Many animals have colored substances in their blood or hemolymph respiratory pigments.
These substances are able to combine with oxygen gas, significantly increasing the body's ability to transport this gas. The main respiratory pigments present in animals are the hemoglobin and the hemocyanin.
If human blood had no hemoglobin it would be able to carry only 2% of the oxygen gas the body needs.
Hemoglobin is a protein consisting of four polypeptide chains associated with a chemical group called heme group, what contains iron. One hemoglobin (Hb) molecule is capable of combining with four oxygen gas molecules, forming oxyhemoglobin.
Hb + 4 O2 -------> Hb (O2)4
Hemoglobin is present in the blood of all vertebrates, housed inside the red blood cells. Some invertebrates such as certain annelid, nematelminth, mollusc and arthropod species have dissolved hemoglobin in the hemolymph.
Hemocyanin is a protein that contains copper atoms in its composition. It is found in many species of mollusks and arthropods dissolved in hemolymph. When combined with oxygen gas molecules, hemocyanin becomes bluish. In its free form, however, it is colorless.
In humans and other mammals, about 5 to 7 percent of the carbon dioxide released by tissues dissolves directly in blood plasma and is thus transported to the lungs. Another 23% are associated with the amine groups of the hemoglobin itself and other blood proteins, being transported by them.
Most of the carbon dioxide released by the tissues (about 70%) penetrates the red cells and is transformed by the action of carbonic anhydrase enzyme , in carbonic acid, which subsequently dissociates into H + ions and bicarbonate.
CO2 + H2O --------> H2CO3 --------> H+ + HCO3
H ions+ are associated with molecules of hemoglobin and other proteins, while bicarbonate ions diffuse into the blood plasma, where they aid in maintenance of the degree of acidity of the blood.
|Dissolved in Plasma||7%|
|In the form of baking soda||70%|
A process opposite to what occurs in the tissue capillaries happens in the lungs. Then the carbon dioxide molecules and the ions H+ dissociate from proteins. Inside the red blood cells H ions+ combine with bicarbonate, reconstituting carbonic acid. This by action of the carbonic anhydrase enzyme, is then decomposed into carbonic gas and water.