Endocrine System By Elaine Bell

Endocrine and Exocrine There are two types of gland in the body. Endocrine glands release hormones directly into the blood. These glands are ductless and consist of a group of cells that produce and release the hormone directly into the capillaries that run through the gland. Example = Pituitary gland in the hypothalamus. Exocrine glands do not release hormones but have a small duct that carries their secretion to another place. Example = Salivary gland that secretes saliva, to be carried along the duct into the mouth.

Signalling The cells receiving the hormone signal must have a specific hormone receptor on the plasma membrane. The hormone will bind with this receptor. Each hormone needs a different receptor, therefore the hormone can travel in the blood without affecting cells without the specific receptor. The cells with the specific receptor are called target cells. These group together to form target tissue. This is how the endocrine system can send different signals all over the body at the same time It can also be used to send specific signals.

Types of Hormone There are two different types of hormone: Protein and peptide hormones, and derivatives of amino acids. Protein and peptide hormones are involved in the control of blood glucose and include insulin and glucagon. Amine hormones include adrenaline from the adrenal gland and thyroxine from the thyroid. Steroid Hormones Include oestrogen, testosterone and corticosteroids. These hormones work in different ways. The proteins are not soluble in the phospholipid bilayer and do not enter cells, however steroids can pass through the membrane and enter the cell where they can have a direct affect on the DNA within the nucleus.

Glands and Hormones Many glands are positioned within the body, each with a different function They all produce different hormones.

Adrenaline As adrenaline is an amino acid derivative it is unable to enter the target cell. The adrenaline receptor on the target cell must be a complementary shape to the hormone, for the hormone to affect the cell. This receptor is associated with an enzyme on the inner surface of the cell surface membrane. This enzyme is called adenyl cyclase.

Action of Adrenaline The adrenaline in the blood binds with its specific receptor site on the cell surface membrane. The adrenaline molecule is called the first receptor. It then binds to the receptor and activates the adenyl cyclase. This enzyme converts ATP to cyclic AMP. The cAMP is the second messenger within the cell. The cAMP can then cause an effect inside the cell by activating enzyme action.

Adrenal Medulla The medulla is found in the centre of the gland The cells manufacture and release the adrenaline hormone in response to stress such as pain or shock. As the effects of adrenaline need to be wide spread, most cells have receptors. The effect is to prepare the body for activity. This includes: Relaxing smooth muscle in the bronchioles. Increasing stroke volume of heart. Increasing heart rate Causing general vasoconstriction to raise blood pressure. Stimulating conversion of glycogen to glucose. Dilating pupils Increasing mental awareness Inhibiting action of the gut. and causing body hair to stand erect.

Adrenal Cortex The cortex uses cholesterol to produce certain steroid hormones These have a variety of roles in the body. The mineralocorticoids help to control the concentrations of sodium and potassium in the blood. The glucocorticoids help to control the metabolism of carbohydrates and proteins in the liver.