An Overview of the Endocrine System

The study of hormones and the glands that secrete them is called [1]. Hormone-producing glands lack the [2] that exocrine glands have, and secrete their products into the bloodstream instead. The blood carries the hormones to their [3] organs, which have receptors to bind them. This is similar to the way neurotransmitters work. It is difficult to sharply distinguish the endocrine and nervous systems from each other, among other reasons because some hormones are secreted by [4] cells, which are nerve cells that release their products into the blood.

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The Hypothalamus and Pituitary Gland

The pituitary gland consists of a division called the [5] that is directly connected to the hypothalamus by a stalk with a nerve tract in it, and a division called the [6] that is connected to the hypothalamus only by way of blood vessels. The most physiologically important parts of these divisions are the pars distalis, or [7], and pars nervosa, or [8].

The 7 secretes six hormones: growth hormone; prolactin; [9], which stimulates the adrenal gland, adipose tissue, and pancreas; [10], which stimulates the thyroid gland; and FSH and LH, collectively called the [11] because they stimulate the ovaries and testes. The relationship between the anterior pituitary and another endocrine gland is called an [12], and the hormones that communicate from the pituitary to another endocrine gland are called [13] hormones. The anterior pituitary is controlled by releasing and inhibiting hormones that come from the [14] and reach it by way of the hypothalamo-hypophyseal [15]. It is also influenced by the hormones secreted by the pituitary's target organs. For example, thyroid hormone may reduce the pituitary's secretion of thyroid-stimulating hormone. This type of control is called [16].

The 8 secretes only two hormones: [17], which plays a role in childbirth and lactation, and [18], which regulates water balance. The release of these hormones is controlled by [19] reflexes.

Unlike most pituitary hormones, growth hormone (GH), or [20], has widespread effects on many target organs and tissues. GH stimulates the breakdown of [21] and synthesis of [22] and reduces urinary excretion of [23]. Two tissues whose growth is especially stimulated by GH are [24] and [25]. Some of the effects of GH may be enhanced by peptides called insulin-like growth factors, or [26]. A deficiency of GH in childhood causes a growth disorder called [27]. Excessive GH secretion causes [28] if it occurs in childhood and [29] if it occurs in adulthood. The main effect of posterior pituitary hyposecretion is [30], in which there is excessive urine output and intense thirst.

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Other Endocrine Glands

The [31] produces serotonin by day and converts it to [32] at night. It may have a role in timing the onset of puberty. The [33], a gland just above the heart, secretes hormones that regulate T lymphocyte activity. The [34], a little higher in the neck, requires the mineral [35] to produce its hormones, T₃ and T₄. T₄ is also called [36]. In the absence of 35, this gland fails to make T₃ and T₄, and then for lack of negative feedback to the pituitary, it becomes enlarged-a condition called [37]. T₃, mainly, raises the body's metabolic rate and has a heat-producing, or [38], effect. This gland also produces [39], a hormone that regulates calcium metabolism in children. On the posterior side of this gland are four small [40] glands, whose hormone strongly affects calcium metabolism in both children and adults.

The [41] glands, located on the superior pole of each kidney, consist of an inner [42] that secretes mainly epinephrine and norepinephrine and an outer [43] that secretes steroid hormones (corticosteroids). These steroids include [44], which regulates Na⁺ and K⁺ balance; testosterone-like sex steroids called [45]; and [46], a collective term for hormones such as cortisol and corticosterone that regulate carbohydrate, fat, and protein metabolism.

The [47] is mainly a digestive gland, but has cell clusters called [48] that secrete two glucose-regulating hormones. One of these, [49], is secreted when blood glucose concentration is high and stimulates its uptake by cells. The other, [50], is secreted when blood glucose concentration is low and stimulates the release of glucose from stored glycogen. Hyposecretion or ineffectiveness of the former hormone causes [51], which is characterized by excessive urine output, or [52]; high blood sugar, or [53]; and glucose in the urine, or [54]. This disease results from a lack of 49 in only 10% of cases. This form of the disease is called [55] and used to be called [56] because it typically begins early in life. The other 90% of cases result from a lack of receptors for 49. This form of the disease is called [57] and usually appears later in life.

Most sex steroids are secreted by the gonads–the [58] secrete estradiol and progesterone and interstitial cells of the testes secrete [59]. The heart secretes the hormone [60], which stimulates the kidneys to excrete more sodium. The kidneys secrete [61], which stimulates the bone marrow to produce more erythrocytes. The digestive tract secretes several [62] hormones, enabling one part of the tract to communicate with another.

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Hormones and Their Actions

Hormones call into three chemical classes: [63], which are cholesterol derivatives; [64], which are synthesized from single amino acids; and [65], which are chains of 3 to 200 or more amino acids. Hormones in the first group are hydrophobic and have to be bound to [66] in the blood plasma to be transported. These transporters also prolong the [67] of these hormones by slowing down their excretion in the urine. Thyroid hormones belong in the second class. The thyroid stores them in the form of a colloidal protein called [68]. When thyroxine is released into the blood, it binds to a transporter called [69]. Hormones in the third class are hydrophilic, and most of them do not require transporters in the blood plasma.

When they reach their target cells, thyroxine and hormones in class 63 readily diffuse through the plasma membrane and bind to receptors in the [70]. Here, they activate the transcription of [71] and thus stimulate the production of new proteins. Peptide hormones and hormones of class 64 other than thyroxine must bind to [72] on the cell surface and activate the formation of [73] messengers within the cell. The best-known of these messengers is [74]. Another is [75] ions, which can act by changing the cell's membrane potential, activating cytoplasmic enzymes, or binding to a receptor called [76], which then activates protein kinases.

Each hormone that binds to a target cell receptor triggers the activation of thousands of enzyme molecules, and each enzyme produces thousands of product molecules. Thus, hormones have a magnifying effect called [77] that allows small amounts of hormones to have very large effects on target organs. When a hormones is highly concentrated for a long period of time, a target cell may reduce the number of receptors for it, a process called [78] that avoids over-stimulation. When hormone concentrations are low, a target cell may show [79], the production of increased receptors, making it more sensitive to the hormone. If two hormones acting together have a stronger effect than the sum of their effects when acting alone, they are said to be [80]. If two hormones oppose each other's effects, they are said to be [81].

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Eicosanoids and Other Chemical Messengers

Chemical signals called local hormones, or [82], neither come from neurons like neurotransmitters nor travel in the bloodstream like hormones; they diffuse only a short distance from source to target cell. Somatostatin in the [83] and histamine from [84] cells are examples. So are the [85], 20-carbon derivatives of arachidonic acid. The enzyme [86] converts arachidonic acid to leukotrienes, which are involved in allergy and inflammation. The enzyme [87] converts arachidonic acid to prostacyclin, thromboxanes, and [88], which were first discovered in bull semen but are now known to stimulate a wide variety of responses in human tissues.

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Stress and Adaptation

The general adaptation syndrome (GAS) is a concept created by biochemist [89]. Following his work, many authorities came to view stress as any situation in which the [90] axis is stimulated. The GAS involves an early stage, the [91], in which CRH and ACTH are secreted, glucocorticoid levels rise, glucose and fatty acids are mobilized, and the sympathetic nervous system and adrenal medulla show elevated activity. In the stage of [92], ideally these responses restore homeostasis. If this fails, however, a life-threatening stage of [93] may follow, in which suppression of the [94] system can leave a person highly vulnerable to infections or cancer.

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