4. Transport of oxygen + carbon dioxide in blood

• • Most specialised transport role of blood = the transport of o2 from lungs to body cells by erythrocytes
  • Erythrocytes are involved in removal of co2 from cells + its transport to lungs for gas exchange
• Adaptations of erythrocytes:

  • Biconcave shape: has a larger SA than a simple disc structure = increasing SA available for diffusion of gases helps them pass through narrow capillaries
  • In adults = they're formed continuously in red bone marrow
  • By the time erythrocytes enter circulation = they've lost their nuclei which =  maximises amount of haemoglobin able to fit in cell, however limits their life
  • They last for 120 in bloodstream
• Adaptations of erythrocytes 2

  • Contain haemoglobin: very large globular conjugated protein made of 4 peptide chains = each with a iron containing prosthetic haem group

  --> 300 haemoglobin molecules in each RBC + each haemoglobin molecules binds to 4 o2 molecules

  • Oxygen binds loosely to haemoglobin forming = oxyhaemoglobin
  • Hb + 4O2 --> Hb(O2)4      
• CARRYING OXYGEN

  • When erythrocytes enter capillaries in lungs = o2 levels in cells are low

  --> creates a steep conc gradient between inside of erythrocytes + air in alveoli

  • O2 moves into erythrocytes + binds with haemoglobin
  • Arrangement of haemoglobin molecules means as soon as 1 o2 molecule binds to a haem group = molecule changes shape, making it easier for next o2 molecules to bind --> positive cooperativity
• CARRYING OXYGEN 2

  • As o2 bound to haemoglobin = free o2 conc in erythrocyte stays low so a steep diffusion gradient is maintained until all haemoglobin's saturated wit o2.
  • When blood reaches body tissues = situation reversed

  --> conc of o2 in cytoplasm of body cells is lower than in erythrocytes

  --> as a result oxygen moves out of erythrocytes down a conc gradient

  • Once first o2 molecule is released by haemoglobin, molecule again changes shape and it becomes easier to remove remaining o2 molecules.
• OXYGEN DISSOCIATION CURVE

  • is an important tool for understanding how blood carries + releases oxygen.
  • Percentage saturation haemoglobin in blood is plotted against = partial pressure of oxygen [pO2]
  • Oxygen dissociation curves = show affinity of haemoglobin for oxygen
  • A very small change in partial pressure of oxygen in surroundings makes a significant difference to saturation of haemoglobin with o2 = as once the first molecule becomes attached = the change in shape of haemoglobin means other O2 molecules are added rapidly
• OXYGEN DISSOCIATION CURVE 2

  • Curve levels out at highest partial pressures of oxygen because all haem groups are bound to oxygen = haemoglobin saturated = cannot take up anymore
  • This means that at high partial pressure of o2 in lungs the haemoglobin  in RBC's is rapidly loaded with oxygen
  • Equally, a small drop in o2 levels in respiring tissues means o2 is = released rapidly from haemoglobin to diffuse into the cells
  • This effect is enhanced by relatively low pH in tissues compared with the lungs
• The effect of carbon dioxide

  • As partial pressure of co2 rises = haemoglobin gives up o2 more easily
  • Change known as Bohr effect, which is important in the body as

  --> active tissues with a high partial pressure of co2, haemoglobin gives up its o2 more readily

  --> in lungs where proportion of co2 in air is low = o2 binds to haemoglobin more easily
• Fetal haemoglobin

  • When a fetus is developing its completely dependent on the mother to provide it with oxygen
  • Oxygenated blood from mother runs close to deoxygenated fetal blood in placenta
  • If blood of fetus has same affinity for o2 as blood of mother = then little or no o2 would be transferred to blood of fetus
  • However, if fetal haemoglobin has a higher affinity for o2 than adult haemoglobin at each point along dissociation curve
  • So it removes oxygen from maternal blood as they move past each other
• Transporting carbon dioxide

  • Co2 is transported from tissues to lungs in 3 different ways:

  1. About 5% is carried dissolved in plasma
  2. 10-20% is combined with amino groups in polypeptide chains of haemoglobin to form a compound called = carbaminohaemoglobin
  3. 75-85% is converted into hydrogen carbonate ions [HCO3-] in cytoplasm of RBC's

   

  • Most of the carbon dioxide that diffuses into blood from cells is transported to lungs in form of HCO3-  ions
  • Co2 reacts slowly with water to form carbonic acid [H2CO3-] 
  • The carbonic acid then dissociates to form H+ ions and HCO3-  ions
• TRANSPORTING CO2

  • CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- 
  • In cytoplasm of RBC's there are high levels of enzyme carbonic anhydrase
  • This enzyme catalyses the reversible reaction between co2 and H2O to form = carbonic acid
  • Which then dissociates to form HCO3-  ions and H+ ions
  • Negatively charged HCO3-  ions move out of RBC's into plasma by diffusion down conc gradient and negatively charge Cl- ions move into RBC's = maintains electrical balance of cell
  • =Chloride shift
  • By removing co2 + converting it to HCO3- ions  = RBC's maintain a steep conc gradient for co2 to diffuse from respiring tissues into RBC's
• TRANSPORTING CO2 2

  • When blood reaches lung tissue where there's low conc of co2 = carbonic anhydrase catalyses reverse reaction = breaking down carbonic acid into co2 + h2o
  • HCO3-  ions diffuse back into RBC's + react with H+ ions to form carbonic acid
  • When this is broken down by carbonic anhydrase it releases = free CO2, which diffuses out of blood into lungs
  • Cl- ions diffuse out of RBC's back into plasma down electrochemical gradient
  • Haemoglobin acts as a buffer + prevents changes in pH by accepting free H+ ions in a reversible reaction to form = haemoglobinic acid