Hormones are the functional molecules of the endocrine system. They are chemicals that are made and released by endocrine glands, and serve as messages that induce effects in various parts of the body. Steroid hormones, protein hormones, and amine hormones are the three specifically designated varieties of hormones. Hormones are created to be exacting in their performance, meaning that the cells and their intention of action is precise and they do not initiate any form of action beyond their target cells.
Some hormones change the permeability of the cell membrane, other hormones can alter enzyme activity, and some hormones stimulate the release of other hormones. One hormone may affect one tissue in a different way than it affects another tissue. It depends on the tissue cells that are programmed to respond differently to the same hormone. A single hormone may also have different effects on the same tissue at different times in life. Some hormone-induced effects require the action of more than one hormone in case one hormone does not do its job properly the others will take over.
Hormones originate primarily from the pituitary, pineal, thymus, thyroid, adrenal glands and pancreas. Women have ovaries and men have testes that activate their reproduction system. Hormones are normally drawn upon for specific short-term tasks. They also stimulate the body for a new phase in life such as development, puberty, pregnancy, sexual function, reproduction, and mood. A lot of these cause noticeable changes in one's appearance. 
Most hormones flow freely throughout the body by the blood stream. Some are carried by a substance to keep them from breaking down, such as a protein molecule, to keep them dissolved in the blood. They flow until they reach their destination by which the brain tells them. They release chemicals that initiate the change. Some hormones, however, are delivered directly to the target tissues instead of circulating throughout the entire bloodstream. Hormones that come from a portion of your brain that controls the endocrine system called the hypothalamus, are sent directly to the adjacent pituitary gland, making it several hundred times higher than in the circulatory system.
There are two major classes of hormones: steroids and proteins (including peptides and modified amino acids). Adrenalin Hormones exist in mammals, including humans, as well as in invertebrates and plants. All but the plant hormone type is either created from the cholesterol naturally produced in the body or derived from naturally occurring amino acids. Amino acid derivatives are responsible for the creation of protein hormones, which are then tied together from peptide chains. Protein hormones account for the vast majority of hormones in the human body.
Growth Hormone is responsible for growth by stimulating the formation of bone,and molecules vital to building muscle and other tissue.
Sex hormones are responsible for sexual organs, sexual behavior, reproduction, and pregnancy. Along with these effects comes more hormones such as: testosterone and estrogens that regulate female sexual development and behavior as well as some aspects of pregnancy. Progesterone, a female hormone secreted in the ovaries, regulates menstruation and stimulates lactation in humans and other mammals.
- Main Article: Digestive system
Secretin was the first hormone to be discovered of the digestive hormones. This hormone came from the duodenum. It causes the pancreas to secrete digestive juices. There are a few hormones that control the secretion of the pancreas. The main job of the secretin is to stimulate the pancreas so that it secretes a solution, which has a strong presence of bicarbonate ions.
Cholecystokinin is a hormone that is secreted by the small intestines. This hormone is used to stimulate the gallbladder to release bile. It also stimulates the pancreas, which then releases digestive enzymes. Together with secretin it slows the movement of the stomach.
Gastrin is hormone that is secreted from the stomach. When the gastrin is released from the stomach it is then combined with the blood. The gastrin is released when the food in the stomach stimulates it. Gastrin starts off in the lower region of the stomach. It the travels up through the stomach and when it finally reaches the upper region of the stomach it then stimulates movement within the stomach. When the contents of the stomach become acidic the gastrin is released.
- Purves, William. Life the Science of Biology. 2005. Couriers Company Inc., p. 978
- Purves, p978
- Purves, p978
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- Human Hormone Listing
- Hormone wikipedia, 17 March 2009.
- Hormones of the human author, publisher, 30 October 2004 .