PHYSIOLOGY OF ENDOCRINE SYSTEM
Endocrine
System vs. Nervous System Function
The endocrine system works alongside of the nervous system to form the control systems of the body. The nervous system provides a very fast and narrowly targeted system to turn on specific glands and muscles throughout the body. The endocrine system, on the other hand, is much slower acting, but has very widespread, long lasting, and powerful effects. Hormones are distributed by glands through the bloodstream to the entire body, affecting any cell with a receptor for a particular hormone. Most hormones affect cells in several organs or throughout the entire body, leading to many diverse and powerful responses.
Hormone
Properties
Once hormones have been produced by glands, they are distributed through the body via the bloodstream. As hormones travel through the body, they pass through cells or along the plasma membranes of cells until they encounter a receptor for that particular hormone. Hormones can only affect target cells that have the appropriate receptors. This property of hormones is known as specificity. Hormone specificity explains how each hormone can have specific effects in widespread parts of the body.
Many hormones produced by the endocrine system are classified as tropic hormones. A tropic hormone is a hormone that is able to trigger the release of another hormone in another gland. Tropic hormones provide a pathway of control for hormone production as well as a way for glands to be controlled in distant regions of the body. Many of the hormones produced by the pituitary gland, such as TSH, ACTH, and FSH are tropic hormones.
Hormonal
Regulation
The levels of hormones in the body can be regulated by several factors. The nervous system can control hormone levels through the action of the hypothalamus and its releasing and inhibiting hormones. For example, TRH produced by the hypothalamus stimulates the anterior pituitary to produce TSH. Tropic hormones provide another level of control for the release of hormones. For example, TSH is a tropic hormone that stimulates the thyroid gland to produce T3 and T4. Nutrition can also control the levels of hormones in the body. For example, the thyroid hormones T3 and T4 require 3 or 4 iodine atoms, respectively, to be produced. In people lacking iodine in their diet, they will fail to produce sufficient levels of thyroid hormones to maintain a healthy metabolic rate. Finally, the number of receptors present in cells can be varied by cells in response to hormones. Cells that are exposed to high levels of hormones for extended periods of time can begin to reduce the number of receptors that they produce, leading to reduced hormonal control of the cell.
Classes
of Hormones
Hormones are classified into 2 categories depending on their chemical make-up and solubility: water-soluble and lipid-soluble hormones. Each of these classes of hormones has specific mechanisms for their function that dictate how they affect their target cells.
Water-soluble hormones: Water-soluble hormones include the peptide and amino-acid hormones such as insulin, epinephrine, HGH, and oxytocin. As their name indicates, these hormones are soluble in water. Water-soluble hormones are unable to pass through the phospholipid bilayer of the plasma membrane and are therefore dependent upon receptor molecules on the surface of cells. When a water-soluble hormone binds to a receptor molecule on the surface of a cell, it triggers a reaction inside of the cell. This reaction may change a factor inside of the cell such as the permeability of the membrane or the activation of another molecule. A common reaction is to cause molecules of cyclic adenosine monophosphate (cAMP) to be synthesized from adenosine triphosphate (ATP) present in the cell. cAMP acts as a second messenger within the cell where it binds to a second receptor to change the function of the cell’s physiology.
Lipid-soluble hormones: Lipid-soluble hormones include the steroid hormones such as testosterone, estrogens, glucocorticoids, and mineralocorticoids. Because they are soluble in lipids, these hormones are able to pass directly through the phospholipid bilayer of the plasma membrane and bind directly to receptors inside the cell nucleus. Lipid-soluble hormones are able to directly control the function of a cell from these receptors, often triggering the transcription of particular genes in the DNA to produce "messenger RNAs (mRNAs)" that are used to make proteins that affect the cell’s growth and function.
