythrocyte stays low, so a steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen

Question 3

The effect of carbon dioxide

Answer 3

• As partial pressure of carbon dioxide rises, haemoglobin gives up oxygen more easily. This change is known as the Bohr effect.
• The Bohr effect is important in the body because as a result:
  1) in active tissues with a high partial pressure of carbon dioxide, haemoglobin gives up its oxygen more readily
  2) in the lungs where the proportion of carbon dioxide is relatively low, oxygen binds to the haemoglobin molecules easily

Question 4

Fetal haemoglobin

Answer 4

• When a fetus is developing in the uterus it is completely dependent on its mother to supply it with oxygen.
• Oxygenated blood from the mother to supply it with oxygen
• Oxygenated blood from the mother runs close to the deoxygenated fetal blood in the placenta
• If the blood of the fetus 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 fetus.
• However fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin at each has a higher affinity for oxygen than adult haemoglobin at each 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 5

Transporting carbon dioxide

Answer 5

• Carbon dioxide is transported from the tissues to the lung in three different ways:
  1) About 5% is carried dissolved in the plasma
  2) 10-20% is combined with the amino groups in the polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin
  3) 75-85% is converted into hydrogen carbonate ions (HcO3-) in the cytoplasm of the red blood cells
• Most of the carbon dioxide reacts slowly with water to form carbonic acid (H2CO3-)
• The carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions
• In the blood plasma this reaction happens slowly.
• However, in the cytoplasm of the red blood cells, there are high levels of the enzymes carbonic anhydrase.
• This enzyme catalyses the reversible reaction between carbon dioxide and water to form carbonic acid.
• The carbonic acid then dissociates to form hydrogen carbonate ions and hydrogen ions.
• The negatively charged hydrogen carbonate ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient and negatively charges chloride ions move into the erythrocytes, which maintains the electrical balance of the cell. This is chloride shift