hormones
Chemicals that guide the changes in our bodies and influence how glands and organs work.
Chemical Nature of Hormones Chemically, hormones may be classified as either proteins or steroids. All of the hormones in the human body, except the sex hormones and those from the adrenal cortex, are proteins or protein derivatives.
Mechanism of Hormone Action Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone. This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. 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.
Control of Hormone Action 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.
Endocrine Glands & Their Hormones
The endocrine system is made up of the endocrine glands that secrete hormones. Although there are eight major endocrine glands scattered throughout the body, they are still considered to be one system because they have similar functions, similar mechanisms of influence, and many important interrelationships.
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.
Pituitary & Pineal Glands
Thyroid & Parathyroid Glands
Adrenal (Suprarenal) Gland
Pancreas --- Islets of Langerhans
Gonads (Testes and Ovaries)
Other Endocrine Glands
HORMONAL CONTROL
Normal menstrual function results from interactions among the central nervous system, hypothalamus, anterior pituitary, ovaries, and associated target tissues.
Although each part of the system is essential to normal function, the ovaries are primarily responsible for controlling the cyclic changes and the length of the menstrual cycle.
In most women in the middle reproductive years, menstrual bleeding occurs every 25 to 35 days, with a median length of 28 days.
The hormonal control of the menstrual cycle is complex.
For example, the biosynthesis of estrogens that occurs in adipose tissue may be a significant source of the hormone.
There is evidence that a certain minimum body weight (48 kg) and fat content (16% to 24%) are necessary for menarche to occur and for the menstrual cycle to be maintained.
This is supported by the observation of amenorrhea in women with anorexia nervosa, chronic disease, and malnutrition and in those who are long-distance runners.
In women with anorexia nervosa, gonadotropin and estradiol secretion, including LH release and responsiveness to the hypothalamic gonadotropin-releasing hormone (GnRH), can revert to prepubertal levels.
With resumption of weight gain and attainment of sufficient body mass, the normal hormonal pattern usually is reinstated.
Obesity or significant weight gain also is associated with oligomenorrhea or amenorrhea and infertility, although the mechanism is not well understood.
Hypothalamic and Pituitary Hormones
Growth, prepubertal maturation, the reproductive cycle, and sex hormone secretion in males and females are regulated by FSH and LH from the anterior pituitary gland.
Because these hormones promote the growth of cells in the ovaries and testes as a means of stimulating the production of sex hormones, they are called the gonadotropic hormones.
The secretion of LH and FSH is stimulated by GnRH from the hypothalamus.
In addition to LH and FSH, the anterior pituitary secretes a third hormone called prolactin.
The primary function of prolactin is the stimulation of lactation in the postpartum period.
During pregnancy, prolactin, along with other hormones such as estrogen, progesterone, insulin, and cortisol, contributes to breast development in preparation for lactation.
Although prolactin does not appear to play a physiologic role in ovarian function, hyperprolactinemia leads to hypogonadism.
This may include an initial shortening of the luteal phase with subsequent anovulation, oligomenorrhea or amenorrhea, and infertility.
The hypothalamic control of prolactin secretion is primarily inhibitory, and dopamine is the most important inhibitory factor.
Hyperprolactinemia may occur as an adverse effect of drug treatment using phenothiazine derivatives (i.e., antipsychotic drugs that block dopamine receptors).
Ovarian Hormones
The ovaries produce estrogens, progesterone, and androgens.
Ovarian hormones are secreted in a cyclic pattern as a result of the interaction between the hypothalamic GnRH and the pituitary gonadotropic hormones, FSH and LH.
The steroid sex hormones enter cells by passive diffusion, bind to specific receptor proteins in the cytoplasm, and then move to the nucleus, where they bind to specific sites on the chromosomes.
These hormones exert their effects through gene-hormone interactions, which stimulate the synthesis of specific messenger ribonucleic acid (mRNA).
In addition, estrogen appears to have the ability to influence cell activity through other non genomic mechanisms.
These non genomic effects take place in cells that have no steroid receptors, possibly mediated by other membrane receptors.
This may explain in part some of the non reproductive effects of estrogen.
An example of a non genomic cardioprotective effect would be the antioxidant activity of estrogen in preventing endothelial injury that can lead to platelet adherence.
The number of hormonal receptor sites on a cell is not fixed; evidence suggests that they are constantly being removed and replaced.
An increase or a decrease in the number of receptors can serve as a mechanism for regulating hormonal activity.
For example, estrogen may induce the development of an increased number of estrogen receptors in some tissues and may stimulate the synthesis of progesterone receptors in others.
In contrast, progesterone may cause a reduction in the number of estrogen and progesterone receptors.
The recent discovery of a second type of estrogen receptor (ER2) that is different in structure, tissue distribution, and expression from ER1 helps to expand our understanding of the mechanism of action of estrogen in the body.
The ER2 appears to be an activator of estrogen response, whereas the ER1 appears to modulate or inhibit the action of estrogen.
Likewise, the progesterone receptor has two major forms (A and B), expressed by a single gene, but promoted differently in a complex system of transcription regulation.