Every reaction in your body, from breaking down food into energy, the mood swings that you have, physiological development of your body, development of your reproductive system, etc. are all carried out by certain chemicals. These certain chemicals are known as hormones in your body. These slow processes that take time to develop are a part of the endocrine system functions. Breathing, body movement, sudden reaction to the surroundings are a part of the nervous system functions.
The endocrine systems consists of hormones and glands. Hormones are the chemical messengers of the body that travel down to the various body parts concerned transferring information. There are many hormones secreted by the endocrine organs, and each individual hormone affects only those body cells that have a genetic program that allows them to react only to those hormones that are related to them. The hormones influence the body to react according to the changes in the balance of fluids and minerals in blood, stress, infection, etc.
The hormones secreted by the endocrine organs are very important for regulating metabolic processes, growth of the body and sexual development. These glands release the hormones into the blood stream and are transported to the various cells and body parts. When the hormones reach the target site, they bind to the receptor cells with a lock and key mechanism. The hormone may be present within the nucleus or on the surface of the cells. Once bound to the receptor, the hormones transmit a signal that triggers an action by the site. Hormones control the organ's function and affect the growth and development of the organs. It is due to the hormones that the sexual characteristics of the organs develop and act accordingly. They also determine the use and storage of energy in the body, regulate the fluid, salt and sugar levels in the blood. Minute amount of hormones trigger large reactions within the body. All hormones are proteins, but all proteins are not hormones. Steroids are not derived from proteins, but from the fatty substances from cholesterol.
The body has a well-controlled feedback system that manages the on/off button of the endocrine gland. When the chemical level or the nutrient level in the body is abnormally high or low, the endocrine glands secrete hormones. Once the level of the body fluids is normal the hormones secretions is shut down. When the glands receive information to secrete hormones, it is a positive feedback mechanism. If the glands receives information to stop the secretions of the hormones, it is known as negative feedback.
Functions
of Endocrine System
The endocrine system is a collection of glands that secrete different hormones for the various functions and chemical reactions occurring within the body. The main function is to maintain a stable environment within the body or homeostasis. For example, maintaining the blood sugar levels according to changes occurring in the body is homeostasis. The other function of is promoting the structural changes of the body. For example, the permanent changes occurring in the body over time like height, development of sexual organs, etc. is a part of the structural changes.
Hypothalamus: A collection of specialized cells that are located in the lower central part of the brain is called the hypothalamus. The hypothalamus is the main link between the endocrine and the nervous systems. The nerve cells of the hypothalamus control the pituitary gland by stimulating or suppressing the hormone secretions.
Pituitary Gland: The pituitary gland is located at the base of the brain just below the hypothalamus. The pituitary gland is the most important part in the endocrine system. The pituitary gland secretes hormones on the basis of the emotional and seasonal changes. The hypothalamus sends information that is sensed by the brain to pituitary triggering production hormones. The pituitary gland is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe of the pituitary gland regulated the activity of the thyroid, adrenals, and the reproductive glands. The anterior lobe also produces hormones like:
Growth Hormone: To stimulate the growth of the bones and tissues. It also plays a role in the body's absorption of nutrients and minerals.
Prolactin: To activate the production of milk in lactating mothers
Thyrotropin: To stimulate the thyroid gland to produce thyroid hormones
Corticotropin: To stimulate the adrenal glands to produce certain hormones.
Endorphins are also secreted by the pituitary that acts on the nervous system and reduces the feeling of pain. The pituitary glands produces hormones that signal the reproductive organs to secrete sex hormones. The menstrual cycle and ovulation in women is also controlled by the pituitary gland. The posterior lobe of the pituitary gland produces antidiuretic hormone that helps to control the water balance in the body. Oxytoxins that trigger the contractions of the uterus in a woman who is in labor is secreted by the posterior lobe.
Thyroid Gland: The thyroid gland is situated in the front part of the lower neck that is shaped like a bow tie or butterfly. The production and secretions of the hormones of the thyroid glands are controlled by thyrotropin secreted by the pituitary gland. Thyroid produces thyroxine and triiodothyronine, that controls the rate at which the cells use up energy from food for production of energy. The thyroid hormones are very important as they help in growth of bones and the development and growth of the brain and nervous system in children. Over or under secretion of thyroid hormones leads to a number of thyroid problems in the body.
