Glands Many hormones are produced by organs called glands . Some hormones are produced by organs that have other purposes as well and are not considered glands. For example, the stomach , the heart , and the small intestine all produce important hormones. Some mechanical body processes work this way also, such as breathing and blood circulation. Cellular processes in living organisms, the level at which hormones operate, are not that simple.
Hormones also come in pairs, with one member opposing the action of the other.
An example is the insulin-glucagon pair produced by the pancreas . By adding first a little of one and then a little of the other hormone, the amount of sugar in the blood is kept nearly constant. Almost all biochemical actions involve a reaction that triggers a second reaction that triggers a third and so forth. For the opposing pairs of hormones, the effect of one hormone includes triggering a small cascade that increases the amount of the other member of the pair. Since the level of one hormone partly controls the amount of the other, the hormones can react to changing conditions.