Endocrine glands are specialized organs that produce and release hormones into the body.  Hormones are the chemical messengers which travel through the blood stream toward the target cells upon which they act.  Hormones have a negative feedback system which signals when there is an excess of a specific hormone and will "shut off" the organ secreting the hormone.  When the hormone levels return to normal, the glands resume their production and release of the hormones.

How Hormones Work

There are two types of hormones: steroid and protein.  Steroids are able to diffuse across the cell membrane because it is lipid-soluble.  Once inside the cell, it binds to a receptor protein in the nucleus and activates specific genes to produce specific proteins.  A proteinous hormone cannot enter the cell because it is not lipid-soluble.  Instead it binds to a receptor outside the cell which stimulates the production of another messenger, cyclic AMP (cAMP).  This molecule stimulates enzymes inside the cell to make specific cellular manipulations.

The Pituitary Gland

The pituitary gland is often called the "master gland" because it produces and releases many hormones to other endocrine glands, stimulating them to produce other hormones to other parts of the body.   The pituitary is divided into two parts:  the anterior and posterior pituitaries.  The anterior pituitary secretes six hormones:  Adrenocorticotropic Hormone (ACTH) - stimulates the Adrenal Cortex to produce and secrete its hormones; Follicle-Stimulating Hormone (FSH) - stimulates the hair follicle to grow; Growth Hormone (GH) - stimulates growth throughout the body; Leutenizing Hormone (LH) - stimulates the release of the ovum during the menstrual cycle; Prolactin - stimulates the mammary glands to produce milk; Thyroid-Stimulating Hormone (TSH) - stimulates the Thyroid to produce and secrete its hormones.  The hypothalamus lies directly above the pituitary gland and regulates the anterior pituitary by secreting hormones to stimulate or inhibit its actions.

The Posterior Pituitary secretes 2 hormones:  Antidiuretic Hormone (ADH), aka vasopressin - causes the kidneys to withhold water; and Oxytocin - causes the uterus to contract.

The Pancreas secretes two hormones which are produced in the islet of Langerhans:  Glucagon - raises the blood sugar level by stimulating the liver to convert glycogen into glucose; and Insulin - lowers the blood sugar level by removing glucose from the bloodstream and stores it in muscle or liver cells.

The Adrenal Glands contain two endocrine glands:   the Adrenal Cortex secretes 3 types of hormones and the Adrenal Medulla secretes 2 types of hormones.  The cortex secretes glucocorticoids that increase the blood sugar level by converting amino acids and fatty acids into glucose; mineralocorticoids that help retain salt and water in the kidneys (They stimulate the retention of sodium (Na⁺) and chlorine (Cl^-) ions which produce salt (NaCl), producing a gradient which causes water to flow toward the salt due to osmosis.); and cortical sex hormones that stimulate the development of secondary sex characteristics - e.g. pubescent male voice change.  The medulla secretes two hormones (Epinephrine and Norepinephrine); both are activated when one is under extreme stress and are involved in the "fight-or-flight" response.  They work together to increase heart rate, metabolic rate, and blood pressure.

The Thyroid releases Thyroxine to regulate the metabolic rate in body tissues.   Two conditions are associated with thyroid hormones:  Hyperthyroidism - too much thyroxine is released, resulting in a fast metabolic rate and usually irritable and nervous behavior; and Hypothyroidism - too little thyroxine is released, resulting in a slow metabolic rate and usually sluggish behavior and being overweight.  The thyroid also releases Calcitonin which decreases the blood's concentration of calcium by storing it in the bones.

The Parathyroids are four small round organs which lie on the thyroid.  Each secretes parathyroid hormone which increases the blood calcium levels by releasing calcium stored in the bones.  Bone remodeling is the process by which calcium is added and released from the bones to the blood stream and vise aversa.

Sex Hormones

Female hormones include estrogen and progesterone.  Estrogen promotes secondary sex characteristics; progesterone prepares the uterus for pregnancy.  The male hormone that promotes secondary sex characteristics is testosterone.

Next:  "Body Systems Review."