Parathyroids: These are four tiny glands that are attached to the thyroid gland. They release the parathyroid hormone that helps in regulating the level of calcium in blood along with another hormone produced by thyroid known as calcitinin.
Adrenal Glands: On each of the two kidneys, there are two triangular adrenal glands situated. The adrenal gland is divided into two parts. The outer part called the adrenal cortex produces corticosteroids, that influence and regulate the salt and water levels. They are also helpful in the body's response to stress, metabolism, immune system and the function and development of sexual organs. The inner part called the adrenal medulla, secretes catecholamines like epinephrine. This hormone is also called the adrenaline, it increases the blood pressure and heart rate when the body is under stress.
Reproductive Glands or Gonads: The gonads are present in males and females and are the main organs producing sex hormones. In men, the gonads are related to testes. The testes are located in the scrotum and secrete androgens. The most important hormone for men testosterone is secreted from the testes. In women, ovaries are the gonads that are located in the pelvis region. They produce estrogen and progesterone hormones. Estrogen is involved during the sexual maturation of the girl, that is, puberty. Progesterone along with estrogen is involved in the regulation of menstruation cycle. These hormones are also involved during pregnancy.
Pancreas: These glands are associated with the digestive system of the human body. They secrete digestive enzymes and two important hormones insulin and glucagon. These hormones work together to maintain the level of glucose in the blood. If these hormones are not secreted in the required levels, it leads to development of diabetes.
Pineal: The pineal gland is located in the center of the brain. Melatonin is secreted by this gland that helps regulate the sleeping cycle of a person.
How Does Endocrine System Function with Other Systems?
The system that helps the body communicate, control and coordinate various functions is the endocrine system. The other systems with which this system interacts includes the nervous system, the reproductive system, liver, gut, pancreas, fat and the kidneys. This interaction is carried out via a network of glands and organs. These glands and organs can produce, store and secrete many types of hormones. Thus, this system helps control and regulate:
Reproductive system: Helps in controlling the formation of gametes
Skeletal system: Helps in controlling the growth of bones
Muscular system: Helps in controlling muscle metabolism
Excretory system: Helps control water in the kidneys
Respiration system: Helps in controlling the rate of respiration
The interaction with these systems helps in maintaining the energy levels within the body. It also affects the growth and development of the body as well as maintaining homeostasis. When one or more than one of the organs stop functioning or function abnormally, it leads to diseases and disorders. It leads to over or under production of hormones, that causes hormonal imbalance. The imbalance sends the normal functioning of other systems and organs to a toss, leading to diseases and disorders. For example, when the pancreas as affected it leads to diabetes.
The hypothalamus oversees many internal body conditions. It receives nervous stimuli from receptors throughout the body and monitors chemical and physical characteristics of the blood, including temperature, blood pressure, and nutrient, hormone, and water content. When deviations from homeostasis occur or when certain developmental changes are required, the hypothalamus stimulates cellular activity in various parts of the body by directing the release of hormones from the anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by one of the following two pathways:
* Communication between the hypothalamus and the anterior pituitary occurs through chemicals (releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to the anterior pituitary through blood vessels. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a capillary network (primary plexus) and transported through veins (hypophyseal portal veins) to a second capillary network (secondary plexus) that supplies the anterior pituitary. The hormones then diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary. The releasing and inhibiting hormones secreted by the hypothalamus and the hormones produced in response by the anterior pituitary are listed in Table 1 . Many of the hormones produced by the anterior pituitary are tropic hormones (tropins), hormones that stimulate other endocrine glands to secrete their hormones.
Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary. Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and transported through the axon and stored in the axon terminals that lie in the posterior pituitary. When the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones, oxytocin and antidiuretic hormone (ADH), are produced and released in this way.
