Are chemical messengers used by the brain to communicate from one neuron to another. Neurotransmitters
dendrites
soma
axon

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)

Small intestine because that is the organ which absorbs nutrients