system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can’t we just make blood? Or better yet why can’t we make a blood replacement that is more efficient?
gpt
It's too complicated.

Blood is made of live cells:  

* Red cells handle the transport of oxygen and carbon dioxide, and they do this by reacting, one atom or molecule at a time, and taking the oxygen and/or carbon dioxide inside the cell, to transport it.  Oxygen specifically it's very reactive, it rusts materials, it burns them, it reacts very easily, so the red cells use complicated organic chemistry to "transport it safely" and deliver it to the cells in your body.

* White cells deal with intruders and infections, detecting all sorts of foreign bacteria, and eating them, and otherwise identifying the cells in your body correctly, and not hurting them.

* Blood has platelets and other organic compounds that make it clot, when you get a cut the blood clots / hardens, blocking the cut and more blood from seeping out.

* Blood transports hormones, nutrients from your digestive system, byproducts of digestion to be taken out by your kidneys, etc.

It's just too complex; we don't have chemistry that's complex enough to handle all these functions with just chemicals, and we don't have the biology that's advanced enough to create designer blood cells to handle these functions.