Anterior
Pituitary Hormones.
(1) Growth hormone: causes growth of almost all cells and tissues of the body.
(2) Adrenocorticotropin: causes the adrenal cortex to secrete adrenocortical hormones.
(3) Thyroid-stimulating hormone: causes the thyroid gland to secrete thyroxine and triiodothyronine.
(4) Follicle-stimulating hormone: causes growth of follicles in the ovaries prior to ovulation, promotes the formation of sperm in the testes.
(5) Luteinizing hormone: plays an important role in causing ovulation; also causes secretion of female sex hormones by the ovaries and testosterone by the testes.
(6) Prolactin: promotes development of the breasts and secretion of milk. Posterior Pituitary Hormones.
(1) Antidiuretic hormone (also called vasopressin): causes the kidneys to retain water, thus increasing the water content of the body; also, in high concentrations, causes constriction of the blood vessels throughout the body and elevates the blood pressure.
(2) Oxytocin: contracts the uterus during the birthing process, thus perhaps helping expel the baby; also contracts myoepithelial cells in the breasts, thereby expressing milk from the breasts when the baby suckles.
Adrenal
Cortex.
(1) Cortisol: have multiple metabolic functions for control of the metabolism of proteins, carbohydrates, and fats.
(2) Aldosterone: reduces sodium excretion by the kidneys and increases potassium excretion, thus increasing sodium in the body while decreasing the amount of potassium.
From this overview of the endocrine system, it is clear that most of the metabolic functions of the body are controlled one way or another by the endocrine glands. For instance, without growth hormone, the person remains a dwarf. Without thyroxine and triidothyronine from the thyroid gland, almost all the chemical reactions of the body become sluggish, and the person becomessluggish as well. Without insulin from the pancreas, the body's cells can utilize very little of the food carbohydrates for energy. And, without the sex hormones, sexual development and sexual functions are absent.
CHEMISTRY OF THE HORMONES
Chemically, the hormones are of three basic types:
(1) Steroid hormones: These all have a chemical structure similar to that of cholesterol and in most instances are derived from cholesterol itself. Different steroid hormones are secreted by
(a) the adrenal cortex (cortisol and aldosterone),
(b) the ovaries (estrogen and progesterone), (c) the testes (testosterone), and (d) the placenta (estrogen and progesterone),
(2) Derivatives of the amino acid tyrosine: Two groups of hormones are derivatives of the amino acid tyrosine. The two metabolic thyroid hormones, thyroxine and triiodothyromine, are iodinated forms of tyrosine derivatives. And the two principal hormones of the adrenal medullae, epinephrine and norepinephrine, are both catecholamines, also derived from tyrosine.
(3) Proteins or peptides: All the remaining important endocrine hormones are either proteins, peptides, or immediate derivatives of these. The anterior pituitary hormones are either proteins or large polypeptides; the posterior pituitary hormones, antidiuretic hormone and oxytocin, are peptides containing only eight amino acids. And insulin, glucagon, and parathormone are all large polypeptides.
HORMONE RECEPTORS AND THEIR ACTIVATION
The endocrine hormones almost never act directly on the intracellular machinery to control the different cellular chemical reactions; instead, they almost invariably first combine with hormone receptors on the surfaces of the cells or inside the cells. The combination of hormone and receptor then usually initiates a cascade of reactions in the cell.
Either all or almost all hormonal receptors are very large proteins, and each cell usually has some 2000 to 10,000 receptors.
Also, each receptor is usually highly specific for a single hormone; this determines the type of hormone that will act on a particular tissue. Obviously, the target tissues that are affected by a hormone are those that contain its specific receptors.
The locations of the receptors for the different types of hormones are generally the following:
(1) In the membrane. The membrane receptors are specific mostly to the protein, peptide, and catecholamine (epinephrine and norepinephrine) hormones.
