Hormones, endocrine glands and the hypothalamus

Nerves are fast, but their effect is short-lived, and nerve fibres do not reach every cell in the body. Many cell activities need steady, longer-lasting control. That is provided by hormones, which give chemical coordination. The neural and endocrine systems work together to control the body.

Endocrine glands have no ducts (they are ductless glands): they pour their secretions, the hormones, straight into the blood. (Exocrine glands, like sweat glands, deliver through ducts.) Today hormones are defined as non-nutrient chemicals that act as messengers between cells and are made in trace (tiny) amounts.

The organised endocrine glands are the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testes and ovaries). Some other organs also make hormones: the gut, liver, kidney and heart.

The pituitary is often called the master gland, but it takes its orders from the hypothalamus, the base of the forebrain. Think of the hypothalamus as the grandfather of the house: he gives the orders, the father (the pituitary) passes them on, and the children (the other glands) do the work. The hypothalamus has special neurosecretory cells that make two kinds of hormones:

• Releasing hormones, which tell the pituitary to release its hormones. For example, GnRH makes the pituitary release gonadotrophins.
• Inhibiting hormones, which tell it to stop. For example, somatostatin stops the release of growth hormone.

These hormones travel from the hypothalamus to the anterior (front) pituitary through a special set of blood vessels, a portal circulatory system. The posterior (back) pituitary is controlled directly by nerves from the hypothalamus.