Human endocrine system

The human body has two distinct kinds of glandular systems. Exocrine glands (such as the salivary and gastric glands) secrete substances like enzymes, saliva or sweat through a duct that carries them to a body surface or cavity. Endocrine glands, by contrast, are ductless: they release their hormone directly into the surrounding tissue fluid, from where it diffuses into nearby capillaries and circulates through the whole body until it reaches its target organ.

Glands whose only job is hormone secretion are called exclusive endocrine glands — in humans these are the pituitary, thyroid, parathyroid, pineal, adrenal and thymus glands (see Figure 11.1 for their locations). The hypothalamus is a special case: because it performs both a neural function and a hormonal one, it is classed as a neuroendocrine gland. A further group of organs, including the pancreas, the epithelium of the gastrointestinal tract, the kidney, the heart, the gonads and the placenta, have hormone-secreting tissue as only part of their overall job — these are called partial endocrine glands. …