(2) In the cytoplasm. The receptors for the different steroid hormones are found almost entirely in the cytoplasm.
(3) In the nucleus. The receptors for the metabolic thyroid hormones (thyroxine and triiodothyronine) are found in the nucleus, believed to be located in direct association with one or more of the chromosomes.
Pharmacology
MECHANISMS OF HORMONAL ACTION
Activation of the Receptors. The receptors in their unbound state usually are inactive, and the intracellular mechanisms that are associated with them are also inactive. However, in a few instances the unbound receptors are in the active form, and when bound with the hormone they become inhibited.
Activation of a receptor occurs in different ways for different types of receptors.
In general, the transmitter substance combines with the receptor and causes a conformational change of the receptor molecule; this in turn alters the membrane permeability to one or more ions, especially sodium, chloride, potassium, and calcium ions. A few of the general endocrine hormones also function in this same way – for instance, the effect of epinephrine and norepinephrine in changing the membrane permeability in certain of their target tissues.
In addition to this occasional direct effect of hormone receptors to change cell membrane permeability, there are also two very important general mechanisms by which a large share of the hormones function: (1) by activating the cyclic AMP system of the cells, which in turn activates multiple other intracellular functions; or (2) by activating the genes of the cell, which cause the formation of intracellular proteins that in turn initiate specific cellular functions. These two general mechanisms are described as follows:
THE CYCLIC AMP MECHANISM FOR CONTROLLING CELL FUNCTION – A "SECOND MESSENGER" FOR HORMONE MEDIATION
Many hormones exert their effects on cells by
first causing the substance cyclic
The cyclic AMP mechanism has been shown to be a way in which all the following hormones (and many more) can stimulate their target tissues:
1. Adrenocorticotropin
2. Thyroid-stimulating hormone
3. Luteinizing hormone
4. Follicle-stimulating hormone
5. Vasopressin
6. Parathyroid hormone
7. Glucagon
8. Catecholamines
9. Secretin
The hypothalamic releasing hormones.
The stimulating hormone first binds with a specific "receptor" for that hormone on the membrane surface of the target cell. The specificity of the receptor determines which hormone will affect the target cell. After binding with the membrane receptor, the combination of hormone and receptor activates the protein enzyme adenyl cyclase. This enzyme is also located in the membrane and is either bound directly with the receptor protein or closely associated with it. However, a large portion of the adenyl cyclase enzyme protrudes through the inner surface of the membrane into the cytoplasm and, when activated, causes immediate conversion of much of the cytoplasmic ATP into cyclic AMP.
Once cyclic AMP is formed inside the cell it activates still other enzymes. In fact, it usually activates a cascade of enzymes. That is, a first enzyme is activated and this activates another enzyme, which activates still a third, and so forth. The importance of this mechanism is that only a few molecules of activated adenyl cyclase in the cell membrane can cause many more molecules of the next enzyme to be activated, which can cause still many times that many molecules of the third enzyme to be activated, and so forth. In this way, even the slightest amount of hormone acting on the cell surface can initiate a very powerful cascading activating force for the entire cell.
The specific action that occurs in response to cyclic AMP in each type of target cell depends upon the nature of the intracellular machinery, some cells having one set of enzymes and other cells having other enzymes. Therefore, different functions are elicited in different target cells— such functions as
(1) initiating synthesis of specific intracellular chemicals,
(2) causing muscle contraction or relaxation,
(3) initiating secretion by the cells,
(4) altering the cell permeability,
(5) and many other possible effects.
ACTION OF STEROID HORMONES ON THE GENES TO CAUSE PROTEIN SYNTHESIS
A second major means by which hormones— specifically the steroid hormones secreted by the adrenal cortex, the ovaries, and the testes—act is to cause synthesis of proteins in the target cells; these proteins then function as enzymes or transport proteins that in turn activate other functions of the cells.
