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Gastrin is released by G cells and stimulates Parietal cells to secrete hydrochloric acid. Why is it not acting in a paracrine fashion instead of endocrine?
Paracrine signalling would mean not merely that gastrin is signalling the same organ, but that it is signalling nearby cells in the same organ - so that a few parietal cells close by respond, while the others perhaps would ignore the signal because maybe the paracrine signal would be degraded. While the distinction is not absolute - there are shades of gray in all these things - blood gastrin tests are commonly done and medically relevant ( https://medlineplus.gov/ency/article/003697.htm ). Because you can look at gastrin level as a whole-body characteristic, it is certainly appropriate to call it an endocrine signal.
Though their role is less emphasized, bear in mind that there are gastrin-producing G cells in the duodenum as well, and gastrin affects other digestive organs.