Saturday, May 14, 2011

Gaseous Exchange in Mammals

(A) Gaseous exchange in mammals
  1. Respiratory tract consists of:  nostril, nasal cavity, pharynx, trachea, bronchi + bronchioles.
  2. Gaseous exchange occurs at the ­­­alveoli.
  3. Alveolar wall = single layer of squamous epithelium cells.
  4. Capillary wall = flattened endothelial cells.
  5. Thin layer of alveolar wall + rich network of blood capillaries à enhance gaseous exchange between alveolar sac + blood.
  6. Deoxygenated blood - enters the lungs - thrugh pulmonary arteries.
  7. Oxygenated blood - leaves the lungs - through pulmonary vein.
  8. The rate of O2 + CO2 diffusion in the alveolar à depends on partial pressure of the gases in the alveolar air.

   9.   In the lung:
à PO2 in the alveolar air is higher than blood
à O2 diffuse rapidly from alveolus
à to blood plasma.

  1. In the peripheral tissues:
à PO2 is lower than blood
à O2 diffuses out from blood
à to tissues.

  1. O2 +  CO2 à low solubility in blood plasma
  2. CO2 à mainly transported as hydrogen carbonate ion (HCO3+).
  3. In the tissues
à PCO2 is higher than blood
à  CO2 diffuses out
à into the capillaries.

(B) Gaseous exchange in mammals

  1. Blood carries CO2 in 3 forms:
·   dissolved gas (8%)
·   HCO3- ion in the plasma (73%)
·   carbamino haemoglobin in the red blood cells (19%)

  1. A large amount of CO2
·   combines with H2O in the red blood cells (RBC)
·   to form carbonic acid (H2CO3)
·   then ionizes to form H+ +  HCO3-
  1. HCO3- from the red blood cell:
·   diffuse out into the plasma.
·   loss of negatively charge HCO3- from the red blood cell
·   balanced by the inward diffusion of -ve chloride ions (Cl-) = chloride shift.

  1. In the lungs.
à PCO2 is lower than in the blood.
à CO2 diffuses out
à into alveoli.

  1. In the plasma:
1.  dissolved CO2 diffuses out into the alveolar air.
2.  Carbamino haemoglobin:
à dissociates
à form CO2 + haemoglobin.
3.  Hydrogen carbonate ions (HCO3-):
à diffuse into the red blood cells
à reacts with H+
à form carbonic acid (H2CO3).
à dissociates
à form H2O + CO2
à CO2 diffuses out into the alveolar air
à exhaled.

(C) Gaseous exchange in mammals

1.  O2  = transported by the haemoglobin molecule in the red blood cells.
2.  Each haemoglobin molecule à carries 4 mol of O2.
3.  Each polypeptide chain contains:

·   2 subunits of alpha polypeptides chains
·   2 subunits of beta polypeptides chains.

4.   Haem:
à prosthetic group of haemoglobin.
à as a porphyrin ring with a ferum ion (Fe2+) in the middle.
à each capable of carrying a single O2 molecule attached to the ferum ion
à porphyrin ring shows strong cooperative O2 binding to the haemoglobin.

5.   When one mol of O2 binds to one of the haem group in the haemoglobin
à produces a conformational change in that subunit
à transmitted to the other 3 subunits
à facilitate O2 binding to the rest of the polypeptide.
à concerted conformational change of the subunits
à increases affinity of haemoglobin for O2
à resulting in sigmoid shape for the O2 dissociation curve of the haemoglobin.

     6. The cooperative effect is reversible
à when one subunit of oxyhaemoglobin unloads its O2,
à other three quickly follow suit
à conformational change
à lowers its affinity for O2.
     7. Relationship between PO2 and % saturation of haemoglobin with O2
         à represented by O2 dissociation curve = sigmoid.

   8.   The O2 dissociation curve shows:
à when haemoglobin is exposed to a gradual increase of PO2,
à it absorbs O2 rapidly at first
à but more slowly as the PO2 continues to rise.

     9. The % of O2 saturation of haemoglobin = 95%
à when blood flows through the lungs (PO2 = 105 mmHg).

    10. The % of O2 saturation of haemoglobin = 70%
à when blood flows through a moderately active/resting muscle (PO2=40 mmHg).

    11. As the blood from the lungs reaches the muscle at rest:
à 25% of the O2 carried in the heamoglobin is unloaded to the surrounding tissues
à for cellular respiration.
à 70% of the oxygen is still retained by the haemoglobin.

    12. The % of O2 saturation of haemoglobin is 40%  
à when the blood flows through the active muscle at PO2 of 20 mmHg.

    13. When the muscle is active:
à haemoglobin readily unloads 55% of the oxygen
à still retains 40%.

    14. During exercise:
à active tissue is in demand for O2 to produce energy.
à slight drops in PO2 (between muscle at rest + muscle during exercise.)
à enough to cause a relatively large increase in the amount of O2 the blood unloads.

    15. The increase of CO2 conc/decrease in blood pH:
à will induce haemoglobin to unload more O2.
à affinity of haemoglobin to O2 is lower.
à O2 dissociation curve shifts to the right = Bohr effect.

  16. Foetal haemoglobin:
à has a higher affinity for O2 than the maternal haemoglobin
à therefore, O2 dissociation curve lies to the left.

   17.  O2 must easily dissociate from the maternal haemoglobin to the foetal haemoglobin
   à therefore, easily transferred from maternal to foetal blood.

  18. Myoglobin:
   à  higher affinity for O2 than haemoglobin.
à  Therefore, dissociation curve lies to the left.
à has a lower affinity for O2 than myoglobin.

19.   In the muscle:
à O2 dissociates from haemoglobin
à transferred to myoglobin to be stored.

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