The endocrine system is composed of endocrine glands. Endocrine glands are glandular tissues that secrete specialised chemical messengers called hormone directly into the bloodstream which act on target tissues to create a response to an initial stimulus. What are hormones? Hormones are the different chemical messengers which are released into the bloodstream and can change a target cell's activity. The two types of hormones include:

• Steroid/fat soluble hormones, which bind with a target tissue receptor and form a complex which enters the cell and has an effect on gene regulation

• Protein or amine/water soluble hormones which bind with target tissue (cells) receptors and initiate a secondary messenger system which leads to enzyme driven cell responses.

Hormones target and activate particular cells and organs, creating a response, and the only cells in the body that have receptors for a particular hormone will respond to that hormone. A target tissue may be a long way from the gland that secretes the hormone; for example, ADH is secreted by the pituitary gland, but it exerts its effects on the kidneys.

Main Endocrine Glands The two main endocrine glands which are also considered the 'master endocrine glands' are the hypothalamus, and the pituitary gland which is further divided into the anterior and posterior gland. Hypothalamus The hypothalamus controls and integrates many basic physiological activities (temperature, food, fluid levels, sleep) as well as controlling autonomic nervous system reflexes. Anterior Pituitary The anterior pituitary gland is connected to the hypothalamus by blood vessels, and is stimulated by neuro-hormones secreted by the hypothalamus (releasing factors). It is the main centre for coordinating nervous and endocrine systems, and many hormones are released from this gland to stimulate other endocrine glands. For example, TSH stands for thyroid stimulating hormone and stimulates the thyroid to produce thyroxine. Posterior Pituitary The posterior pituitary gland is an extension of the hypothalamus and is attached to it via a network of nerve fibres, and it secreted neurohormones produced by the hypothalamus. It does not produce its own hormones. The only hormones it secretes are oxytocin, and ADH (anti-diuretic hormone) which is important in osmoregulation.