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Ch.20 Organization and Acute and Chronic Responses to Physical Activity -…
Ch.20 Organization and Acute and Chronic Responses to Physical Activity
Endocrine System
The endocrine system (endocrine means "hormone secreting") consists of a host organ (gland), minute quantities of chemical messengers (hormones) and a target or receptor organ.
Some glands serve both endocrine and exocrine functions.
There are six major endocrine organs; the pineal, pituitary, thyroid, parathyroid, thymus, and adrenal glands.
Several other organs/areas contain discrete areas of endocrine tissue or cells throughout the body.
Endocrine glands secrete substances directly into extracellular spaces around the gland.
Exocrine glands, in contrast, contain secretory ducts that carry substances directly to a specific compartment or surface.
In 2003, a humoral factor (a cytokine) was first identified as produced and released from contracting muscle cells that appeared to exhibit strong metabolic effects.
These muscle secreted cytokines (referred to as myokines) and other muscle produced peptides, produced, expressed, and released by muscle fibers, exert autocrine, paracrine, or endocrine effects.
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The liver secretes somatomedin, which affect growth of muscle, cartilage and other tissues.
Hormones, chemical substances synthesized by specific host glands, enter the bloodstream for transport throughout the body.
Hormones generally fit into one of two categories; 1) steroid-derived hormones, 2) amine and polypeptide hormones (synthesized from amino acids)
In contrast to steroid hormones, amine and peptide hormones are soluble in blood plasma. Which allows easy uptake by target cells.
The term half-life describes the time required to reduce a hormone's blood concentration by one half.
A hormones half-life gives a good indication of how long its effect persists.
Hormone-receptor binding represents the first step in initiating hormone action. The extent of a target cell's activation by a hormone depends on three factors: 1) hormone concentration in the blood, 2) Number of target cell receptors for the hormone, 3) sensitivity or strength of the union between hormone and receptor.
Up-regulation describes the state whereby target cells form more receptors in response to increasing hormone levels to increase the hormone's effect.
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Prolactin (PRL) initiates and supports milk secretion from the mammary glands.
Greater increases in PRL occur in women who run without wearing an undergarment support
Either fasting or consuming a high-fat diet enhances release of this hormone.
Gonadotropic hormones stimulate the male and female sex organs to grow and secrete their hormones at a faster rate.
the two gonadotropic hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Inconsistent reports describe short-term exercise-associated alterations in FSH and LH.
LH release is normally pulsatile, making it difficult to separate any specific exercise related change from the normal pulsatile pattern. Generally, LH concentration rises before movement begins and peaks during recovery.
Thyroid stimulating hormone (TSH) influences the thyroid gland.
The thyroid gland secretes calcium-regulating hormone calcitonin and two protein-iodine-bound hormones, thyroxine (T4) and triiodothyronine (T3, the active form of thyroid hormone).
T3 and T4 are often referred to as major metabolic hormones.
Through its stimulating effect on enzyme activity, T4 secretion raises metabolism of all cells except in the brain, spleen, testes, uterus and thyroid gland itself.
Abnormally high T4 raises BMR fourfold, while depressed levels blunt BMR and leads gains in BW and BF.
Fewer than 3% of obese persons show abnormal thyroid functions, so depressed thyroid activity cannot explain excessive body fat gain in most individuals.
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Adrenal Hormones
The adrenal glands are tissues situated just above each kidney.
The glands have two distinct parts; medulla (inner portion that secretes the catecholamines) and cortex (outer portion that secretes the mineralocorticoids, glucocorticoids, and androgens).
The adrenal medulla makes up part of the sympathetic nervous system.
It secretes the catecholamines, epinephrine and norepinephrine.
Epinephrine represents 80% of adrenal medulla secretions, whereas norepinephrine provides the principle neurotransmitter released from the sympathetic nervous system.
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The adrenal cortex, stimulated by corticotropin, secretes adrenocortical hormones.
