They're red because they contain a special protein called hemoglobin which is very concentrated inside the cell.

红细胞的红色源于,其内部一种叫做血红蛋白的特殊蛋白质,血红蛋白在细胞内的浓度很高

耶鲁公开课 - 生物医学工程探索课程节选

Hemoglobin, as you know, is an oxygen carrier and so red blood cells concentrate in bags of hemoglobin which circulate in your blood and they carry oxygen.

众所周知,血红蛋白可以携带氧气,含有大量血红蛋白的红细胞,在全身循环,以携带氧气

耶鲁公开课 - 生物医学工程探索课程节选