The Endocrine System and Hormonal Control Endocrine glands produce and secrete hormones directly into the bloodstream. Exocrine glands secrete through ducts (like saliva glands). hormones are chemical messengers released by certain cells and circulate in the blood to exert an effect on cells in other parts of the body. the endocrine and nervous systems work together to monitor the ever-changing internal environment of the body. they control things like: blood pressure, heart rate, reproduction, growth and development, the immune system. How do Hormones Work? there are two classes of hormones: steroid hormones - all derived from cholesterol, have a multi-ringed structure (progesterone, cortisol). These are lipid soluble. protein hormones - made of chains of amino acids (insulin, growth hormone). These are water soluble. the mechanism of action for each type of hormone is different. (see diagram) steroid hormones can diffuse through the cell membrane and in combination with a receptor molecule, initiates transcription of a certain gene. protein hormones attach to specific receptors on the cell membrane, which causes the cell to produce cyclic AMP, which kick starts production of enzymes to make a particular product. The Pituitary Gland located in the brain, attached to the hypothalamus (which controls its action). it consists of two lobes, the anterior (front) and posterior (back), which release different hormones directly into the bloodstream. the posterior lobe stores and releases hormones made by specialized nerve cells in the hypothalamus (antidiuretic hormone, ADH and oxytocin). the anterior lobe makes its own hormones (GH, FSH, LH, TSH) and releases them into the bloodstream, but the release is controlled by the hypothalamus, which sends hypothalamic hormones to either stimulate or inhibit the release of a certain hormone from the anterior lobe.

Regulation of Blood Sugar the pancreas is a small gland made of two kinds of tissues that independently function as exocrine and endocrine glands. the endocrine tissue, called the islets of Langerhans is made of two types of cells: Beta Cells - produce insulin, which is released when the blood sugar level increases. insulin causes cells of muscle, liver (convert to glycogen) and other organs to take up glucose, which lowers the level in the blood. Alpha Cells - produce glucagon, which is realeased when blood sugar levels are low, causes glycogen to be converted to glucose and increases the level of glucose in the blood.

The Adrenal Glands located above each kidney, they consist of: the adrenal medulla - regulated by the nervous system, it makes two hormones, adrenaline (epinephrine) and noradrenaline (norepinephrine). in times of stress, sympathetic nerves cause the release of both hormones into the bloodstream. this causes the conversion of glycogen to glucose (for extra energy). they also increase heart rate, breathing rate, dilation of blood vessels, dilation of the pupils. the adrenal cortex - produces some sex hormones and two other different types of hormones: glucocorticoids - various hormones that are designed to help the body meet the demands of stress. cortisol, is an example, and it increases the levels of amino acids (for conversion to glucose), fatty acids (for energy) and overall, the level of glucose in the bloodstream. mineralocorticoids - aldosterone is the best example, it is involved in water reabsorption.