There must be some relationship between "passion", adrenalin and endorphin. What could this be?

Let's define first what is passion, adrenalin and endorphin.

Passion is an emotion, a strong liking or desire for or devotion to some activity, object, or concept.

Adrenaline or Epinephrine is a "fight or flight" hormone which is released from the adrenal glands whenever danger threatens. When secreted it floods out the endocrine gland into the bloodstream to instantly prepare the body for action in emergency situations. The hormone boosts the supply of oxygen and energy-giving glucose to the muscles making the individual more mentally alert and physically strong; only vital bodily processes occur.

Endorphins (or more correctly Endomorphines) are endogenous opioid biochemical compounds. They are peptides produced by the pituitary gland and the hypothalamus in vertebrates, and they resemble the opiates in their abilities to produce analgesia and a sense of well-being.

There seems to be a cycle of influence among the three - passion, endorphine and adrenalin. Passion is the feeling itself. It started by a stimulation (by a stimulus). If the stimulation is right, the person achieved an endorphine rush. Combine with the rush of adrenalin, it becomes passion.

combination of chemical and physical response.

heighten physical activity can increase endorphin levels as well as adrenalin levels

so can the thrills of passion

so imagine the rush of passion mixed with someone who is ready for action.

another benefit of staying in shape and keeping the blood flowing.

The endorphins did it.

That's part of how the parasympathetic nervous system works.

...3 is better than 1

hormones

summing up your three keywords, it is physical attraction, or lust.

instinct

Lust