The body runs two separate long-distance communication systems, and they work in almost opposite ways. The nervous system sends electrical signals down dedicated wires — individual nerve fibres — directly to a specific target, arriving in milliseconds. The endocrine system instead releases chemical messengers called hormones directly into the bloodstream, where they travel to essentially every cell in the body rather than a single addressed target, taking anywhere from seconds to hours to have an effect and lasting far longer once they do.
A broadcast system instead of a wired one
Because hormones are released into the general circulation rather than delivered down a dedicated line, they physically reach nearly every cell in the body, not just the ones they're meant to affect. What determines whether a given hormone actually does anything to a particular cell is whether that cell happens to carry the right receptor for it — a cell without the matching receptor is bathed in the hormone just like every other cell, but has no way to respond to it. This is a fundamentally different targeting mechanism from the nervous system's wired approach, and it's part of why hormonal effects tend to be broader and more diffuse than a nerve signal's precisely localised one: a single hormone released from one gland can simultaneously affect the liver, the muscles, and the brain, provided each of those tissues happens to carry cells with the appropriate receptor.
Slower, but built for effects that need to last
The nervous system's speed makes it well suited to responses that need to happen essentially instantly, like pulling a hand back from something hot. The endocrine system's slower, broader signalling is instead well suited to processes that unfold over a longer timescale and need to stay coordinated across many different tissues at once — regulating metabolism, growth, or the body's response to prolonged stress, for instance, where a signal that persists for minutes or hours, rather than milliseconds, is actually the more useful property, not a limitation. The two systems aren't really in competition with each other; they're specialised for different jobs, and they work together closely in practice, with the nervous system directly triggering the release of certain hormones from endocrine glands when a fast initial response needs to be followed by a longer, more sustained one.
What we're still unsure about
The basic contrast between wired, fast nervous signalling and broadcast, slower hormonal signalling is well-established physiology, confirmed across a huge range of experimental and clinical evidence. What's less settled, and remains an active area of endocrinology research, is exactly how a given tissue's sensitivity to a hormone changes over time — the number of receptors a cell displays for a given hormone isn't fixed, and can shift in response to how much of that hormone the cell has recently been exposed to, in ways that aren't yet fully mapped out for every hormone and every tissue combination, making a cell's actual real-world responsiveness harder to predict from hormone levels alone than the basic broadcast model might suggest.
This sits inside Endocrine System & Hormones, one of eight topics in Human Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.