Under normal circumstances, low plasma volume is sensed in kidney (and in heart and aorta) and stimulates increase in plasma renin activity (renin), angiotensin II (A-II), and aldosterone (ALDO). Increase in plasma renin activity and aldosterone promotes salt and water retention, which leads to increase in extracellular fluid volume and plasma volume.
There is failure to sense and appropriately respond to low plasma volume. There is no appropriate increase in plasma renin activity, angiotensin-II, and aldosterone given the hypovolemia. Because plasma renin activity and aldosterone are not increased, salt and water retention is not increased, and plasma volume is not increased.