Hormones: provide ways for cells to communicate with one another.

Hormone- a chemical messenger with a specific shape and travels in blood stream.

-go to target cells or target organ.

When it gets to where it needs to go it will activate a gene in the cell to make other compounds.

ex: pituitary gland: gland at base of brain make hormone LH (lutenizing hormone), go through bloodstream to stimulate ovary to produce yellow tissue, to produce hormone progesterone, which maintains the thickness of the uterus lining.