High blood glucose levels make it difficult for the kidneys to recover all the glucose from nascent urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced; this may result in dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system.
Oversecretion of insulin can cause hypoglycemia , low blood glucose levels. This causes insufficient glucose availability to cells, often leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.
When blood glucose levels decline below normal levels, for example between meals or when glucose is utilized rapidly during exercise, the hormone glucagon is released from the alpha cells of the pancreas.
Glucagon raises blood glucose levels, eliciting what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose.
This process of glucose synthesis is called gluconeogenesis. Glucagon also stimulates adipose cells to release fatty acids into the blood. These actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising blood glucose levels inhibit further glucagon release by the pancreas via a negative feedback mechanism. In this way, insulin and glucagon work together to maintain homeostatic glucose levels, as shown in Figure 5.
Pancreatic tumors may cause excess secretion of glucagon. Type I diabetes results from the failure of the pancreas to produce insulin. Which of the following statement about these two conditions is true? The basal metabolic rate, which is the amount of calories required by the body at rest, is determined by two hormones produced by the thyroid gland: thyroxine , also known as tetraiodothyronine or T 4 , and triiodothyronine , also known as T 3.
These hormones affect nearly every cell in the body except for the adult brain, uterus, testes, blood cells, and spleen. They are transported across the plasma membrane of target cells and bind to receptors on the mitochondria resulting in increased ATP production.
In the nucleus, T 3 and T 4 activate genes involved in energy production and glucose oxidation. T 3 and T 4 release from the thyroid gland is stimulated by thyroid-stimulating hormone TSH , which is produced by the anterior pituitary. TSH binding at the receptors of the follicle of the thyroid triggers the production of T 3 and T 4 from a glycoprotein called thyroglobulin.
Thyroglobulin is present in the follicles of the thyroid, and is converted into thyroid hormones with the addition of iodine. Iodine is formed from iodide ions that are actively transported into the thyroid follicle from the bloodstream.
A peroxidase enzyme then attaches the iodine to the tyrosine amino acid found in thyroglobulin. T 3 has three iodine ions attached, while T 4 has four iodine ions attached. T 3 and T 4 are then released into the bloodstream, with T 4 being released in much greater amounts than T 3. As T 3 is more active than T 4 and is responsible for most of the effects of thyroid hormones, tissues of the body convert T 4 to T 3 by the removal of an iodine ion.
Most of the released T 3 and T 4 becomes attached to transport proteins in the bloodstream and is unable to cross the plasma membrane of cells. These protein-bound molecules are only released when blood levels of the unattached hormone begin to decline.
The follicular cells of the thyroid require iodides anions of iodine in order to synthesize T 3 and T 4. Iodides obtained from the diet are actively transported into follicle cells resulting in a concentration that is approximately 30 times higher than in blood. The typical diet in North America provides more iodine than required due to the addition of iodide to table salt. Inadequate iodine intake, which occurs in many developing countries, results in an inability to synthesize T 3 and T 4 hormones.
The thyroid gland enlarges in a condition called goiter , which is caused by overproduction of TSH without the formation of thyroid hormone. Thyroglobulin is contained in a fluid called colloid, and TSH stimulation results in higher levels of colloid accumulation in the thyroid. In the absence of iodine, this is not converted to thyroid hormone, and colloid begins to accumulate more and more in the thyroid gland, leading to goiter. Disorders can arise from both the underproduction and overproduction of thyroid hormones.
Hypothyroidism , underproduction of the thyroid hormones, can cause a low metabolic rate leading to weight gain, sensitivity to cold, and reduced mental activity, among other symptoms.
In children, hypothyroidism can cause cretinism, which can lead to mental retardation and growth defects. Hyperthyroidism , the overproduction of thyroid hormones, can lead to an increased metabolic rate and its effects: weight loss, excess heat production, sweating, and an increased heart rate.
Regulation of blood calcium concentrations is important for generation of muscle contractions and nerve impulses, which are electrically stimulated. If calcium levels get too high, membrane permeability to sodium decreases and membranes become less responsive. If calcium levels get too low, membrane permeability to sodium increases and convulsions or muscle spasms can result.
