Willy, good evening I hope you are well and having a great night.  With regard to your question, consider the following as it pertains to a proper warm up.  During exercise, blood flow patterns change. Through the action of the sympathetic nervous system, blood is redirected away from areas where it is not essential to those areas that are active during exercise. Only 15 to 20% of resting cardiac output go to muscle, but during exhaustive exercise the muscles receive 80 to 85% of the cardiac output. This shift is accomplished by reducing blood flow to the kidneys, liver, stomach and intestines, which cause the feelings of nausea associated with intense workouts. Also, the red blood cells which carry oxygen to the working muscles as well as the entire body gets diverted to the working muscles which leads to feelings of being light-headed due to lack of sufficient oxygen to other systems. In addition, most physiologists agree that moderate to intense exercise affect intestinal absorption and blood flow to the gut. Since substances must be absorbed from the intestinal wall into the blood, reductions in blood flow suggest fewer opportunities for absorption. Abdominal cramping is an indication of an interruption in oxygen (and hence blood supply) to the gastrointestinal area when the body prioritizes oxygen to the working muscles instead of the gastrointestinal area.

We hope this helps.  Take care and have a great Wednesday night.