Final answer:
ANP is a hormone secreted by the heart that has the opposite function to the hormone secreted by the outermost zone of the adrenal cortex. It reduces water reabsorption by the kidneys and prevents water retention.
Step-by-step explanation:
ANP, or atrial natriuretic peptide, is a hormone secreted by the heart in response to increased blood volume. It acts on the kidneys to reduce the reabsorption of Na and water, leading to their excretion in the urine. This hormone also reduces the release of renin and aldosterone by the adrenal cortex, which further prevents water retention. Thus, ANP has the opposite function to the hormone secreted by the outermost zone of the adrenal cortex, which promotes water retention.