People with congestive heart failure (CHF) are advised by doctors to restrict fluid intake. For example, while 8 cups of fluid per day is recommended for healthy individuals, people with CHF are recommended not to exceed 4 cups a day. The explanation, according to a medical student friend, is so that the blood volume is reduced, and therefore the heart doesn't have to work that hard. I am baffled by such an explanation. According to physics, the mechanical power output of the heart is given by the volume of blood passing through the heart per second times the pressure difference between the inlets and outlets of the heart. If I understand correctly, a reduced blood volume affects neither the hemoglobin concentration nor the blood pressure due to homeostasis, so a reduced blood volume shouldn't alter the heart's workload. Am I mistaken or is my friend's explanation wrong?