Carbon Dioxide Transport

Question 1

transport of carbon dioxide

Answer 1

Co2 is taken up by blood in tissue capillaries and transported to the lungs where it is released into the alveoli.

Co2 Uptake :- Co2 diffuses from the cells (PCO2 50 mmHg) into tissue fluid (Pco2 46 mmHg) and then into blood in tissue capillaries (Pco2 40 mmHg) along the pressure gradient.

Transport in Blood :- In the blood, Co2 is transported in different forms as follows :

• in dissolved form in plasma: 7%
  the solubility coefficient of co2 is 20 times that of oxygen
• only 0.3 ml of 100 ml of blood
• due to absence of carbonic anhydrase in plasma, carbonic acid formation is less in plasma.
• however a small amt of co2 combines with plasma proteins to carbamino-protein complexes

— transport as bicarbonate ions
- about 70%
- H2CO3 then quickly dissociates to form H and HCO3 ions. H ions are buffered by Hb to form HHB.
- Some HCO3 ions combine with K ions to form KHCO3 and are transported in this form in the RBCs.
- Majority of the formed HCO3 ions diffuse out into plasma and combine with Na ions to form NaHCO3 which is transported in plasma.
- When HCO3 ions diffuse out of cells, chloride ions move into the cells to maintain electro neutrality.
- This is called Hamburger’s Chloride shift and is brought about by a protein called bicarbonate chloride carrier
protein.
- When this venous blood reaches lungs, opposite changes take place. - Oxygenation of Hb releases H ions which
combine with HCO3 ions to form H2CO3 which the associates to form CO2 and H2O, Co2 diffuses out into the
alveolar air.
- HCO3 ions from plasma enter the red cells continuing the above reaction and Cl ions diffuse out of cells

— as carbamino compound
carbon dioxide in red cells also reacts with free amine groups (nh2) of haemoglobin to form carbamino haemoglobin (hbNHCOOH)

CO2 + HbNH2 ——–> HbNHCOOH

deoxygenated haemoglobin binds more co2 than the oxygenated haemoglobin. about 23% of co2 is transported in this form.

Question 2

co2 dissciation curve

Answer 2

co2 dissciation curve is plotted in a similar fashion to that of oxygen.
but unlike that of oxygen this is related Directly to the pco2. therefore co2 dissociation curve is almost a straight line in the normal arterial pco2 range.

major features:
1) linear relationship between co2 content and pco2 : within the physiological range of pco2 (40-46) the curve is more linear and steeper.

2) transfer of large quantity of co2 : because of shape of tbe curve large amounts of co2 can be loaded and unloaded from the blood with a small change in pco2. also blood can hold much more co2 than o2

3) inverse relationship between po2 and co2 content:
the co2 dissciation curve shifts downward and to the right at a higher po2. the effect of change in po2 (or oxyhaemoglobin saturation) on co2 content is known as the haldane effect. the relationship between po2 and co2 content is inverse.
at any level of pco2, the total co2 content increases as po2 falls.

4) the advantage of this is that it allows the blood to load more co2 in the tissues and unload more co2 in the lungs.
- thus when blood enters capillaries in the tissues and releases o2 the co2 carrying capacity increases and conversely when blood enters pulmonary capillaries and picks up oxygen, the co2 carrying capacity decreases.

Question 3

haldane effect

Answer 3

when blood passes through pulmonary capillaries, oxygen diffuses into the blood and forms oxy hb
- oxygenation of hb shifts the co2 dissociation curve to the right and hb begins to lose co2

• thus loading of o2 facilitates unloading of co2
• this is haldane effect, named after the great ancient Respiratory physiologist john scott haldane
• as deoxyhaemoglobin binds h+ more actively than that of oxyhaemoglobin binding of o2 with haemoglobin decreases its affinity for co2