The sequence of events in steroid function is the following:
1. The steroid hormone enters the cytoplasm of the cell, where it binds with a specific receptor protein,
2. The combined receptor protein/hormone then diffuses into or is transported into the nucleus.
3. The combination now activates the transcription process of specific genes to form messenger RNA.
4. The messenger RNA diffuses into the cytoplasm where it promotes the translation process at the ribosomes to form new proteins.
To give an example, aldosterone, one of the hormones secreted by the adrenal cortex, enters the cytoplasm of renal tubular cells, which contain its specific receptor protein. Therefore, in these cells the above sequence of events ensues. After about 45 minutes, proteins begin to appear in the renal tubular cells that promote sodium reabsorption from the tubules and potassium secretion into the tubules. Thus, there is a characteristic delay in the beginning action of the steroid hormone of 45 minutes and up to several hours or even days for full action, which is in marked contrast to the almost instantaneous action of some of the peptide and amino acid-derived hormones, such as vasopressin and norepinephrine.
CONTROL OF PITUITARY SECRETION BY THE HYPOTHALAMUS
Almost all secretion by the pituitary is controlled by either hormonal or nervous signals from the hypothalamus. Indeed, when the pituitary gland is removed from its normal position beneath the hypothalamus and transplanted to some other part of the body, its rates of secretion of the different hormones (except for prolactin) fall to low levels – in the case of some of the hormones, almost to zero.
Secretion from the posterior pituitary is controlled by nerve fibers originating in the hypothalamus and terminating in the posterior pituitary. In contrast, secretion by the anterior pituitary is controlled by hormones called hypothalamic releasing and inhibitory hormones (or factors) secreted within the hypothalamus itself and then conducted to the anterior pituitary through minute blood vessels called hypothalamic-hypophysial portal vessels. In the anterior pituitary these releasing and inhibitory hormones act on the glandular cells to control their secretion.
Hypothalamic
nuclei
The hypothalamus receives signals from almost all possible sources in the nervous system. Thus, when a person is exposed to pain, a portion of the pain signal is transmitted into the hypothalamus. Likewise, when a person experiences some powerful depressing or exciting thought, a portion of the signal is transmitted into the hypothalamus. Olfactory stimuli denoting pleasant or unpleasant smells transmit strong signal components directly and through the amygdaloid nuclei into the hypothalamus. Even the concentrations of nutrients, electrolytes, water, and various hormones in the blood excite or inhibit various portions of the hypothalamus. Thus, the hypothalamus is a collecting center for information concerned with the internal well-being of the body, and in turn much of this information is used to control secretions of the many globally important pituitary hormones.
THE HYPOTHALAMIC-HYPOPHYSIAL PORTAL SYSTEM
The anterior pituitary is a highly vascular gland with extensive capillary sinuses among the glandular cells. Almost all the blood that enters these sinuses passes first through a capillary bed in the tissue of the lower tip of the hypothalamus and then through sma:ll hypothalamic-hypophysial portal vessels into the anterior pituitary sinuses. Small blood vessels project into the substance of the median eminence and then return to its surface, coalescing to form the hypothalamic-hypophysial portal vessels. These in turn pass downward along the pituitary stalk to supply blood to the anterior pituitary sinuses.
Secretion of Hypothalamic Releasing and Inhibitory Hormones into the Median Eminence. Special neurons in the hypothalamus synthesize and secrete hormones called hypothalamic releasing and inhibitory hormones (or releasing and inhibitory factors) that control the secretion of the anterior pituitary hormones. These neurons originate in various parts of the hypothalamus and send their nerve fibers into the median eminence and the tuber cinereum, the hypothalamic tissue that extends into the pituitary stalk. The endings of these fibers are different from most endings in the central nervous system in that their function is not to transmit signals from one neuron to another but merely to secrete the hypothalamic releasing and inhibitory hormones (factors) into the tissue fluids. These hormones are immediately absorbed into the capillaries of the hypothalamic-hypophysial portal system and carried directly to the sinuses of the anterior pituitary gland.
