Anatomy & Physiology

Anatomy & Physiology Study Guide suprarenal glands are highly vascularized. The suprarenal gland is divided into two parts with separate endocrine functions: a superficial suprarenal cortex and an inner suprarenal medulla. The Suprarenal Cortex The suprarenal cortex produces more than two dozen steroid hormones, collectively called adrenocortical steroids, or simply corticosteroids . In the bloodstream, these hormones are bound to transport proteins called transcortins. Corticosteroids are vital. If the suprarenal glands are destroyed or removed, the individual will die unless corticosteroids are administered. Corticosteroids, like other steroid hormones, exert their effects by determining which genes are transcribed in the nuclei of their target cells, and at what rates. Deep to the suprarenal capsule are three distinct regions, or zones, in the suprarenal cortex: an outer zona glomerulosa; a middle zona fasciculata; and an inner zona reticularis. Each zone synthesizes specific steroid hormones. The zona glomerulosa , the outer area of the suprarenal cortex, produces mineral corticoids , steroid hormones that affect the electrolyte composition of body fluids. Aldosterone is the principal mineralocorticoid produced by the suprarenal cortex. The zona fasciculata produces steroid hormones collectively known as glucocorticoids . When stimulated by ACTH from the adenohypophysis, the zona fasciculata primarily secretes cortisol , also called hydrocortisone, along with smaller quantities of the related steroid corticosterone . The liver turns some of the circulating cortisol to cortisone, another active glucocorticoid. Glucocorticoids accelerate the rates of glucose synthesis and glycogen formation, especially in the liver. Glucocorticoids also show anti- inflammatory effects. The zona reticularis forms a narrow band bordering each suprarenal medulla. This zone accounts for only about seven percent of the total volume of the suprarenal cortex. Under stimulation by ACTH, the zona reticularis normally produces small quantities of androgens, the sex hormones produced in large quantities by the testes. Once in the bloodstream, some of the androgens released by the zona reticularis are transformed into estrogens, the dominant sex hormone in females. Suprarenal androgens encourage the development of pubic hair in boys and girls before puberty. While not important in adult men, in adult women suprarenal androgens promote muscle mass, blood cell formation, and support the libido. The Suprarenal Medulla The boundary between the suprarenal cortex and the suprarenal medulla is irregular, and the supporting connective tissues and blood vessels are extensively interconnected. The sympathetic department of the autonomic nervous system controls secretory activities of the suprarenal medullae. The suprarenal medulla contains two populations of secretory cells. One produces epinephrine (adrenaline), the other norepinephrine (noradrenaline). Evidence suggests that the two types of cells are distributed in different areas of the medulla and that their secretory activities can be independently controlled. The hormones in these vesicles are continuously released at low levels by exocytosis. Epinephrine comprises 75–80 percent of the secretions from the suprarenal medullae, the rest being norepinephrine. Activation of the suprarenal medullae has the following effects:

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