21.1: What is the Endocrine System?
The endocrine system sends hormones—chemical signals—through the bloodstream to target cells—the cells the hormones selectively affect. These signals are produced in endocrine cells, secreted into the extracellular fluid, and then diffuse into the blood. Eventually, they diffuse out of the blood and bind to target cells which have specialized receptors to recognize the hormones.
While most hormones travel through the circulatory system to reach their target cells, there are also alternate routes to bring hormones to target cells. Paracrine signaling sends hormones out of the endocrine cell and into the extracellular fluid where they affect local cells. In a form of paracrine signaling, called autocrine signaling, hormones secreted into the extracellular fluid affect the cell that secreted them.
Another type of signaling, synaptic signaling, involves the release of neurotransmitters from neuron terminals into the synapse—a specialized junction that relays information between neurons—where they bind to receptors on neighboring neurons, muscle cells, and glands. In neuroendocrine signaling, neurosecretory cells secrete neurohormones that travel through the blood to affect target cells. Overall, endocrine signaling has a slower effect than other types of signaling because it takes longer for hormones to reach the target cells, but the effects typically also last longer.
Hormones directly diffuse into the extracellular fluid surrounding the endocrine glands because they have no ducts. In comparison, exocrine glands, like the salivary gland, have ducts that secrete a targeted dose directly onto a surface or into a cavity. In addition to being found in specialized endocrine glands, endocrine cells can also be located in organs like the stomach, among cells with different functions.
A hormone has specific target cells that have receptors that recognize the hormone. It can be thought of like a lock and key where the receptors on a target cell are the lock and will only recognize the hormone, the key, that fits it. Target cells can be very close to the endocrine cells that produce the hormone or very far away but must be transported through the bloodstream. For instance, enteroendocrine cells in the stomach and small intestine release hormones that can alter gastric acid secretion by stomach cells. On the other hand, hormones released by the pituitary gland located at the base of the brain can affect urine production by acting on kidney cells.