Function of the Releasing and Inhibitory Hormones. The function of the releasing and inhibitory hormones is to control the secretion of the anterior pituitary hormones. For each type of anterior pituitary hormone there is usually a corresponding hypothalamic releasing hormone; for some of the anterior pituitary hormones there is also a corresponding hypothalamic inhibitory factor. For most of the anterior pituitary hormones it is the releasing hormone that is important; but, for prolactin, an inhibitory hormone probably exerts most control. The hypothalamic releasing and inhibitory hormones (or factors) that are of major importance are:
1. Thyroid-stimulating hormone releasing hormone (TRH), which causes release of thyroid-stimulating hormone
2. Corticotropin -releasing (CRF), which causes release of adrenocorticotropin
3. Growth hormone releasing hormone (GHRH), which causes release of growth hormone, and growth hormone inhibitory hormone (GHIH), which is the same as the hormone somatostatin and which inhibits the release of growth hormone
4. Luteinizing hormone releasing hormone (LRH), which causes release of both luteinizing hormone and follicle-stimulating hormone – this hormone is also called gonadotropin-releasing hormone (GnRH)
5. Prolactin inhibitory factor (PIF), which causes inhibition of prolactin secretion
In addition to these more important hypothalamic hormones, still another excites the secretion of prolactin, and several hypothalamic inhibitory hormones inhibit some of the other anterior pituitary hormones. Each of the more important hypothalamic hormones will be discussed in detail at the time that the specific hormonal system controlled by them is presented in this and subsequent chapters.
Other Hypothalamic Substances That May Have Hormonal Effects. Multiple other substances, especially many small peptides, are found in the neurons of the hypothalamus. However, functions for these as hormones are only speculative. Yet, because they are of research interest they are listed here: (1) substance P, (2) neurotensin, (3) angiotensin II, (4) enkephalins, (5) endorphins, (6) uasoactive inhibitory polypeptide, and (7) cholecystokinin-8. Many of these same substances are also found in neurons elsewhere in the brain, suggesting that they may function as neurotransmitters both in the hypothalamus and elsewhere. In addition, some of them are in the neurons of the enteric nervous system of the gastrointestinal tract, functioning there also as neurotransmitters possibly as hormones released into the circulating blood from the nerve endings.
Thyroid Gland. (1 and 2) Thyroxine and triidothyronine: increase the rates of chemical reactions in almost all cells of the body, thus increasing the general level of body metabolism.
(3) Calcitonin: promotes the deposition of calcium in the bones and thereby decreases calcium concentration in the extracellular fluid.
Islets of Langerhans in the Pancreas.
Insulin: promotes glucose entry into most cells of the body, in this way controlling the rate of metabolism of most carbohydrates.
(2) Glucagon: increases the release of glucose from the liver into the circulating body fluids.
Ovaries. (1) Estrogens: stimulate the development of the female sex organs, the breasts, and various secondary sexual characteristics.
(2) Progesterone: stimulates secretion of "uterine milk" by the uterine endometrial glands; also helps promote development of the secretory apparatus of the breasts.
Testes. (1) Testosterone: stimulates growth of the male sex organs; also promotes the development of male secondary sex characteristics.
Parathyroid Gland. (1) Parathormone: controls the calcium ion concentration in the extracellular fluid by controlling (a) absorption of calcium from the gut, (b) excretion of calcium by the kidneys, and (c) release of calcium from the bones.
The Thyroid Metabolic Hormones The thyroid gland, which is located immediately below the larynx on either side of and anterior to the trachea, secretes two significant hormones, thyroxine and triiodothyronine, that have a profound effect on the metabolic rate of the body. It also secretes calcitonin, an important hormone for calcium metabolism.
FUNCTIONS OF THE THYROID HORMONES IN THE TISSUES
The thyroid hormones have two major effects on the body: (1) an increase in the overall metabolic rate, and (2) in children, stimulation of growth.