ENDOCRINE SYSTEM
 © 2003 Theresa Lorraine
 

The nervous system sends electrical messages to control and coordinate the body. The endocrine system has a similar job, but uses chemicals to “communicate”. These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by a group of specialized cells called an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel to elsewhere in the body to target organs, upon which they act. Note that this is in contrast to our digestive glands, which have ducts for releasing the digestive enzymes.

 

 

 

 

Organs of the Endocrine System

Glands

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. For instance, exocrine glands, such as the sweat and salivary glands, release secretions in the skin or inside the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

Hormones

transfer information and instructions from one set of cells to another


There are three general classes (groups) of hormones. These are classified by chemical structure, not function.

Steroid Hormones including prostaglandins which function especially in a variety of female functions (aspirin inhibits synthesis of prostaglandins, some of which cause “cramps”) and the sex hormones all of which are lipids made from cholesterol,

Amino Acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine, and

Peptide Hormones (like insulin) which is the most numerous/diverse group of hormones.

1. The Hypothalamus is a collection of specialized cells that is located in the lower central part of the brain, is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

2.. Pituitary gland  Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. The anterior lobe produces hormones such as:

Growth hormone which stimulates the growth of bone and other body tissues and plays a role in    the body's handling of nutrients and minerals
Prolactin, which activates milk production in women who are breastfeeding
Thyrotropin, which stimulates the thyroid gland to produce thyroid hormones
Corticotropin, which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins, chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic  hormone, which helps control the balance of water in the body. Antidiuretic hormone also affects the production of oxytocin , which triggers the contractions of the uterus in a woman having a baby.

3. Thyroid gland  The thyroid , located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine  and triiodothyronine . These hormones control the rate at which cells burn fuels from food to produce energy. The production and release of thyroid hormones is controlled by thyrotropin , which is secreted by the pituitary gland. The more thyroid hormone there is in a person's bloodstream, the faster chemical reactions occur in the body.

Why are thyroid hormones so important? There are several reasons - for example, they help kids and teens develop strong bones, and they also play a role in the development of the brain and nervous system in kids.

4.  Parathyroid glands  Attached to the thyroid are four tiny glands that function together called the parathyroids . They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin , which is produced in the thyroid.

5.  Pancreas  The pancreas is also part of the body's hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas produces (in addition to others) two important hormones, insulin and glucagon . They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.

6.  Adrenal Glands  The body also has two triangular adrenal  glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part, the adrenal cortex, produces hormones called corticosteroids  that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function. The inner part, the adrenal medulla , produces catecholamines , such as epinephrine (. Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress.

7.  Gonads or sex organs  The gonads are the main source of sex hormones.

The male gonads, or testes , are located in the scrotum. They secrete hormones called androgens , the most important of which is testosterone .  A woman's gonads, the ovaries , are located in her pelvis. They produce eggs and secrete the female hormones estrogen and progesterone .

8.  Pineal Gland  The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate when you sleep at night and when you wake in the morning.