Transport of oxygen and carbon dioxide in the blood

• Carrying oxygen:

  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.
• Carrying oxygen:

  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 molecule to bind. This is known as positive cooperativity. Because 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.
• When the blood reaches the body tissues, the situation is reversed. The 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.
• The effect of carbon dioxide:

  As the 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:

  • 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
• Fetal haemoglobin:

  When a foetus is developing in the uterus it is completely dependent on its mother to supply it with oxygen. Oxygenated blood from the mother runs close to the deoxygenated fetal blood in the placenta. Fetal 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.
• Transporting carbon dioxide:

  • 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 carbaminohaemoglobin
  • 75-85% is converted into hydrogen carbonate ions in the cytoplasm of the red blood cells
• Most of the carbon dioxide that diffuses into the blood from the cells is transported to the lungs in the form of hydrogen carbonate ions. Carbon dioxide reacts slowly with water to form carbonic acid. The carbonic acid then dissolves to form hydrogen ions and hydrogen carbonate ions.
• In the cytoplasm of the red blood cells there are high levels of an enzyme called carbonic anhydrase. This enzyme catalyses the reversible reaction between carbon dioxide and water to form carbonic acid. This means carbon dioxide is converted into hydrogen carbonate ions quicker in the cytoplasm of red blood cells than in the blood plasma.
• The negatively charged hydrogen carbonate ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient and negatively charged chloride ions move into the erythrocytes, which maintains the electrical balance of the cell. This is known as the chloride shift.
• By removing the carbon dioxide and converting it to hydrogen carbonate ions, the erythrocytes maintain a steep concentration gradient for carbon dioxide to diffuse from the respiring tissues into the erythrocytes.
• When the blood reaches the lung tissue where there is a relatively low concentration of carbon dioxide, carbonic anhydrase catalyses the 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. When this is broken down by carbonic anhydrase it releases free carbon dioxide, which diffuses out of the blood into the lungs.
• The Bohr effect= when a high carbon dioxide concentration causes the oxyhemoglobin curve to shift to the right
• The forman ovale is open in a fetus as the fetus is not breathing in the womb so the lungs are not functioning. Therefore the lungs are bypassed.
• Fetal haemoglobin has a higher affinity for oxygen. This is essential for the fetus as fatal haemoglobin must be able to bind to oxygen when adult haemoglobin dissociates.
• Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin so adult haemoglobin will release oxygen in the placenta whereas metal haemoglobin is still able to take up some oxygen in the placenta. So the fatal haemoglobin curve is to the left of the adult haemoglobin curve.
• Erythrocytes:

  • biconcave - large SA:V ratio so oxygen can diffuse in and out rapidly
  • no nucleus - more volume for haemoglobin
• Haemoglobin = conjugated protein
• One molecule of haemoglobin can combine with four oxygen molecules.
• At low partial pressures of oxygen, haemoglobin has a low affinity for oxygen. Once one oxygen molecule is bound, the affinity of haemoglobin for oxygen increases this means it is easier for further oxygen molecules to bind.
• haemoglobin has four polypeptides - each polypeptide contains a haem group which can bind oxygen.
• It takes a relatively high partial pressure of oxygen for the first oxygen molecule to bind to a haem group. When one oxygen bind the quaternary structure of the haemoglobin molecule changes, this increases the affinity of the haem groups for oxygen.
• The fourth haem group only binds to oxygen at fairly high partial pressures of oxygen as 3 of the 4 haem groups have been filled so the chances of an oxygen molecule colliding with the fourth haem group is relatively low.
• As red blood cells move into body tissues, the partial pressure of oxygen decreases as the tissues are carrying out aerobic respiration, one oxygen molecule now unloads from the haemoglobin. This changes the haemoglobin's quaternary structure decreasing the oxygen affinity of the remaining haem groups.