The Endocrine system.
How:
Hypothalamus and Pituitary Gland
The hypothalamus is the link between the nervous and endocrine systems. It is a section of the brain responsible for maintaining homeostasis. The hypothalamus sends out nerve messages that control sleep, hunger, and other basic body processes. It also controls the pituitary gland, which is right next to it in the brain.
The pituitary gland is a pea-sized structure attached to the brain. It's considered the "master gland" because it produces hormones that control other endocrine glands. It also regulates processes such as growth, blood pressure, and water balance.
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