endocrine system

If the nervous system is the body's high-speed telephone network — fast, point-to-point, electrical — then the endocrine system is its postal service: slower, broadcast widely through the blood, but able to deliver lasting instructions to almost every cell at once. Together they form the two great communication systems that coordinate the body.

The endocrine system is the collection of ductless glands and hormone-secreting tissues that regulate the body through hormones. Its classic members include the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pancreatic islets, and gonads. But it also includes scattered hormone-producing cells in the gut, heart, kidney, and fat — the so-called diffuse endocrine system.

These glands rarely act alone. They are wired into feedback loops and hierarchical axes that keep hormone levels within healthy ranges. A useful caveat: the boundary between endocrine and nervous systems is blurry — the hypothalamus, for instance, is brain tissue that also acts as a master endocrine gland, a crossover captured by the field of neuroendocrinology.