Figure 6. Parathyroid hormone PTH is released in response to low blood calcium levels. It increases blood calcium levels by targeting the skeleton, the kidneys, and the intestine. Blood calcium levels are regulated by parathyroid hormone PTH , which is produced by the parathyroid glands, as illustrated in Figure 6. PTH triggers the formation of calcitriol, an active form of vitamin D, which acts on the intestines to increase absorption of dietary calcium.
PTH release is inhibited by rising blood calcium levels. Hyperparathyroidism results from an overproduction of parathyroid hormone. This results in excessive calcium being removed from bones and introduced into blood circulation, producing structural weakness of the bones, which can lead to deformation and fractures, plus nervous system impairment due to high blood calcium levels. Hypoparathyroidism, the underproduction of PTH, results in extremely low levels of blood calcium, which causes impaired muscle function and may result in tetany severe sustained muscle contraction.
The hormone calcitonin , which is produced by the parafollicular or C cells of the thyroid, has the opposite effect on blood calcium levels as does PTH. Calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and stimulating calcium excretion by the kidneys. This results in calcium being added to the bones to promote structural integrity. Calcitonin is most important in children when it stimulates bone growth , during pregnancy when it reduces maternal bone loss , and during prolonged starvation because it reduces bone mass loss.
In healthy nonpregnant, unstarved adults, the role of calcitonin is unclear. Hormonal regulation is required for the growth and replication of most cells in the body. Growth hormone GH , produced by the anterior portion of the pituitary gland, accelerates the rate of protein synthesis, particularly in skeletal muscle and bones. A rise in blood glucose levels triggers release of insulin from the pancreas.
This mechanism of hormone production is stimulated by: humoral stimuli hormonal stimuli neural stimuli negative stimuli Which mechanism of hormonal stimulation would be affected if signaling and hormone release from the hypothalamus was blocked? Compare and contrast hormonal and humoral stimuli. Answers Patient A has symptoms associated with decreased metabolism, and may be suffering from hypothyroidism.
Patient B has symptoms associated with increased metabolism, and may be suffering from hyperthyroidism. A B Hormone production and release are primarily controlled by negative feedback. In negative feedback systems, a stimulus causes the release of a substance whose effects then inhibit further release. Increasing levels of these hormones in the blood then feed back to the hypothalamus and anterior pituitary to inhibit further signaling to the thyroid gland. The term humoral is derived from the term humor, which refers to bodily fluids such as blood.
Humoral stimuli refer to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. Hormonal stimuli refer to the release of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by hormones released by other endocrine organs. For example, the hypothalamus produces hormones that stimulate the anterior pituitary. For example, the anterior pituitary releases thyroid-stimulating hormone, which stimulates the thyroid gland to produce the hormones T 3 and T 4.
The complex interplay of the actions of various neurotransmitters regulates the production and release of hormones from the hypothalamus. The hypothalamic hormones are released into blood vessels that connect the hypothalamus and the pituitary gland i. Because they generally promote or inhibit the release of hormones from the pituitary gland, hypothalamic hormones are commonly called releasing or inhibiting hormones.
The major releasing and inhibiting hormones include the following also see table , p. Corticotropin-releasing hormone CRH , which is part of the hormone system regulating carbohydrate, protein, and fat metabolism as well as sodium and water balance in the body. Gonadotropin-releasing hormone GnRH , which helps control sexual and reproductive functions, including pregnancy and lactation i.
Thyrotropin-releasing hormone TRH , which is part of the hormone system controlling the metabolic processes of all cells and which contributes to the hormonal regulation of lactation. Somatostatin, which also affects bone and muscle growth but has the opposite effect as that of GHRH.
Dopamine, a substance that functions primarily as a neurotransmitter but also has some hormonal effects, such as repressing lactation until it is needed after childbirth. The pituitary also sometimes called the hypophysis is a gland about the size of a small marble and is located in the brain directly below the hypothalamus. The pituitary gland consists of two parts: the anterior pituitary and the posterior pituitary.
