 Dear students, in this topic we shall discuss the respiratory pigments. The substances which transport the respiratory gases, oxygen and carbon dioxide in the blood are known as respiratory pigments. Respiratory pigments are complexes of protein and metallic ions. They have characteristic colors that change with the level of oxygen attached or unattached with them. There are many types of respiratory pigments found in animal kingdom. The most common are the hemoglobin, myoglobin, hemocyanin, hemerythrin and chlorochlorine. We shall discuss these pigments one by one. First we are going to discuss hemoglobin in detail. Hemoglobin is the most common respiratory pigment which is found in all vertebrates and many invertebrates. It is colored bright red when it is loaded with oxygen while its color is dark maroon when it is deoxygenated. It is known as a refer to as oxyhemoglobin when oxygen is bound and is known as deoxyhemoglobin when oxygen is not bound or absent. Dear student, a molecule of hemoglobin has four iron containing heme groups which are attached to a tetrameric globin protein that has four chains, two alpha and two beta chains. Iron in the hemoglobin is in ferrous state that is a valency of plus two. Dear students, the presence of hemoglobin in blood increases the oxygen binding capacity of blood 70 times. For example, a blood with hemoglobin in it, its capacity to carry oxygen is even 20 millilitre oxygen per 100 ml of blood at 30, 70 centigrade and at 1 atmospheric pressure. Without hemoglobin, only plasma can carry only 0.3 millilitre of oxygen. Dear students, oxygen binding does not affect the ferrous state of oxyhemoglobin. When oxyhemoglobin is formed, ferrous iron remains in the ferrous form. But there are some substances which if bind to hemoglobin, they change the state of this ferrous iron into ferric iron. This is oxidation of hemoglobin and it results in the formation of methemoglobin. Methemoglobin is non-functional, it does not bind and carry any oxygen. If there is more concentration of methemoglobin in blood, it impairs the transport of oxygen by the blood. Red blood cells have an enzyme called methemoglobin reductase. It reduces this type of methemoglobin and converts the iron into ferrous iron. This is methemoglobin reductase enzyme. There are many compounds which increase the level of methemoglobin in the blood. The affinity of hemoglobin for carbon monoxide is about 200 times greater than it is for oxygen. If carbon monoxide is present in the air, it displaces oxygen and saturates hemoglobin. As a result, carboxyhemoglobin is formed. Carboxyhemoglobin also reduces the transport of oxygen from the blood. As a result, anoxia is caused, it impairs brain functions and it may prove lethal. Dear students, the second respiratory pigment, myoglobin, has much higher oxygen affinity than the hemoglobin. It is not found in the blood or any circulatory fluids. It is found in vertebrate muscles and is used to store oxygen. These stores are used in the times of necessity or need. Hemocyanin is a large copper containing respiratory pigment which transports oxygen in many mollusks and arthropods. It is coloured light blue when it is oxygenated and colour less when it is deoxygenated. The reason is that such animals who have hemocyanin as a respiratory pigment are caused by their blood to be blue. Hemerothrin is another respiratory pigment which is coloured violet pink when it is oxygenated. It is colour less when it is deoxygenated. It is used for oxygen transport in only one member of phylum anelida while the marine invertebrate phyla priapolida and brekyopoda are found in all of these members. Dear students, the chelorochlorine respiratory pigment is a dichromatic compound which appears in dilute solutions in green colour and is seen in concentrated solutions in light red colour. It is a respiratory pigment of many anelids, particularly marine polyketes which have chelorochlorine respiratory pigment.