Let's break it down. What is plasma? What is plasma volume?
Plasma is defined as the liquid portion of blood. It's the portion in which blood cells are suspended and other substances (proteins, sugars, ions, hormones, CO2, etc) are dissolved. Plasma volume is the amount of plasma, and it's important to know that it can vary. It can be increased or decreased. The activity of certain hormones induce such increases/decreases in plasma volume. Take ADH for example. ADH increases the amount of water reabsorbed at the collecting duct of nephrons. More water in the blood means a relatively greater plasma volume. Similarly, aldosterone, a steroid hormone, functions by increasing sodium absorbtion and potassium secretion in the kidney, this too leads to an increase in plasma volume (osmosis).