The anterior pituitary produces several important hormones that either stimulate target glands e. The pituitary hormones include adrenocorticotropic hormone ACTH ; gonadotropins; thyroid-stimulating hormone TSH , also called thyrotropin; growth hormone GH ; and prolactin.
Thus, ACTH stimulates the adrenal cortex to produce corticosteroid hormones—primarily cortisol—as well as small amounts of female and male sex hormones. The gonadotropins comprise two molecules, luteinizing hormone LH and follicle-stimulating hormone FSH. These two hormones regulate the production of female and male sex hormones in the ovaries and testes as well as the production of the germ cells—that is, the egg cells i.
TSH stimulates the thyroid gland to produce and release thyroid hormone. The remaining two pituitary hormones, GH and prolactin, directly affect their target organs.
GH is the most abundant of the pituitary hormones. For example, it stimulates the linear growth of the bones; promotes the growth of internal organs, fat i. Accordingly, the GH levels in the blood are highest during early childhood and puberty and decline thereafter. Nevertheless, even relatively low GH levels still may be important later in life, and GH deficiency may contribute to some symptoms of aging.
In addition to its growth-promoting role, GH affects carbohydrate, protein, and fat i. Thus, GH increases the levels of the sugar glucose in the blood by reducing glucose uptake by muscle cells and adipose tissue and by promoting glucose production i. GH also enhances the uptake of amino acids from the blood into cells, as well as their incorporation into proteins, and stimulates the breakdown of lipids in adipose tissue.
To elicit these various effects, GH modulates the activities of numerous target organs, including the liver, kidneys, bone, cartilage, skeletal muscle, and adipose cells. For some of these effects, GH acts directly on the target cells. In other cases, however, GH acts indirectly by stimulating the production of a molecule called insulin-like growth factor 1 IGF-1 in the liver and kidneys.
The blood then transports IGF-1 to the target organs, where it binds to specific receptors on the cells. This interaction then may lead to the increased DNA production and cell division that underlie the growth process. This regulatory mechanism also involves a short-loop feedback component, by which GH acts on the hypothalamus to stimulate somatostatin release. In addition, GH release is enhanced by stress, such as low blood sugar levels i.
Acute and chronic alcohol consumption have been shown to reduce the levels of GH and IGF-1 in the blood. Both effects have been observed in animals as well as in humans. Those deleterious effects of alcohol may be particularly harmful to adolescents, who require GH for normal development and puberty. Together with other hormones, prolactin plays a central role in the development of the female breast and in the initiation and maintenance of lactation after childbirth.
Several factors control prolactin release from the anterior pituitary. For example, prolactin is released in increasing amounts in response to the rise in estrogen levels in the blood that occurs during pregnancy. In nursing women, prolactin is released in response to suckling by the infant. Several releasing and inhibitory factors from the hypothalamus also control prolactin release.
The most important of those factors is dopamine, which has an inhibitory effect. Alcohol consumption by nursing women can influence lactation both through its effects on the release of prolactin and oxytocin see the following section and through its effects on the milk-producing i. The posterior pituitary does not produce its own hormones; instead, it stores two hormones—vasopressin and oxytocin—that are produced by neurons in the hypothalamus.
Both hormones collect at the ends of the neurons, which are located in the hypothalamus and extend to the posterior pituitary. Thus, AVP release promotes the reabsorption of water from the urine in the kidneys. Through this mechanism, the body reduces urine volume and conserves water. AVP release from the pituitary is controlled by the concentration of sodium in the blood as well as by blood volume and blood pressure.
For example, high blood pressure or increased blood volume results in the inhibition of AVP release. Consequently, more water is released with the urine, and both blood pressure and blood volume are reduced. Alcohol also has been shown to inhibit AVP release. Conversely, certain other drugs e. Oxytocin, the second hormone stored in the posterior pituitary, stimulates the contractions of the uterus during childbirth.
