The endocrine system is a system of glands that secretes hormones that affect the body at the cellular level. The hypothalamus in the brain send signals to the pituitary gland that is underneath the brain. The pituitary then sends hormones into the bloodstream, and the hormones from the pituitary either affect the body or causes other glands to secrete their hormones, which then affect the body. For example, the pituitary gland can secrete growth hormone, which goes off and affects the body directly, or it can secrete adrenocorticotropic hormone, which affects the adrenal glands, and those glands secrete their own hormones. Those hormones affect the body at a cellular level.