The adrenal glands are endocrine glands located directly on top of the kidneys. Their main function is to secrete vital hormones and steroids during times of stress, as well as govern many different bodily functions. The hormones that the adrenal glands secrete are primarily derived from amino acids or lipids. The right adrenal gland is pyramid-shaped, while the left adrenal gland is shaped like a crescent moon. They have an irregular surface configuration, are enclosed in a capsule made of connective tissue, and have a liberal blood supply from the superior, middle, and inferior suprarenal arteries.
The outer portion of each gland is called the adrenal cortex, and it has three zones of cells. Each zone can be identified under a microscope based on different structural features; additionally, the zones can be distinguished solely based on their different functions. The outermost layer is called the zona glomerulosa, and it primarily produces mineralcorticoids to regulate blood pressure. When the body experiences low blood pressure or a low sodium concentration, aldosterone is released to the kidneys, and more specifically the distal convoluted tubule and collecting duct of the renal pyramid, to cause sodium reabsorption, which then leads to water reabsorption. This portion of the gland is also controlled by the renin-angiotensin system with angiotensin II being a major stimulus. The middle layer, called the zona fasciculata, produces glucocorticoids such as cortisol to mobilize nutrients and enhance the effects of glucagons and catecholamines. It can also secrete increased levels of cortisol when it is stimulated by the adrenocorticotropic hormone, ACTH, which is secreted by the anterior pituitary gland. The inner most layer, called the zona reticularis, mainly produces androgens.
The inner portion of each gland is called the adrenal medulla, which is made up of chromaffin cells that produce catecholamine hormones such as epinephrine and norepinephrine. These two hormones target heart rate and are heavily relied upon by the body during a fight-or-flight response. The medulla is also highly vascular with preganglionic sympathetic fibers throughout to facilitate communication between the hypothalamus and the sympathetic division of the autonomic nervous system.
In some cases, the body can produce unfavorable amounts of cortisol. When a patient has too much cortisol, they are diagnosed with Cushing’s syndrome; whereas when a patient is seen with extremely low levels of cortisol, they are diagnosed with adrenal insufficiency. Other disorders include: hyperaldosteronism, when the adrenal glands produce too much aldosterone; pheochromocytoma, when the adrenal glands secrete excessive amounts of catecholamines.
Image Caption: Adrenal gland. Credit: Wikipedia