While the nervous system uses electrical signals, the endocrine system uses chemical messengers called hormones. Secreted by glands like the pituitary, thyroid, and adrenals, hormones travel through the bloodstream to regulate metabolism, growth, sleep cycles, and reproduction. 6. The Cardiovascular System
: Your heart is a hardworking muscle that beats roughly 115,000 times a day to keep blood moving. The Filter
The most complex structure known to humanity is not a particle collider or a space station; it is the three-pound organ inside your skull: the brain.