Description
The endocrine glands include the pineal gland, pituitary gland, thyroid gland, parathyroid gland, thymus, adrenal gland, pancreas, ovary, andtestis. Endocrine glands release hormones into the blood and these molecules travel throughout the body to reach sites where they are active.
Some other organs secrete hormones. For instance, the stomach releases hormones that affect digestion and hunger such as gastrin and ghrelin.
Role of the Hormonal System in the Body
The endocrine system, along with the nervous system, functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion. The effect is of short duration, measured in seconds, and localized. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities. The action of the endocrine system is measured in minutes, hours, or weeks and is more generalized than the action of the nervous system.
Some of the functions regulated by the endocrine system are growth and development, hunger, mood, metabolism, and reproduction.
Hormones
The secretory products of endocrine glands are called hormones and are secreted directly into the blood and then carried throughout the body where they influence only those cellsthat have receptor sites for that hormone.
Action hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells respond to a given hormone because they have receptors for that hormone, which work as a lock and key mechanism. If the hormone (key) fits the receptor (lock), then the hormone will have an effect on the cell. If a hormone and a receptor site do not match, then there is no reaction. All the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected. Hormones bring about their characteristic effects on target cells by modifying cellular activity.
Protein hormones react with receptors on the surface of the cell, and the sequence of events that results in hormone action is relatively rapid. Steroid hormones typically react with receptor sites inside a cell. Because this method of action actually involves synthesis of proteins, it is relatively slow.
Negative feedback
Hormones are very potent substances, which means that very small amounts of a hormone may have profound effects on metabolic processes. Because of their potency, hormone secretion must be regulated within very narrow limits in order to maintain homeostasis in the body.
Many hormones are controlled by some form of a negative feedback mechanism. In this type of system, a gland is sensitive to the concentration of a substance that it regulates. A negative feedback system causes a reversal of increases and decreases in body conditions in order to maintain a state of stability or homeostasis. Some endocrine glands secrete hormones in response to other hormones. The hormones that cause secretion of other hormones are called tropic hormones. A hormone from gland A causes gland B to secrete its hormone. A third method of regulating hormone secretion is by direct nervous stimulation. A nerve stimulus causes gland A to secrete its hormone.
Non-endocrine functions
Some glands also have non-endocrine regions that have functions other than hormone secretion. For example, the pancreas has a major exocrine portion that secretes digestive enzymes and an endocrine portion that secretes hormones. The ovaries and testes secrete hormones and also produce the ova and sperm. Some organs, such as the stomach,intestines, and heart, produce hormones, but their primary function is not hormone secretion.
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