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Blood typically carries small amounts of molecular oxygen (O2) dissolved in the plasma and large amounts chemically combined with hemoglobin. Partial pressure depends only on physically dissolved oxygen, which determines how much oxygen will combine with hemoglobin. The blood needs an O2 carrier in humans because this gas is not sufficiently soluble in the blood plasma to satisfy the body's requirement. At 37 ºC, a liter of blood only dissolves 2.3 mL of O2. However, one liter of blood contains 150 g of hemoglobin, and since each gram of hemoglobin dissolves 1.34 mL of O2, 200 mL of O2 are transported per liter of blood. This quantity is 87 times more than what plasma could carry by itself. Without an O2 carrier like hemoglobin, blood would have to circulate 87 times faster, requiring a high-pressure pump, turbulent flow, and a considerable ventilation-perfusion mismatch. This is why other transporter molecules exist in various organisms, giving each of them a representative color. This is how it is in the human being that blood is red; in spiders, crustaceans, mollusks, octopuses, and squids, the blood is blue; in some segmented worms, some leeches, and some marine worms, there is green blood; finally, in marine worms and brachiopods, the blood is violet.
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