The Adrenal Gland
Contents
Introduction to the adrenal gland
These glands, although small in size are nevertheless essential for life. As their name suggests they are located (AD) above the (RENAL) kidneys. Each gland has a dual structure with separate functions together with its own embryology. In high power cross section the gland can be seen to comprise of three outer cortical layers and an inner medullary layer. The three cortical layers, which comprise 90% of the glandular mass are divided into the following:
and finally
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Adrenal medulla
The adrenal medulla is:
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The Adrenal Cortex
The adrenal cortex is involved in the production of numerous hormones in each of the three zones. The major secretion of the zones differ as do the mechanisms responsible for the modulation of their activity. Although the understanding of steroidogenesis (40+ hormones so far) may be required for the biochemistry course only rudementary knowledge is necessary to understand the function of the cortical hormones. In terms of synthesis the hormones can be divided into three groups depending upon the number of carbon atoms in their molecule;
These hormones can also be divided according to their primary action mineralo-(salt), gluco-(metabolism) or sex.
The mechanism of action of steroid hormones was covered in "Introduction to Endocrinology - lecture 1" remember that steroid hormones act on target cells by passing through the membrane and acting on an internal receptor.
NOTE: when answering an exam question regarding the action of hormones it is important to consider the following;
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Aldosterone - a mineralocorticoid
The main rôle of this mineralocorticoid is in the long term control of the cardiovascular system cf. Kidney lectures. Its origins are in the zona glomerulosa of the adrenal cortex where it is synthesised from cortisol originating in the zona fasciculata. The majority of the aldosterone that is released into the circulation (60%) is bound to a plasma protein, an alpha globulin called "transcortin" or corticosteroid-binding protein. The half life of the hormone is approximately 15-30 minutes following release principally by the renin / angiotensin system with some involvement of ACTH (see figure). The actions of aldosterone are varied;
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Cortisol/ corticosterone - glucocorticoids
Cortisol and corticosterone are both formed in the zona fasciculata of the adrenal cortex. In man, the gland produces these hormones in the ratio 10:1 cortisol:corticosterone. These hormones are 21 carbon steroids which are 90-95% bound to the plasma protein transcortin. This protein acts as a buffer of the hormone, giving it a half life of 60-90 minutes. Cortisol and corticosterone are released following acute and long term stress and may also be released following phase changes in time producing the familiar feelings known as "jet lag". The actions of the hormones can be summarised as follows:-
carbohydrate actions:-
Protein actions:-
Fat actions:-
Further actions:-
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Androgens/ oestrogens - sex steroids
Androgens and oestrogens are both formed in the zona reticularis of the adrenal cortex. Although these hormones are produced in the adrenal their production is insignificant when compared to the production of sex steroids by the gonads (ovaries and testes). The release of sex steroids from the adrenal cortex is governed purely by the action ACTH from the anterior pituitary.
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Summary of the actions of sex steroids:-
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Abnormal adrenal functions - medulla:-
Abnormal adrenal functions can be associated with the adrenal medulla or cortex.
Overproduction of catecholamines is usually associated with catecholamine-secreting tumours such as phaeochromocytoma. The increased plasma levels of these hormones will produce severe hypertension (high blood pressure) and hyperglycaemia.
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