adrenal gland

adrenal gland. The adrenal glands are a pair of endocrine organs located superior to the kidneys and are crucial for the body's response to stress. They consist of two distinct regions, the outer cortex and inner medulla, which produce different sets of vital hormones. These hormones regulate a wide range of physiological processes, including metabolism, immune function, and blood pressure.

Anatomy

Each adrenal gland is situated within the perinephric fat atop a kidney in the retroperitoneal space. The right gland is pyramidal and lies close to the inferior vena cava, while the left is semilunar and neighbors the pancreas and splenic artery. The gland is encased in a fibrous capsule and receives its blood supply from branches of the abdominal aorta, namely the superior, middle, and inferior suprarenal arteries. Venous drainage is primarily via a single suprarenal vein, with the right draining into the inferior vena cava and the left into the left renal vein. The gland is divided into a yellow outer cortex, derived from mesoderm, and a reddish-brown inner medulla, which originates from neural crest cells and is essentially a specialized sympathetic ganglion.

Physiology

The cortex secretes steroid hormones, synthesized from cholesterol, and is divided into three histological zones. The zona glomerulosa produces mineralocorticoids like aldosterone, which regulates electrolyte and water balance by acting on the distal convoluted tubule of the nephron. The zona fasciculata produces glucocorticoids such as cortisol, which influences glucose metabolism, suppresses the immune system, and aids the stress response. The zona reticularis secretes weak androgens like dehydroepiandrosterone (DHEA). The medulla produces catecholamines, primarily epinephrine and norepinephrine, which are released in response to stimulation by the splanchnic nerves and mediate the fight-or-flight response by increasing heart rate and blood glucose.

Disorders

Pathologies of the adrenal gland include both hypofunction and hyperfunction. Addison's disease, first described by Thomas Addison, results from adrenal insufficiency, often due to autoimmune destruction or tuberculosis, leading to a deficiency in cortisol and aldosterone. Conversely, Cushing's syndrome, associated with Harvey Cushing, involves glucocorticoid excess, which can be caused by a pituitary adenoma (Cushing's disease), an adrenal adenoma, or exogenous steroid use. A pheochromocytoma is a tumor of the medulla that secretes excessive catecholamines, causing paroxysmal hypertension. Congenital adrenal hyperplasia, often due to a deficiency in 21-hydroxylase, disrupts steroid synthesis and can cause virilization and salt-wasting crises. Primary hyperaldosteronism (Conn's syndrome), frequently caused by an adrenal adenoma, leads to hypertension and hypokalemia.

History

The adrenal glands were first identified by Bartolomeo Eustachi in the 16th century, but their function remained unknown for centuries. In the 19th century, Thomas Addison of Guy's Hospital in London provided the first clinical description of adrenal insufficiency. Later, Charles-Édouard Brown-Séquard demonstrated the glands' necessity for life through adrenalectomy experiments on animals. The distinction between the cortex and medulla and their separate functions was elucidated in the early 20th century. The isolation and synthesis of adrenal hormones, such as cortisol and aldosterone, were major achievements of mid-20th century endocrinology, with key contributions from scientists like Edward Calvin Kendall, Tadeus Reichstein, and Philip Showalter Hench, who shared the Nobel Prize in Physiology or Medicine in 1950.

In other animals

Adrenal glands are present in all vertebrates, but their anatomy varies. In fish, the cortical and medullary tissues are often separate, known as the interrenal tissue and chromaffin tissue, respectively. In amphibians and reptiles, the two tissues intermingle. In most mammals, the glands are discrete capsular organs as in humans, though their shape and relative size of the cortex can differ; for instance, in rodents, the zona glomerulosa is not always distinct. The hormones produced, such as cortisol in most mammals and corticosterone in rodents and birds, are conserved in function, playing critical roles in stress, metabolism, and osmoregulation across species.

Category:Endocrine system Category:Glands