Human Anatomy and Physiology - N. I. Fedyukovich 2003

Endocrine Glands
Adrenal Gland

The Adrenal Glands (glandulae suprarenales) are paired Organs located in the retroperitoneal space, directly above the superior pole of each Kidney. Each gland weighs approximately 12–13 g, with a length of 40–60 mm and a width of 2–8 mm.

The Adrenal gland has the shape of an anteroposteriorly flattened cone, featuring anterior, posterior, and inferior (renal) surfaces. The glands are situated at the level of the XI–XII thoracic vertebrae. The right adrenal gland lies slightly lower than the left. The posterior surface of the right adrenal gland abuts the lumbar part of the Diaphragm, its anterior surface contacts the visceral surface of The Liver and the duodenum, and its inferior concave surface touches the upper pole of the right kidney. The anterior surface of the left adrenal gland adjoins the tail of the Pancreas and the cardiac part of The Stomach, its posterior surface contacts the diaphragm, and its inferior surface meets the upper pole of the left kidney.

The surface of the adrenal gland is nodular. On the anteromedial surface, There is a deep fissure known as the hilum of the organ, through which the central vein and lymphatic vessels emerge. Externally, the adrenal gland is enclosed within a fibrous capsule that firmly adheres to the parenchyma and extends numerous Connective Tissue septa deep into the gland. Beneath the fibrous capsule lies the cortex (adrenal cortex), which consists of three distinct zones. Outermost, adjacent to the capsule, is the zona glomerulosa, followed by the middle and widest zona fasciculata, and finally the innermost zona reticularis, which borders the medulla.

The adrenal cortex produces Hormones collectively known as corticosteroids. These are divided into two main groups: 1) glucocorticoids (corticosterone, cortisol, hydrocortisone, and cortisone), which are synthesized in the zona fasciculata; and 2) mineralocorticoids (aldosterone), secreted by the Cells of the zona glomerulosa. Additionally, the adrenal cortex—primarily the zona reticularis—secretes small amounts of sex hormone-like substances structurally and functionally similar to androgens, as well as estrogens and progesterone.

The central portion of the adrenal gland contains the medulla, formed by large cells that stain yellowish-brown with chromium salts. Two Types of these cells are distinguished: epinephrocytes, which make up the bulk of the tissue and secrete adrenaline, and norepinephrocytes, which produce noradrenaline.

Glucocorticoids exert diverse metabolic effects. They stimulate the synthesis of Glycogen from glucose and Proteins and promote glycogen storage in Muscles while simultaneously elevating Blood glucose levels; they significantly influence cellular and humoral Immunity and possess potent anti-inflammatory properties. Changes in glucocorticoid concentrations are particularly pronounced during stress. According to the stress theory formulated by H. Selye, stress comprises three phases: alarm, resistance, and exhaustion. A stress response may pass without lasting effects if the stimulus is mild; upon repeated exposure, adaptation to the stimulus is possible. However, if the stress is highly intense, it can lead to the depletion of all glucocorticosteroid reserves in the adrenal cortex and its structural degradation.

Alterations in glucocorticoid levels—whether increased (hyperfunction) or decreased (hypofunction)—lead to severe physiological disorders. Excessive cortisol secretion results in obesity, enhanced protein breakdown (catabolic effect), Water retention, Hypertension, and other complications. Conversely, adrenocortical insufficiency, characterized by reduced corticosteroid production, causes Addison's disease—a severe pathology marked by bronze Skin pigmentation, chronic fatigue, hypotension, and myocardial weakness.

Mineralocorticoids (aldosterone) regulate Na⁺ and K⁺ balance by acting primarily on the Kidneys. Excess hormone leads to elevated blood Na⁺ concentration and reduced IC, increases osmotic pressure, promotes fluid retention in the body, and raises ARTERIAL BLOOD PRESSURE. Conversely, hormone deficiency results in decreased Na⁺ levels in the blood and Tissues alongside elevated K⁺ levels. The loss of Na⁺ is accompanied by fluid depletion from tissues, leading to dehydration.

Adrenaline affects The Cardiovascular system by increasing blood pressure, Heart rate, and myocardial contractility, while dilating Skeletal Muscle Blood Vessels and the smooth musculature of the Bronchi. Furthermore, it elevates blood glucose levels and enhances cellular oxidative processes. The release of adrenaline into the bloodstream is mediated by the sympathetic Nervous system.

Noradrenaline helps maintain vascular tone and participates in the transmission of nerve impulses from sympathetic nerve fibers to their target organs.