In single celled animals like protozoans the respiratory gases are directly diffused into or outside the body but in complex animals like mammals the respiratory gases (O2 and CO2) are transported with the help of circulatory system in which the blood is circulatory fluid (in most of the cases particularly in higher animals).
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Blood itself is not the carrier of the respiratory gases but contains a respiratory pigment which actually acts as the carrier of the respiratory gases because this pigment has a special affinity for respiratory gases.
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The respiratory pigment in its nature varies in different animals. The different respiratory pigments found in the animals are as follows:
Of these the first two pigments are found either in solution or in corpuscles in the plasma of the blood and are widely distributed where as the rest two are in solution in the plasma of the blood.
In addition to the above mentioned pigments there are certain other miscellaneous pigments which are sporidically met in the animals.
These are pinnaglobio, echhwciirome, vanadium and molpadinro, their functions are not clear so far.
1. Haptoglobin:
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It is the most important and commonest respiratory pigment which is widely distributed in the animals.
It is found in the dissolved condition in the plasma of the blood of molluscs, annelids and arthropods but in the corpuscles (R.B.C.) in the chordates. It is even found in the body fluid of some protozoans.
It is also found in the muscles of birds and mammals and is known as myoglobin.
Haemoglobin is a red coloured pigment formed of two distinct components, the haeme and globin.
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The haeme is iron containing component or the prosthetic group of iron while the globin is the conjugated protein. Each molecule of haemoglobin contains four haeme groups, each of molecular weight 872.
An oxygen molecule may unite reversibly by combining with one of the four iron atoms that is attached by valency bonding to four pyrrole groups that make up the haeme molecule.
A remaining sixth valency bond of iron apparently attaches with the globin. The percentage of haemoglobin in the blood varies from animal to animal.
In the human blood every 20 cc of blood approximately contains 10 gms of haemoglobin.
The most important property of haemoglobin is that it has a special affinity towards respiratory gases, O2 and CO2.
It combines at normal temperature with oxygen- and carbon dioxide when it comes in contact with them and also readily dissociates itself from them.
The haemoglobin when combined with oxygen is called oxy- haemoglobin. The reaction is represented as follows :
Hb + O2ß—————————–àHbO2
Haemoglobin+Oxygen Oxyhaemoglobin
Oxyhaemoglobin on reaching at cellular level, where partial pressure of the oxygen is very low, dissociates into free oxygen and reduced haemoglobin.
This is really a reduction process. Thus the reaction is reversible as shown above.
The amount of oxygen to which haemoglobin combines depends upon the partial pressure of oxygen available.
At maximum oxygenation human haemoglobin will combine with upto four oxygen molecules which it does at a partial pressure of oxygen of approximately 70 mm Hg.
The saying mean of this is that the haemoglobin fully combines with oxygen at that partial pressure at which it is available in the environment.
Similarly it must dissociate at the partial pressure of oxygen existing in the animal tissues.
2. Haemerythrin:
This is also a iron containing pigment of violet colour, found commonly in the polychaetes, worm Magelona, in the sipunculid worms Sipunculus, Dendrostomum and Glofingia.
This is found in corpuscles. It is less efficient in its oxygen carrying capacity when compared with haemoglobin.
3. Haemocyanin:
This pigment is next in importance to haemoglobin. It is found in many crustaceans and some mollscus such as cephalopods (squids).
It is not confined to corpuscles like haemoglobin but exists freely in the blood plasma.
This is colourless or blue copper containing pigment which is capable of accepting and transporting oxygen by conversions of part of the cuprous to cupric state.
The physiology and biochemistry of this pigment have been studied in detail by Redfield. It has the same shape of dissociation curve as of haemoglobin but is not able to transport equivalent volumes of oxygen.
4. Chlorocruorin:
This is green coloured pigment found in polychaete annelids particularly of the families Sabellidae and Serpulidae.
It is green in dilute solutions but red in concentrated ones. It occurs in dissolved condition in plasma.
This pigment was discovered by Milne-Edwards in polychaetes in the year of 1838. Later Dujardin and Quadrifages confirmed the occurrence of this pigment in other annelids.
Its respiratory properties have been studied by Lankester and according to him the pigment exists in two forms, one oxidized and the other reduced.