An element that speeds up chemical reactions is called a/an
a. substrate.
b. enzyme.
c. calorie.
d. solvent.

I think it is enzymes.Inside our body most of the chemical reactions are speeds up by enzymes.These enzymes decreases the minimum energy that is required for the reaction to happen. ;-]. Actually it works like a catalyst.

decreased osmolality of extracellular fluids

Explanation:

When there is decreased osmolality of extracellular fluids, there will be Volume depletion, or extracellular fluid (ECF) volume contraction, which occurs as a result of loss of total body sodium. Causes include vomiting, excessive sweating, diarrhea, burns, diuretic use, and kidney failure. In this way, sodium loss always causes water loss.