What are humoral stimuli?

This is perhaps the simplest system. Some endocrine glands secrete hormones directly according to the concentration of various ions and nutrients in the blood surrounding them. For example, when the concentration of calcium in the blood bathing the parathyroid gland cells is low, they are stimulated to produce parathyroid hormone (PTH) which causes a rise in blood calcium. Conversely, when the concentration of calcium is high around the parathyroid cells, PTH release is inhibited.