Human Anatomy - Kotsan I. Y. 2009

Endocrine glands
Adrenal gland

The Adrenal Gland (glandula suprarenalis) is a paired organ located in the retroperitoneal space directly above the superior pole of the corresponding Kidney (Fig. 228). The mass of a single adrenal gland in an adult is about 12–13 g, its length is 40–60 mm, height is 20–30 mm, and thickness (anteroposterior dimension) is 2–8 mm. The mass and dimensions of the right adrenal gland are slightly smaller than those of the left. When viewed from the front, the right adrenal gland has a triangular shape with rounded corners. In the left adrenal gland, the apex is blunted, giving it a crescent-like shape. Each adrenal gland presents three surfaces: anterior (facies anterior), posterior (facies posterior), and renal (facies renalis), as well as two borders: superior (margo superior) and medial (margo medialis).

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Fig. 228. Adrenal gland (left)

1 — superior suprarenal Arteries and Veins; 2 — inferior suprarenal arteries and veins; 3 — adrenal gland; 4 — renal artery; 5 — kidney; 6 — renal vein; 7 — INFERIOR VENA CAVA; 8 — aorta; 9 — central suprarenal vein

The Adrenal Glands are located at the level of the XI–XII thoracic vertebrae. The right adrenal gland lies slightly lower than the left. Its posterior surface rests against the lumbar part of the Diaphragm, its anterior surface contacts the visceral surface of The Liver and the duodenum, and its inferior concave renal surface abuts the upper pole of the right kidney. The medial border of the right adrenal gland neighbors the inferior vena cava. The medial border of the left adrenal gland contacts the aorta, while its anterior surface adjoins the tail of the Pancreas and the cardiac part of The Stomach. The posterior surface of the left adrenal gland touches the diaphragm, and its inferior surface contacts the upper pole and medial border of the left kidney. Each adrenal gland is embedded within the perirenal adipose body. The anterior surfaces of both the left and right adrenal glands are partially covered by the parietal Peritoneum. The surface of the adrenal glands is somewhat nodular, and a furrow known as the hilum is visible on the anterior surface of each gland, through which the central vein and Lymphatic vessels emerge from the organ. Arterial branches and nerve trunks may penetrate the parenchyma from both the anterior and posterior surfaces of the organ.

Externally, the adrenal gland is covered by a fibrous capsule that tightly adheres to the parenchyma and sends numerous Connective Tissue trabeculae deep into the organ. The parenchyma of the adrenal gland consists of two layers: an outer cortex and an inner medulla, which differ in their development, Structure, and Functions.

The cortex (cortex) belongs to the so-called interrenal system, which develops from the mesoderm between the pronephroi (hence the name of the system). It has a yellowish-brown color, a complex histological structure, and consists of three zones: the zona glomerulosa, zona fasciculata, and zona reticularis. The zona glomerulosa lies closest to the capsule and is formed by small Cells arranged in clusters. Following the zona glomerulosa is the middle and widest zone, the zona fasciculata. It is formed by large, light-colored cells (filled with lipid droplets) arranged in long cords running perpendicular to the organ's surface. The zona reticularis is located at the border with the medulla. It consists of small cells that form small clusters (Cell groups).

The aforementioned zones are quite distinctly separated anatomically and, according to modern data, produce different Hormones (corticosteroids): the zona glomerulosa produces mineralocorticoids (aldosterone); the zona fasciculata produces glucocorticoids (cortisone, hydrocortisone, and corticosterone); and the zona reticularis produces androgens, estrogens, and progesterone (the latter two in small amounts). Mineralocorticoids regulate Water-Salt METABOLISM (the concentration of calcium, sodium, and chloride salts). Glucocorticoids participate in the REGULATION OF CARBOHYDRATE, protein, lipid, and water-electrolyte metabolism. They affect the cardiovascular and central nervous systems, enhance the body's resistance to various harmful influences, and play an active role in adapting the Organism to environmental changes; therefore, hydrocortisone and cortisone are also referred to as "adaptive hormones." They selectively act on Collagen tissue, inhibit The Development of the Extracellular matrix of connective tissue, and decrease capillary permeability, which is why they are also called "anti-inflammatory hormones." Androgens are structurally quite similar to Sex Hormones and thus promote the development of reproductive Organs in childhood as well as the Formation of secondary sex characteristics. Hyperfunction of the adrenal cortex leads to precocious Puberty in children and the development of male secondary sex characteristics in adult women. The functions of estrogen and progesterone are described below in the section on the Endocrine portion of the female Gonads.

The medulla has a common origin with The Nervous system. It develops from the ectoderm—specifically from embryonic Nerve Cells known as sympathoblasts, which migrate from the primordia of the Sympathetic trunk ganglia and differentiate into chromaffinoblasts, which in turn mature into the chromaffin Cells of the medulla that intensely stain with chromic acid salts (hence the name chromaffin or adrenergic system). The medulla is located in the center of the adrenal gland and has a yellowish-brown hue. It is formed by Two Types of cells: epinephrocytes, which make up the bulk of the tissue and secrete the hormone adrenaline, and norepinephrocytes, which are scattered throughout the medulla in small groups and secrete the hormone noradrenaline. Adrenaline stimulates the Sympathetic division of the Autonomic nervous system, increases systolic Blood pressure, accelerates Heart rate, dilates coronary vessels, increases BLOOD FLOW IN the liver, skeletal Muscles, and Brain, elevates blood glucose levels, and enhances lipolysis. The effects of noradrenaline on the body are similar to those of adrenaline. However, the effects of these hormones on certain organs can be completely opposite; for example, noradrenaline slows down The Heart rate.

Age-related Features of the adrenal glands. The mass of a single adrenal gland in a newborn is 8–10 g, significantly exceeding that of an infant in the first year of life. During the first three months after birth, the mass of the adrenal gland decreases sharply (to 3.5 g), primarily due to the thinning and remodeling of the cortical substance, after which it gradually recovers (by age 5) and continues to grow. The final Formation of the adrenal cortex is completed by 8–12 years of age. By age 20, the mass of each gland increases by one and a half times (compared to the newborn mass) and reaches its maximum dimensions of 12–13 g. In subsequent age periods, the mass and dimensions of the adrenal glands remain virtually unchanged. In women, the adrenal glands are slightly larger than in men. After age 70, a slight decrease in the mass and dimensions of the adrenal glands is observed.

Blood supply to the adrenal glands. Each adrenal gland receives up to 25–30 arteries. The largest among them are the superior suprarenal artery (from the inferior phrenic artery), the middle suprarenal artery (from the Abdominal Aorta), and the inferior suprarenal artery (from the renal artery). Sinusoidal blood capillaries form tributaries of the central vein, which empties into the inferior vena cava on the right side and into the renal vein on the left. The lymphatic Vessels of the adrenal glands drain into the lumbar Lymph Nodes.

The Innervation of the adrenal glands involves the vagus nerves as well as fibers originating from the celiac plexus, which contain preganglionic fibers for the medulla.