There is a very complex interrelationship between the glands of the endocrine system. The hypothalamus of the brain sends signals to the pituitary gland situated under the brain, and the pituitary gland is the master gland of the endocrine system. It can send out hormones that directly affect the body or it can send hormones that cause other glands to secrete their hormones, or even hormones that affect other organs. For example, it can secrete growth hormones, which directly affects the body. It can secrete adrenocorticotropic hormone, which affects the adrenal glands and causes them to secrete their hormones. It can secrete antidiuretic hormone, which affects the kidneys and causes them to retain water. There are multiple glands that secrete hormones and affect they body at the cellular level.