The two adrenal glands are flattened bodies that sit on top of each kidney. Each gland is made up of an inner core called the adrenal medulla and an outer shell called the adrenal cortex.
两个肾上腺体为扁平体，位于各肾顶部。各腺由一个称为肾上腺髓质和内核和一个称为肾上腺皮质的外壳组成。
The adrenal medulla is actually part of the sympathetic nervous system (SNS). It is a ganglion of neurons that releases the neurotransmitters norepinephrine and epinephrine into circulation when the SNS is stimulated. The secretion of these neurotransmitters directly into the bloodstream allows them to act as hormones, traveling from the adrenal medulla to react with specific receptor sites throughout the body. This is thought to be a backup system for the sympathetic system, adding an extra stimulus to the fight-or-flight response.
The adrenal cortex surrounds the medulla and consists of three layers of cells, each of which synthesizes chemically different types of steroid hormones that exert physiological effects throughout the body. The adrenal cortex produces hormones called corticosteroids. There are three types of corticosteroids: androgens, glucocorticoids, and mineralocorticoids. Androgens are a form of the male sex hormone testosterone, both men and women produce these hormones. They affect electrolytes, stimulate protein production, and decrease protein breakdown. They are used pharmacologically to treat hypogonadism or to increase protein growth and red blood cell production.
The adrenal cortex responds to adrenocorticotropic hormone (ACTH) released from the anterior pituitary. ACTH, in turn, responds to corticotropin-releasing hormone (CRH) released from the hypothalamus. This happens regularly during a normal day in what is called diurnal rhythm. A person who has a regular cycle of sleep and wakefulness will produce high levels of CRH during sleep, usually around midnight. A resulting peak response of increased ACTH and adrenocortical hormones occurs sometime early in the morning, around 6 to 9 AM. This high level of hormones then suppresses any further CRH or ACTH release. The corticosteroids are metabolized and excreted slowly throughout the day and fall to low levels by evening. At this point the hypothalamus and pituitary sense low levels of the hormones and begin the production and release of CRH and ACTH again. This peaks around midnight, and the cycle starts again. It is thought that this diurnal rhythm is partly in response to exposure to light. People who work night shifts and have a different exposure to light cycling do not have the same diurnal schedule as people who are awake in daylight and sleep at night.
Activation of the stress reaction through the SNS bypasses the usual diurnal rhythm and causes release of ACTH and secretion of the adrenocortical hormones—an important aspect of the stress (“fight-or-flight”) response. The stress response is activated with cellular injury or when a person perceives fear or feels anxious. These hormones have many actions, including the following:
l Increasing the blood volume (aldosterone effect)
增加血容量（醛固酮效应）
l Causing the release of glucose for energy
释放葡萄糖提供能量
l Slowing the rate of protein production (which preserves energy)
蛋白质生产速度放慢（保存能量）
l Blocking the activities of the inflammatory and immune systems (which preserves a great deal of energy)
阻滞炎症和免疫系统活动（此举保存大量能量）
These actions are important during an acute stress situation, but they can cause adverse reactions in periods of extreme or prolonged stress. For instance, a postoperative patient who is very fearful and stressed may not heal well because protein building is blocked; infections may be hard to treat in such a patient because the inflammatory and immune systems are not functioning adequately.
Aldosterone is also released without ACTH stimulation when the blood surrounding the adrenal gland is high in potassium, a direct stimulus for aldosterone release. Aldosterone causes the kidneys to reabsorb sodium with a resultant excretion of potassium to restore homeostasis.
1. The mineralocorticoids produced by the adrenal glands are produced within the?
A. Parafollicular cells
B. Zona reticularis
C. Zona glomerulosa
D. Zona fasciculate
2. Based on the diurnal rhythm, adrenocortical hormone release peaks:
A. Early in the morning
B. Around 6 to 9 AM
C. Around evening
D. Around midnight
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答案Answers 1. C. Zona glomerulosa Rationale: The zona glomerulosa is responsible for synthesis of aldosterone as well as some other corticosteroids such as glucocorticoid while Zona reticularis (B) acts in collaboration with the Zona fasciculate (D) and is primarily involved in the synthesis as well as secretion of different sex hormones that work as a substitute for gonadal hormones. Calcitonin is produced by the parafollicular cells (A) found in the connective tissues between the follicles. 2. D. Around midnight Rationale: Response of increased ACTH and adrenocortical hormones peaks sometime early in the morning, around 6 to 9 AM and the hypothalamus and pituitary sense low levels of the hormones and begin the production and release of CRH and ACTH, which peaks around midnight.
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