Endocrine glands are glands that secrete their product, (hormones), directly into the blood rather than through a duct. This group contains the glands of the Endocrine system. The main Endocrine glands include the pituitary gland, the pancreas,the ovaries, the testes, the thyroid gland and the adrenal glands. Other organs which are not so well known for their endocrine activity include the stomach, which produces such hormones as ghrelin.

The pituitary gland hangs from the base of the brain by a stalk and is enclosed by bone. It consists of a hormone-producing glandular portion (anterior pituitary) and a neural portion (posterior pituitary), which is an extension of the hypothalamus.

The hypothalamus a) regulates the hormonal output of the anterior pituitary via releasing and inhibiting hormones and b) synthesizes two hormones that it exports to the posterior pituitary of storage and later release.

1. Endocrine organs are ductless, well-vascularized glands that release hormones directly into the blood or lymph. They are small and widely separated in the body.

2. The major endocrine organs are the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands, as well as the pancreas and gonads. The hypothalamus is a neuroendocrine organ.

3. Local chemical messengers, not generally considered part of the endocrine system, include autocrines, which act on the cells that secrete them, and paracrines, which act on a different cell type nearby.

Chemistry:

1. Most hormones are steroids or amino acid based.

2. Hormones alter cell activity by stimulating or inhibiting characteristic cellular processes of their target cells.

3. Cell responses to hormone stimulation may involve changes in membrane permeability; enzyme synthesis, activation, or inhibition; secretory activity; gene activation; and mitosis.

4. Second-messenger mechanisms employing intracellular messengers and transduced by G proteins are a common means by which amino acid–based hormones interact with their target cells. In the cyclic AMP system, the hormone binds to a plasma membrane receptor that couples to a G protein. When the G protein is activated it, in turn, couples to adenylate cyclase, which catalyzes the synthesis of cyclic AMP from ATP. Cyclic AMP initiates reactions that activate protein kinases and other enzymes, leading to cellular response. The PIP-calcium signal mechanism, involving phosphatidyl inositol, is another important second-messenger system. Other second messengers are cyclic GMP and calcium.

5. Steroid hormones (and thyroid hormone) enter their target cells and effect responses by activating DNA, which initiates messenger RNA formation leading to protein synthesis.