What is the color of blood? While the instinctive response for many of us may be “red,” the truth is that nature’s artistry extends far beyond our imagination. Let’s delve into the astonishing world of blood's vibrant hues – where blues, purples, greens, and even the absence of color thrive [1, 2]. Determinant of Blood Color: Respiratory Pigment The key to this breathtaking diversity of animal blood colors lies in the type(s) of respiratory pigment in our blood. Respiratory pigments are metal-containing proteins that help transport respiratory gasses such as oxygen and carbon dioxide (footnote 1). The wellknown hemoglobin in our blood is just one example of such a pigment, along with lesser-known ones like chlorocruorin, hemocyanin, and hemerythrin. These pigments differ in their chemical structures and use different metal complexes to bind oxygen molecules. Together, these subtle variations cause each pigment to absorb and reflect unique wavelengths of light, resulting in a vast array of blood colors. Be careful: When we talk about blood color, we are referring to the color of the oxygenated pigments rather than the deoxygenated ones. When oxygen binds to the metals within these pigments, it alters the threedimensional structures of the whole pigments and, in some cases, changes the oxidation states of the metals [2]. This, in turn, results in a shift in their light absorption and reflection spectra, changing the blood colors we see. Blue Blood In certain invertebrates such as squid, octopus, lobster, and horseshoe crab, the presence of the respiratory pigment hemocyanin gives their blood a distinct blue color [2]. Unlike hemoglobin, which utilizes iron (Fe2+) to bind oxygen, hemocyanin relies on copper (Cu2+) for oxygen transportation in these marine animals. The copper (II) ion strongly absorbs red light while reflecting blue light, resulting in the characteristic blue appearance of their blood. These blue-blooded invertebrates have evolved to use hemocyanin for two reasons [3]. Firstly, the effectiveness of hemoglobin to transport oxygen decreases at low temperatures, such that hemocyanin outperforms hemoglobin in the deep sea. Secondly, hemoglobin exhibits superior efficiency in binding oxygen compared to hemocyanin but only in oxygenrich environments, because the binding of every new oxygen molecule can facilitate that of the next oxygen molecule, until the four vacancies in the hemoglobin are taken up. However, in oxygen-deprived settings, hemoglobin’s oxygen-binding efficiency diminishes, and hemocyanin proves to be more effective. Hence, the switch to hemocyanin gives these marine animals an edge in obtaining oxygen in the ocean. Purple Blood Blood naturally takes on a purple color in lamp shells and certain marine worms [1, 2]. These marine invertebrates use neither hemoglobin nor hemocyanin, but hemerythrin. While hemerythrin, like hemoglobin, uses iron as the oxygen-binding material, it gives a violet-pink color instead of bright red in its oxygenated state, and is colorless when no oxygen is bound. Green Blood For some earthworms and leeches, green blood is the norm [1, 2]. These invertebrates contain chlorocruorin, another iron-based pigment that makes their blood look green. While chlorocruorin is typically linked to green hues, its color is concentrationdependent – lower amounts appear green, but higher concentrations cause the pigment to take on red coloration. By Helen Wong 王思齊 絢爛多彩的動物血液 The Astonishing Spectrum of Nature’s Palette 大自然的調色盤: Animal Blood Colors
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