Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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Such is the history of the changes which blood-pigment undergoes within the living body. To a certain extent its chemistry can be followed in the laboratory; but it must be remembered, when we are treating of the chemistry of a substance as complex as hæmoglobin, that the products which can be obtained from it in the laboratory are not necessarily those into which it is transformed in the body. In the laboratory oxyhæmoglobin is easily changed into methæmoglobin, a substance of the same percentage composition, but with its oxygen more firmly fixed. Methæmoglobin can be decomposed into a proteid substance and hæmatin. Hæmatin, when acted on by reducing agents, becomes hæmochromogen. Hæmochromogen, when subjected to such a reducing agent as a mixture of tin and hydrochloric acid, gives rise to coloured bodies closely resembling bile-pigments—not as they are secreted by the bile, but as they appear in the urine. It is impossible to prove that the changing colours of a bruise indicate a sequence of chemical transformations from hæmoglobin to bile-pigment, but it is not improbable that such a description is correct. The test commonly used to ascertain the presence of bile-pigment, i.e., bilirubin, is the play of colours which it exhibits when oxidized by fuming nitric acid. From yellow it turns to green, to blue, and then to purple, more or less reversing the colours of the bruise. It is fairly certain that effused blood undergoes changes along lines which, if not identical with those through which blood passes on its road to bile-pigment, are at any rate very similar.