Blood plasma is the yellow-colored liquid component of blood, in which blood cells are suspended.
Hypovolemia (also hypovolaemia or oligemia) is a state of decreased blood volume; more specifically, a decrease in the volume of blood plasma.
It is thus the intravascular component of volume contraction (or loss of blood volume due to things such as bleeding or dehydration), but, as it also is the most essential one, hypovolemia and volume contraction are sometimes used synonymously.
see Plasma cell.