In nursing women, the hormone activates milk ejection in response to suckling by the infant i. The adrenal glands are small structures located on top of the kidneys. Structurally, they consist of an outer layer i.
The adrenal cortex produces numerous hormones, primarily corticosteroids i. The cortex is also the source of small amounts of sex hormones; those amounts, however, are insignificant compared with the amounts normally produced by the ovaries and testes. The adrenal medulla generates two substances—adrenaline and noradrenaline—that are released as part of the fight-or-flight response to various stress factors.
The primary glucocorticoid in humans is cortisol also called hydro-cortisone , which helps control carbohydrate, protein, and lipid metabolism. For example, cortisol increases glucose levels in the blood by stimulating gluconeogenesis in the liver and promotes the formation of glycogen i. Cortisol also reduces glucose uptake into muscle and adipose tissue, thereby opposing the effects of insulin.
Furthermore, in various tissues, cortisol promotes protein and lipid breakdown into products i. In addition to those metabolic activities, cortisol appears to protect the body against the deleterious effects of various stress factors, including acute trauma, major surgery, severe infections, pain, blood loss, hypoglycemia, and emotional stress. All of these stress factors lead to drastic increases in the cortisol levels in the blood.
For people in whom cortisol levels cannot increase e. Finally, high doses of cortisol and other corticosteroids can be used medically to suppress tissue inflammation in response to injuries and to reduce the immune response to foreign molecules.
Its principal functions are to conserve sodium and to excrete potassium from the body. For example, aldosterone promotes the reabsorption of sodium in the kidney, thereby reducing water excretion and increasing blood volume. Similarly, aldosterone decreases the ratio of sodium to potassium concentrations in sweat and saliva, thereby preventing sodium loss via those routes. The effect can be highly beneficial in hot climates, where much sweating occurs.
In contrast to the glucocorticoids, pituitary, or hypothalamic, hormones do not regulate aldosterone release. Instead, it is controlled primarily by another hormone system, the reninangiotensin system, which also controls kidney function. In addition, the levels of sodium and potassium in the blood influence aldosterone levels.
The gonads i. First, they produce the germ cells i. Second, the gonads synthesize steroid sex hormones that are necessary for the development and function of both female and male reproductive organs and secondary sex characteristics e. Three types of sex hormones exist; each with different functions: 1 estrogens e. In addition to the reproductive functions, sex hormones play numerous essential roles throughout the body.
For example, they affect the metabolism of carbohydrates and lipids, the cardiovascular system, and bone growth and development. The major estrogen is estradiol, which, in addition to small amounts of estrone and estriol, is produced primarily in the ovaries.
Other production sites of estrogens include the corpus luteum, 2 the placenta, and the adrenal glands. In men and postmenopausal women, most estrogens present in the circulation are derived from the conversion of testicular, adrenal, and ovarian androgens. The conversion occurs in peripheral tissues, primarily adipose tissue and skin.
Pituitary gland - The pituitary gland receives signals from the hypothalamus. This gland has two lobes, the posterior and anterior lobes. The posterior lobe secretes hormones that are made by the hypothalamus. The anterior lobe produces its own hormones, several of which act on other endocrine glands.
Thyroid gland - The thyroid gland is critical to the healthy development and maturation of vertebrates and regulates metabolism. Adrenal glands - The adrenal gland is made up of two glands: the cortex and medulla. These glands produce hormones in response to stress and regulate blood pressure, glucose metabolism, and the body's salt and water balance. Pancreas - The pancreas is responsible for producing glucagon and insulin.
Both hormones help regulate the concentration of glucose sugar in the blood. Gonads - The male reproductive gonads, or testes, and female reproductive gonads, or ovaries, produce steroids that affect growth and development and also regulate reproductive cycles and behaviors. The major categories of gonadal steroids are androgens, estrogens, and progestins, all of which are found in both males and females but at different levels. Scientific research on human epidemiology, laboratory animals, and fish and wildlife suggests that environmental contaminants can disrupt the endocrine system leading to adverse-health consequences.
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