1. Removal of oxygen from respiratory surface:
Oxygen from the external environment is diffused into blood through the respiratory surface which is permeable to respiratory gases.
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A small quantity of diffused oxygen is carried by the respiratory pigment present in the blood.
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It has been estimated that oxygen capacity of haemoglobin in cubic centimeters per 100 ml of blood is 9 .0 in fishes, 12.0 in amphibians, 9 .0 in reptiles, 18. 5 in birds and 25. 0 in mammals. In annelids and molluscs it is 6 .5 and 1-5 respectively.
The quantity of oxygen carried by haemoglobin is affected by the partial pressure of the oxygen in the blood plasma.
As the oxygen diffused into the blood, it combines with the respiratory pigment of the blood to form a temporary compound.
If the respiratory pigment is haemoglobin than it combines with oxygen to form oxyhacmcglcbin. Thus oxygen is carried by the blood in the form of oxyhaemoglobin.
2. Diffusion of oxygen from the blood into the tissue cells:
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As it reaches the different body tissues which are in need of oxygen, the oxyhaemoglobin present in the blood dissociates into free oxygen and reduced haemoglobin which is ready to combine with molecules of oxygen.
The oxygen, so liberated, is used in the oxidation of digested food to liberate energy in the tissue cells.
The decomposition of oxyhaemoglobin in the tissue cell is caused due to low partial pressure of the oxygen and high partial pressure of carbon dioxide in the tissue cells.
Oxygen dissociation curves:
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Each haemoglobin molecule, as being composed of four globin-haeme units, combines with four molecules of oxygen, enabling whole blood to carry about 60 times as much oxygen as could be transported by an equal volume of water or plasma.
The amount of oxygen carried by the haemoglobin molecules in the blood is related to the partial pressure of oxygen (Po,) in the blood.
By exposing blood at a constant pH to different partial pressures of oxygen and, following equilibration, determining the oxygen content of the red cells, a curve representing the combining capacity at varying partial pressures of oxygen is obtained.
Oxygen content may be expressed as per cent saturation. If, after exposure to pure oxygen, the oxygen content of haemoglobin was found to be 19 ml/ 100 ml of blood and this blood is said to be 100% saturated, a similar sample exposed (o a lower Po, and containing 9 5 ml 0,/100% ml would be 50% saturated. Such plots are called oxygen dissociation carves.
These curves serve to express the importance of oxygen tension on both loading and unloading there is a series of curves which are sigmoid in shape rather than linear expressing that at high Po2 (100 mm Hg) the blood is 100 saturated.
As the Po2 drops, the oxygen in, the haemoglobin molecules are given up as a result of which the blood is found less saturated.
This is believed to be due to the so-called haeme-haeme interaction, which phenomenon, although not yet fully clear, seems to occur when oxygen is taken up and the conformation of all the four subunits of haemoglobin, or more specially the /S-chains, is altered.
There is then some separation of the subunits which, in turn, increase the rate of uptake of oxygen by the four-haeme groups.
The curves also show the influence of different carbon dioxide pressures in the dissociation of oxyhaemoglobin of human blood.
Thus, if curve O is followed from right to left, when there is no carbon dioxide the blood is fully saturated with oxygen at 100 mm oxygen pressure; at 40 mm oxygen pressure it is about 96% saturated; at 20 mm ; it is 83% saturated ; and at 0 mm it does not contain oxygen.
This shows that when the Po, increases, the above reaction proceeds to the right and oxyhaemoglobin is formed in great amount, as in the lungs.
When Po, decreases, as in the tissues, the reaction proceeds toward the left and more oxygen is liberated.
As the carbon- dioxide partial pressure increases the dissociation curves are shifted to the right.
This states that if more carbon dioxide is present the haemoglobin can hold less oxygen, a phenomenon known as the Bohr effect, after its discoverer.
The decrease in oxygen saturation with an increase in carbondioxide partial pressure may be due to some change in the conformation of the α and β-chains of the globin moiety of haemoglobin by addition of hydrogen ion concentration associated with the increase in the carbon dioxide partial pressure.
Comparing the same curve (curve O) with no carbon dioxide and the one at 40 mm carbon dioxide (curve 40), we see that, at 100 mm oxygen partial pressure, both are practically completely saturated with oxygen; i.e., the haemoglobin is almost all present as oxyhaemoglobin.
At 90 mm oxygen partial pressure, which is the pressure in the arteries, they are still nearly the same, curve O being about 99% and curve 40 about 95% saturated.
At 40 mm oxygen partial pressure, which is the pressure of veins, the 0-curve still shows about 95% saturation while curve 40 is down to 72% saturation; i.e., the presence of 40 mm carbon dioxide has caused the oxyhaemoglobin to dissociate 23% of its oxygen.
This shown that the effect of carbon dioxide pressure is just opposite that of oxygen pressure, and both has desirable physiologic effect.
In the tissues with low oxygen and high carbon dioxide partial pressures, oxyhaemoglobin dissociates more readily and oxygen becomes available for tissue needs.
In the lung the Po, is high and oxyhaemoglobin is formed readily despite the high carbon dioxide pressure.
pH, temperature, and the presence of electrolytes are also some other factors which influence the transport of oxygen by haemoglobin to a great extent. Slight decrease in pH (more acidic) increases the dissociation of oxyhaemoglobin.
Thus the slightly more acid pH in tissues due to carbon dioxide favours the release of oxygen to the tissues.
The slight increase in temperature and the presence of electrolytes also have the similar effect on the dissociation of haemoglobin and transport of oxygen by blood
Very recently Beneschs, 1968, found that there are certain organic phosphate compounds, mainly diphosphoglyceric acids, which have a marked effect on the oxygen-binding power of haemoglobin.
The high concentration of 2, 3 diphosphoglycerate (DPG) in the erythrocytes initiates the readily dissociation of oxyhaemoglobin.
Conversely, the low-concentration of DPG causes the more production of oxyhaemoglobin. DPG in the erythrocytes is formed from glucose and phosphate.