The mention of pigments in living organisms ordinarily brings to mind the varied colors of plants and their flowers rather than the colored compounds in tissues and body fluids of animals. Nevertheless pigmented compounds do occur throughout the body and either by their own virtue or because of accompanying substances they are of considerable significance. The characteristic behavior of hemoglobin in combining with oxygen in the lungs and releasing it in the tissues together with its concomitant change in base binding power is of fundamental importance to life. The pigment in the urine bears some relationship to the metabolism of the organism, the quantity excreted daily being augmented with an increased metabolism and diminishing when the metabolism is decreased. The bile pigments usually are looked on as hemoglobin which has been altered prior to excretion by the liver, but there are data which suggest that these compounds exert an influence
VITAMIN A IN THE RETINA. JAMA. 1931;97(9):646–647. doi:10.1001/jama.1931.02730090060010
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