These corticosteroid hormones, each produced in a different zone (layer) of the adrenal cortex, fit functionally into one of three groups: 1) mineralocorticoids, 2) glucocorticoids, 3) androgens
Mineralocorticoids regulate the mineral salts sodium and potassium in the extracellular fluid.
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Glucocorticoids
Cortisol (hydrocortisone), the major glucocorticoid of the adrenal cortex, affects glucose, protein and free fatty acid metabolism in six ways:
1) promotes breakdown of protein to amino acids in all cells except liver
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Gonadocorticoids
The reproductive organs (gonads) provide the major source of the so-called sex steroids, but the adrenal cortex produces androgen hormones (gonadocorticoids) with similar actions.
The adrenal cortex produces dehydroepiandrosterone, which exerts effects similar to the dominant male hormone testosterone.
Gonadal Hormones
The male testes and female ovaries are the respective endocrine reproductive glands.
These glands produce hormones that promote sex-specific physical characteristics and initiate and maintain reproductive function.
No distinctly "male" or "female" hormones exist, but rather general differences in hormone concentrations between the sexes.
Testosterone is the most important androgen secrete by testes; initiates sperm production and stimulates development of male secondary characteristics (body hair, vocal cord etc).
Testosterone's anabolic, tissue-building role contributes to male-female differences in muscle mass and strength that emerge at puberty.
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Pancreatic Hormones
Insulin regulates glucose entry into all tissues (primarily muscle and adipose) except the brain.
In process of facilitated diffusion, glucose combines with a carrier protein on the cell's plasma membrane for transport into cells. This is how insulin regulates glucose metabolism.
Any glucose not immediately catabolized for energy either stores as glycogen or synthesis to triacylglycerol.
Without insulin, only trace amounts of glucose enter the cells.
Without insulin, fatty acids metabolize as the primary energy substrate.
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Type 1 diabetes is absolute insulin deficiency that develops early in life.
Type 2 diabetes is relative insulin resistance and deficiency that develops later in life and associates with obesity, diet, and sedentary living.
A large glucose uptake for a given insulin concentration reflects increased insulin sensitivity.
Increased insulin release to a constant glucose condition relates to augmented insulin responsiveness.
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Diagnosis of metabolic syndrome (lifestyle disease) includes having three or more of the following five indicators; 1) elevated blood glucose, 2) overweight with large waist girth, 3) high triacylglycerols, 4) low levels of high-density lipoprotein cholesterol, 5) hypertension
Some racial percentages for the diagnosis Mexican Americans 31.9%, whites 23.8%, African Americans 21.6%, people reporting "other" 20.3%. African American women had 57% prevalence than men.
Glucagon is the insulin "antagonist" hormone
Glucagon stimulates both glycogneolysis and gluconeogenesis by the liver and increases lipid catabolism.
A decrease in blood glucose concentration from prolonged intense physical activity or food (or carbohydrate) restriction stimulates glucagon release.
Exercise Training And Endocrine Function
The magnitude of hormonal response to a standard exercise load generally declines with endurance training.
A similar level of hormonal response occurs regardless of training state when subjects exercise at same relative level. With maximal exertion trained subjects exhibit the same or somewhat greater hormonal response compared to the untrained.
Anterior Pituitary Hormones
Compared with the untrained, endurance-trained individuals show less rise win blood GH levels at a given intensity- a response attributed to reduced stress as training progresses and fitness improves.
Regardless of training status, women typically maintain higher GH levels at rest than men.
Integrated GH concentrations (exercise plus recovery) for the group average 45% lower than pertaining values at both training measures. Responses for plasma catecholamines and blood lactate paralleled the decrease in GH.
Because the constant-load exercise test represented less physiologic demand after training (reflected by lower catecholamine and lactate levels), a similar release of GH after training probably requires higher absolute exercise intensity.
Training increases ACTH release during physical activity- a response that stimulates adrenal gland activity to promote fat catabolism and spare glycogen.