If a hormone is released into the bloodstream and therefore comes into contact with many cells, what determines which cells in the body respond to the hormone?

THE CELL RECEPTOR DETERMINES WHICH CELL RESPOND TO AN HORMONE.

Explanation:

In the endocrine system, the hormones are usually released into the blood stream from where they traveled to their target organs. Even though the hormones meet a lot of cells on their way, they do not bind with just any cell. Hormones bind specifically to cells that have receptors that is meant specifically for the hormone. The cell receptor enables the binding together of the hormone and the cell membrane or the cell surface. After the binding, the hormone then elicit the needed response in the cell.  

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.

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