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The adrenals are a pair of glands located retroperitoneally above the kidneys. They are critical for salt and water regulation and metabolism, response to stress and the synthesis and breakdown of fat and protein. Adrenalectomy is followed by collapse and death if hormones are not replaced.
==Vasculature The arteries and veins do not run together. The arterial supply is abundant; 50-60 terminal artieries enter from the phrenic and renal arteries and the aorta. The venous blood is channeled entirely through a single trunk within the medulla which empties on the right side directly into the vena cava and on the left into the left renal vein. The direction of blood flow within the gland is outward to inward, fromt he capsule to the center, thus the medulla is exposed to high levels of steroids produced by the cortex. ==Adrenal CortexEmbryology:During fetal life, 80% of the adrenal is the fetal adrenal cortex. It is deficient in 3βol-dehydrogenase and thus does not produce cortisol. It produces sulfate conjugates of androgens which are converted in the placenta to androgens and estrogens which enter the maternal circulation and form the process of maternal-fetal communication. ==The outer adrenal cortex secretes steroid hormones. It is divided into three zones:
==All three zones secrete cortisterone, but the enzymatic mechaniosm for converting cortisterone to aldosterone is limited to the zona glomerulosa, while enzymatic mechanism for forming cortisol and sex hormones is found only in the inner two zones. The zona glomerulosa regenerates new cortical cells for all three zones. The outer zone is regulated mostly independently from the pituitary while the two inner zones atrophy after pituitary removal. (Thus in hypopituitarism, the ability to conserve Na+ is normal). Hormones of the Cortex The steroid hormones of the adrenal cortex, as well as cholesterol, bile acids, vitamin D and gonadal steroid hormones are synthesized in a series of enzymatic steps from a common 4-ring cyclopentanoperhydrophenthrene nucleus. C19 steroids have androgenic activity C21 steroids are classified as mineralcorticoids, glucocorticoids or progestins. All secreted C21 steroids have mineralcorticoid and glucocorticoid activity. C18 steroids are estrogens, only a small amount is made in the adrenal. ==Some of the the cholesterol used to produce steroids is synthesized from acetate, much is taken up from LDL in circulation. LDL receptors are abundant in adrenalcortical cells. Cholesterol is converted to pregnenolone which then proceeds to different pathways, via different enzymatic mechanisms, to glucocorticoids, mineralcorticoids or androgens.
Category: Anatomy | Endocrinology | |