why is it better to have haemoglobin inside the rbc rather than free in the plasma?

你的问题是为什么血红蛋白在红细胞而不在血浆中.
这是因为一般说来血浆就是我们所说的血液,而血红蛋白是我们身体的所需O2的载体,其由造血干细胞分化而来,存在于血细胞中.而血浆中,除了血细胞以外,还有大量的水,纤维蛋白原（凝血用）,血小板（纤维蛋白原在伤口处合成纤维蛋白,就像给伤口处编了一张网,而血小板就是填充物,填补缺口）等等；而我们说的血清就是没有纤维蛋白原的血浆.
英》在线译的,不准,看中文就行了
This is because the plasma in general is what we call blood,and hemoglobin is required for our physical carrier O2,and its differentiation from hematopoietic stem cells from cells found in blood.And plasma,in addition to blood cells,there are a lot of water,fibrinogen (clotting purposes),and platelets (fibrinogen synthetic protein in the wounds,as to the wounds for a network,and platelet fillers is to fill the gap) and so on; and what we are arguing is that there is no serum plasma fibrinogen