ENDOCRINE SYSTEM

There are two systems that regulate homeostasis in the body. The nervous system relays information to the body using electrical impulses that travel very fast, but the messages are short-lived. The endocrine system uses ___hormones__, or chemical messages that travel through the blood to relay information to the body. The endocrine system is slower-acting, but the action of the hormones is longer lasting. The ___hypothalamus___, which is the part of the brain that maintains homeostasis in the body, controls most of the glands of the endocrine system. I. HORMONES

A. Hormone Production Hormones help regulate growth, metabolism, appearance, behavior, growth, reproduction, and fluid balance. Most hormones are produced by endocrine glands. Endocrine glands are unique because they are ductless; in other words, hormones are secreted into the __bloodstream__ and the ___circulatory___ system transports the hormone to its destination. Its destination is known as its target. A hormone may only have one target, or it may have several.

B. Control of Hormone ProductionHormone release is regulated by a mechanism known as __negative feedback__. Information about the effect of the hormone is “fed back” to the gland. When the desired condition is reached, the gland _shuts off_ hormone production and vice versa.

II. GLANDS OF THE ENDOCRINE SYSTEM This is not a complete list of the endocrine glands or the hormones that they secrete. The following are just a few examples.

A. Pituitary GlandLocated in the brain. It is often called the “master gland” because it sends chemical signals to many of the other endocrine glands.1. Antidiuretic Hormone (ADH) – increases the reabsorption of water by the kidneys when water

content in the body decreases due to sweating or diarrhea. 2. Growth Hormone – stimulates growth by increasing the rate of protein synthesis and mitosis.

B. Thyroid Gland

Located below the larynx and wraps around the upper part of the trachea. Secretes __thyroxine_, which increases the rate of cellular respiration and cellular metabolism. Iodine is required for normal thyroid function.

C. Thymus GlandThe thymus is a hat shaped gland located on top of the heart. It is part of the Lymphatic system as well as the endocrine system. The endocrine function of the thymus gland is to produce hormones that __aid in immunity__. It is much larger in an infant and becomes reduced in size as an adult.

D. Adrenal GlandsLocated on top of each __kidney_. Secrete ___epinephrine___(also called adrenaline), commonly known as the “fight or flight” hormone. Epinephrine is released to help an individual respond to potentially life-threatening situations. Heart rate is increased, pupils dilate to increase airflow, and

glycogen in the liver is broken down to glucose for cellular respiration. Blood flow to the skeletal muscles increases and blood flow to the digestive system is decreased.

E. PancreasTucked under the stomach, the pancreas is part of both the digestive & endocrine systems. Its endocrine role is to regulate the blood __glucose___ concentration through the secretion of 2 hormones:1. Insulin – Secreted when blood glucose concentration is __HIGH_. Enhances the transport of

glucose from the blood into the body cells by increasing the permeability of the cell membranes to glucose. Insulin stimulates the _liver_ and muscle cells to store glucose as __glycogen__. In Diabetes mellitus Type I, the pancreas does not produce insulin and body cells are unable to metabolize glucose. As a result, glucose concentration is __high__ in the blood and is excreted in the urine. Insulin injections are required.

2. Glucagon – Secreted when blood glucose concentration is _LOW__. Stimulates the __liver__ to convert __glycogen__ to glucose, which then raises the blood glucose level to have glucose available to use for cellular respiration.

F. Testes Reproductive organ found in males. Acts as an endocrine structure through the secretion of testosterone. Testosterone promotes the development of male secondary sex characteristics such as hairiness, increased muscle, deep voice, etc.

G. OvariesReproductive organ found in females. Acts as an endocrine structure through the secretion of estrogen. Estrogen promotes female secondary sex characteristics such as breast development.