However, sympathetic nerve stimulation also constricts arteries, which increases blood pressure. The increase in pressure forces the heart to work harder and use more oxygen, which is thought to cause further deterioration of the heart over time. The body then decreases the blood flow to the kidneys. The decreased blood flow causes the kidney to release an enzyme called renin . Angiotensin II is a powerful constrictor of both arteries and veins and stimulates the adrenal gland to secrete a hormone called aldosterone.
Aldosterone causes the kidneys to retain salt and water which increases blood volume.
The increased blood volume helps to maintain cardiac output by increasing the filling of the heart. However, the increased blood volume, along with the vasoconstriction , also increases blood pressure. This increased pressure causes edema and an increased workload, which may further weaken the heart. This causes the kidney to retain more fluid, which increases blood volume and helps cardiac output, but also increases blood pressure, which makes the weakened heart work even harder. There are also changes in the heart muscle itself.