I'm hoping someone here can give me an answer to my question.

Why am I completely exhausted after exercise?

I did a very intense workout in the pool this morning for an hour and just barely made it up the two flights of stairs I had to climb to get out of the building. When I got home I was still too exhausted to even shower. Had to sit for about 45 minutes and wait for some degree of energy to return.

Is there anything I can do to avoid this feeling after a workout?