bioreading lecture9

bioreading lecture9 - 41.3.9 The adrenal gland is two...

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41.3.9 The adrenal gland is two glands in one Title [X] content » link [Notes/Highlighting] Bold Italic Link Clear Revert SAVE [Resource Table of Contents] An adrenal gland sits above each kidney, just below the middle of your back. Functionally and anatomically, each adrenal gland consists of a gland within a gland (Figure 41.11). The core, called the adrenal medulla, produces the hormone epinephrine (also known as adrenaline) and, to a lesser degree, norepinephrine (or noradrenaline), which also acts as a neurotransmitter in the nervous system. Surrounding the medulla is the adrenal cortex, which produces steroid hormones. The medulla develops from nervous tissue and is under the control of the nervous system; the cortex is under hormonal control, largely by corticotropin (ACTH) from the anterior pituitary. Figure 41.11 The Adrenal Gland Has an Outer and an Inner Portion An adrenal gland, consisting of an outer cortex and an inner medulla, sits above each kidney. The medulla and the cortex produce different hormones. THE ADRENAL MEDULLA The adrenal medulla produces epinephrine and norepinephrine in response to stressful situations, arousing the body to action. As we saw earlier in this chapter, epinephrine increases heart rate and blood pressure and diverts blood flow to active muscles and away from the gut. Epinephrine and norepinephrine are both amine hormones— derivatives of the amino acid tyrosine. They are water-soluble, and both bind to the same receptors on the surfaces of target cells. These receptors can be grouped into two general types, α -adrenergic and β - adrenergic receptors, which stimulate different actions within cells (see Figure 41.17 later in this chapter). Epinephrine acts equally on both types, but norepinephrine acts mostly on α -adrenergic receptors.
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Therefore, drugs called beta blockers, so named because they selectively block β -adrenergic receptors, can reduce the fight-or-flight responses to epinephrine without disrupting the physiological regulatory functions of norepinephrine. Beta blockers are commonly prescribed to reduce the symptoms of anxiety, such as dry mouth and elevated heart rate (heart palpitations). THE ADRENAL CORTEX The cells of the adrenal cortex use cholesterol to produce three classes of steroid hormones, collectively called corticosteroids: * The glucocorticoids influence blood glucose concentrations as well as other aspects of fat, protein, and carbohydrate metabolism. * The mineralocorticoids influence the ionic balance of extracellular fluids. * The sex steroids play roles in sexual development, sex drive, and anabolism. The adult adrenal cortex secretes sex steroids in only negligible amounts. The major producers of sex steroids are the gonads, as we will see in the following section.
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