The adrenal glands are well supplied with blood vessels, including a branch from the aorta and a branch from the renal artery. The blood entering the adrenal cortex is drained out through the veins of the adrenal medulla; see Figure 1.
Fig1. Structural details of the human adrenal glands. A. Anatomical relationship between kidneys and their partner adrenal glands. The two adrenal glands have different shapes depending on their anatomical location. The right gland is shaped like a pyramid. In contrast, the left gland is crescent shaped. Each adrenal gland sits on top of its partner kidney. B. The adrenal gland has three distinct layers. The capsule is the outer protective covering of the adrenal gland. Immediately below the capsule is the cortex, which is the location of three zona layers. Each of the zona layers produces a different steroid hormone family. They are the mineralocorticoids (aldosterone) from the zona glomerulosa, the glucocorticoids (cortisol) from the zona fasciculata, and the adrenal androgens (dehydroepiandrosterone and testosterone) from the zona reticularis. C. Function of the medulla layer. Below the cortex is the medulla layer. The medulla is the site of production of catecholamines, e.g., epinephrine and norepinephrine. Readers who want in-depth information on epinephrine and norepinephrine should go to Chapter 11, “Hormones of the Adrenal Medulla.”
The adrenal gland consists of two major segments, an outer cortex and the central medulla. The three consecutive layers of the cortex are each responsible for the biochemical formation of a different family of steroid hormones. Starting with (a) the outer layer, zona glomerulosa produces aldosterone; (b) the middle zona fasciculata produces cortisol and corticosterone; and (c) the third innermost zona reticularis produces testosterone and dehydroepiandrosterone (DHEA), which are both androgens. Each of these steroid hormone families requires its own steroid metabolic pathway that has the appropriate steroid enzymes. The production of all steroid families starts with cholesterol, followed by three to five successive enzyme alterations that produce a specific steroid hormone. More detail on this process is provided in section III, “Biochemistry and Biosynthesis of Adrenal Steroids and Testosterone Steroids.”
The bottom or most interior layer of the adrenal is the medulla. The medulla produces catecholamines, particularly epinephrine and norepinephrine; see Chapter 11 for more details.
In fetal life the human adrenal is relatively large in size. The cortex present at birth is often referred to as the “fetal cortex” because of its transient life. At birth and for most of the first postnatal year, the primary cortex begins to atrophy while the cells of the secondary cortex actively divide. As the fetal cortex disappears, the mature zona glomerulosa appears immediately adjacent to the capsule. With continued proliferation, the next inward zone, the zona fasciculata, arises and within a few months after birth the most inward layer of cells, the zona reticularis, is finally formed.
Also low amounts of corticosterone are secreted. The zona fasciculata is the largest of the three zones in the cortex. The zona reticularis, the innermost layer, is adjacent to the medulla.
