Transport of oxygen and carbon dioxide

• Erythrocytes adaptations:


  • Biconcave shape which increases its surface area for diffusion of gases. This also helps them to pass through narrow capillaries.
  • Formed in the red bone narrow, by the time they mature they have lost their nuclei which maximised the amount of haemoglobin that fits into the cell.
• Haemoglobin

  = Red pigment that carries oxygen.

  • 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.
  • Oxygen bind to haemoglobin to form oxyhaemoglobin.
  • Hb+4O2- Hb(O2)4
• Carrying oxygen

  Erythrocytes enter capillaries in lungs the oxygen levels in the cell are low. This makes a steep concentration gradient between the inside of erythrocytes and the air in alveoli. Oxygen binds to haemoglobin.

  • Arrangement of haemoglobin 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= positive cooperatively.
  • Because oxygen is bound to haem group, the free oxygen concentration is low so steep concentration gradient is maintained until haemoglobin is saturated with oxygen.
• Oxygen dissociation curve


  = Show the affinity of haemoglobin for oxygen.

  • A small change in partial pressure of oxygen make a significant difference to saturation of haemoglobin with oxygen, because once the first molecule becomes attached the change in the shape of the haemoglobin molecule means other oxygen molecules are added rapidly.
  • Curve levels out when all haem groups are bound to oxygen.
  • So at high pressure of oxygen in lungs the haemoglobin is rapidly loaded with oxygen.
• Effect of carbon dioxide

  As pressure of carbon dioxide rises haemoglobin gives up oxygen more easily= Bohr effect.

  • In active tissues with a high pressure of carbon dioxide, haemoglobin gives up oxygen more readily.
  • In lungs where proportion of carbon dioxide in air is low, oxygen binds to haemoglobin easily.
• Fetal haemoglobin


  If blood of fetus had same affinity for oxygen as the blood of the mother, then little or no oxygen would be transferred to the blood of the fetus.

  Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. So it removes oxygen from the maternal blood as they move past each other.
• Transporting carbon dioxide 


  • 5% carried dissolved in plasma.
  • 10-20% combined with amino groups in polypeptide chain of haemoglobin to form carbaminohaemoglobin.
  • 75-85% converted into hydrogen carbonate ion in cytoplasm of red blood cells.
  • Carbon dioxide reacts slowly with water to form carbonic acid. The carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions.
• Chloride shift

  In cytoplasm of red blood cells there is high levels of enzyme carbonic anhydrase which catalyses the reversable reaction between carbon dioxide and water to form carbonic acid. This then dissociates to form hydrogen carbonate ions and hydrogen ions.

  • The negatively charged hydrogen carbonate ions move out the erythrocytes into plasma by diffusion and negatively charged chloride ions move into erythrocyte, which maintains the electrical balance of the cell= chloride shift
• What is the process by which erythrocytes maintain a steep concentration gradient for carbon dioxide?


  Chloride shift
• How do erythrocytes remove carbon dioxide?


  By converting it to hydrogen carbonate ions
• What happens when blood reaches lung tissue regarding carbon dioxide concentration?

  There is a low concentration of carbon dioxide
• What role does carbonic anhydrase play in the lungs?


  It catalyses the breakdown of carbonic acid into carbon dioxide and water
• What do hydrogen carbonate ions do when they diffuse back into erythrocytes?

  They react with hydrogen ions to form more carbonic acid
• What is the result of the breakdown of carbonic acid by carbonic anhydrase?


  It releases carbon dioxide which diffuses out of blood into lungs
• What happens to chloride ions during the chloride shift?


  Chloride ions diffuse out
• How does haemoglobin help maintain pH levels in the blood?


  By accepting free hydrogen ions to form haemoglobinic acid
• What is the nature of the reaction between haemoglobin and hydrogen ions?


  It is a reversible reaction
• What are the main steps involved in the chloride shift process?


  1. Erythrocytes remove carbon dioxide and convert it to hydrogen carbonate ions.
  2. In the lungs, carbonic anhydrase catalyses the breakdown of carbonic acid into carbon dioxide and water.
  3. Hydrogen carbonate ions diffuse back into erythrocytes and react with hydrogen ions to form carbonic acid.
  4. Carbonic acid is broken down, releasing carbon dioxide which diffuses into the lungs.
  5. Chloride ions diffuse out of the erythrocytes.
  6. Haemoglobin acts as a buffer by accepting hydrogen ions to form haemoglobinic acid.