A gland is just any structure that makes and secretes a hormone and the master gland in your body is the pituitary which produces many hormones that signal other glands like the thyroid, parathyroid, adrenal and pineal glands to make their own hormones, the endocrine system also includes a few organs like the gonads, the pancreas and the placenta in pregnant women, all of which have some other non-hormonal functions and are made up of multiple tissue types and technically the hypothalamus in your brain is in the endocrine club too since in addition to all of its busy brain duties, it does produce and release hormones so thanks to all these glands and organs. You’ve got these hormones diffusing through your blood doing all sorts of different things but the thing to remember about them is that a hormone can only trigger a reaction in specific cells, their so-called target cells that have the right receptors for it, so just like some keys can open many locks while others only work with one, so too can the hormone-target cell relationship either be widespread or localized, you’re probably gonna want an example of that so your thyroid at the bottom of your throat produces the hormone thyroxine which stimulates metabolism and binds to receptors in most of the cells in your body but your pituitary which is nestled all comfy under your brain produces follicle stimulating hormone which helps regulate growth and trigger sexual maturity and it only targets specific cells in the ovaries and testes, so how do hormones bind to their target cells, well chemically most hormones are either made of amino acids including their more complex structures like peptides or proteins or they’re derived from lipids like cholesterol and this is key because a hormone’s chemical structure determines if it’s water soluble like most amino acid based ones are or lipid soluble like steroids are. Solubility is important because your cell membranes are made of lipids, that means that water soluble ones can’t get across them so target cells for those kinds of hormones have receptors on the outside of their membranes, lipid-soluble hormones on the other hand can just basically glide right through that cell membrane so their receptor sites are inside the target cells, either way, when a target cell is activated, the hormone alters its activity by either increasing or decreasing some of its functions usually with the goal of maintaining your body’s homeostasis in one way or another, so if hormones are keeping your body in balance, what’s putting your body out of balance, i don’t know….could i interest you in some pie? if you have a couple of nice generous helpings of strawberry rhubarb pie and just to make things interesting let’s say they’re a la mode, your blood glucose level is going to go through the roof and the pancreas regulates your blood sugar by releasing two different hormones, insulin and glucagon, once you have a belly full of that pie, beta cells in your pancreas release insulin which helps lower your blood sugar by increasing the rate at which your cells store the sugar either as glycogen or as fat for later use. Now let’s say you’ve done the opposite, you’ve eaten no pie, you’re pilos, in fact you’ve eaten nothing for hours, if your blood sugar drops too low then alpha cells in the pancreas will instead send out glucagon which helps raise your blood sugar levels in part by decreasing the storage of sugar in your cells and triggering the release of glucose back into the blood.