If you crash a bicycle and cut your knee, it bleeds. You can apply pressure to the wound or otherwise aid in clotting it, but you can't fully control the blood. You can't think, "Body! I command you not to bleed!" Nor can you directly say, "I choose not to bleed" through pure will alone.
This is easy enough to understand. We don't have direct control over our blood. We can apply some measure of indirect to it -- taking aspirin might thin the blood, breathing deeply and relaxing might slow the pulse and the flow of blood slightly -- but we do not have direct and instant control over the flow of our blood.
That's our blood. It's quite a personal thing, when you think about it.
At the same time, there's a view that we have full control and choice over our actions in a given situation.
I no longer believe this to be the case.