Human blood is red primarily because of hemoglobin, a protein in red blood cells that contains iron. When hemoglobin binds¹ with oxygen, it forms oxyhemoglobin, which gives blood its bright red color. In contrast, deoxygenated blood (with less oxygen) appears darker red. The iron in hemoglobin is crucial for oxygen transport and is responsible for the red hue².

血液呈红色主要是因为红细胞中的血红蛋白含有铁元素。当血红蛋白与氧气结合时，会形成氧合血红蛋白，使血液呈现鲜红色；而缺氧的血红蛋白则呈暗红色。铁的存在不仅帮助运输氧气，也决定了血液的红色外观。