The adrenal medullary hormones are not essential for

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The adrenal medullary hormones are not essential for life because they also are secreted by other body tissues, but they do play a role in the physiologic stress response.
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The adrenal medulla secretes norepinephrine (NE) and epinephrine, in proportions of 15% and 85%, respectively. The effects of these hormones vary according to the specific receptor in the cell membranes of the target tissue. two types: alpha adrenergic and beta adrenergic Catecholamines Catecholamines exert their actions on many target organs (Table 64-4 ). Activation of the sympathetic nervous system, which then releases adrenal medullary catecholamines, is an important part of the body's response to stress. Catecholamines are secreted in small amounts at all times to maintain homeostasis. Severe stress triggers increased secretion of these hormones. This sympathetic activation results in the “fight-or-flight” response, a state of heightened physical and emotional awareness. The thyroid gland is in the anterior neck, directly below the cricoid cartilage (Fig. 64-6 ). It has two lobes joined by a thin strip of tissue (isthmus) in front of the trachea. Thyroid Gland The thyroid gland has a rich blood supply and is composed of follicular and parafollicular cells. Follicular cells produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3) . Parafollicular cells produce and secrete thyrocalcitonin (calcitonin [TCT]), which helps regulate serum calcium levels. Control of metabolism occurs through T 3 and T 4
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Both hormones increase metabolism, which causes an increase in oxygen use and heat production in all tissues. The two hormones differ in structure, but their functions are the same. Most circulating T 4 and T 3 are bound to plasma proteins. The proportion of bound hormone is in balance with the free hormone. The free hormone moves into the cell, where it binds to its receptor in the cell nucleus. Once in the cell, T 4 is converted to T 3 , the most active thyroid hormone. The conversion of T 4 to T 3 is impaired by stress, starvation, dyes, beta blockers, amiodarone, corticosteroids, and propyl-thiouracil (PTU). Cold temperatures increase the conversion. Table 64-5 lists thyroid hormone functions. Secretion of T 3 and T 4 is controlled by the hypothalamic-pituitary-thyroid gland axis feedback mechanism The hypothalamus secretes thyrotropin-releasing hormone (TRH). TRH triggers the anterior pituitary gland to secrete thyroid-stimulating hormone (TSH), which then stimulates the thyroid gland to make and release thyroid hormones. If thyroid hormone levels are high, TSH release is inhibited. If thyroid hormone levels are low, TSH release is increased. This is an example of a negative feedback system. Cold and stress are two factors that cause the hypothalamus to secrete TRH, which then stimulates the anterior pituitary to secrete TSH.
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