The suprarenal glands, also known as adrenal glands, belong to the endocrine system. They are a pair of triangular-shaped glands, each about 2 in long and 1 in wide, that sit on top of the kidneys (see the image below). The suprarenal glands are responsible for the release of hormones that regulate metabolism, immune system function, and the salt-water balance in the bloodstream; they also aid in the body’s response to stress. 
Each suprarenal gland is composed of 2 distinct tissues: the suprarenal cortex and the suprarenal medulla. The suprarenal cortex serves as the outer layer of the suprarenal gland, and the suprarenal medulla serves as the inner layer. These 2 major regions are encapsulated by connective tissue known as the capsule (see the images below). [2, 3]
The suprarenal cortex is the largest part of the gland and is composed of 3 zones: the zona glomerulosa (outer zone), the zona fasciculata (middle zone), and the zona reticularis (inner zone). The zona glomerulosa is responsible for the production of mineralocorticoids, mainly aldosterone, which regulates blood pressure and electrolyte balance.
The zona fasciculata, is responsible for the production of glucocorticoids, predominantly cortisol, which increases blood sugar levels via gluconeogenesis, suppresses the immune system, and aids in metabolism. This zone secretes cortisol both at a basal level and as a response to the release of adrenocorticotropic hormone (ACTH) from the pituitary gland.
The zona reticularis produces gonadocorticoids and is responsible for administering these hormones to the reproductive regions of the body. Most of the hormones released by this layer are androgens. The main androgen produced by this layer is dehydroepiandrosterone (DHEA), which is the most abundant hormone in the body and serves as the starting material for many other important hormones produced by the suprarenal gland, such as estrogen, progesterone, testosterone, and cortisol.
The suprarenal medulla is composed of special cells called chromaffin cells, which are organized in clusters around blood vessels. The cells in the suprarenal medulla produce epinephrine (also known as adrenaline) and norepinephrine. These 2 hormones prepare the body for the fight-or-flight response by increasing the heart rate, constricting blood vessels, increasing the metabolic rate, heightening cognitive awareness, and increasing the respiratory rate.
The suprarenal glands require a large supply of blood and release hormones directly into the bloodstream. The suprarenal glands are among the most extensively vascularized organs in the body. Three sources of arteries maintain blood supply to the suprarenal glands. The superior suprarenal arteries are multiple small branches from the inferior phrenic artery, whereas the middle suprarenal artery is a direct branch from the abdominal aorta. An inferior suprarenal artery, sometimes multiple, arises from the renal artery on each side. After the suprarenal glands have been supplied with blood from these arteries, the blood drains through the suprarenal vein to the left renal vein or directly to the inferior vena cava on the right side.
Addison disease is known as primary suprarenal (adrenal) insufficiency. It is a disease in which the suprarenal glands do not produce enough of the hormone cortisol and often the hormone aldosterone. Addison disease results from damage to the suprarenal cortex, usually as a result of an autoimmune disease in which the immune system attacks the gland. Although less likely, damage to the suprarenal cortex can also occur from a tumor or through certain infections, such as HIV infection or tuberculosis. [4, 5, 6, 7]
Cushing syndrome is a disorder in which the suprarenal glands produce too much of the hormone cortisol. This disorder can be caused by Cushing disease, in which the pituitary gland makes too much adrenocorticotropic hormone (ACTH), which signals the suprarenal glands to produce cortisol. [5, 8, 7]
Suprarenal fatigue can occur when prolonged stress and malnutrition weaken the suprarenal glands. When stress occurs over a prolonged period of time, the suprarenal glands can either diminish in size or enlarge. If exposed to an extended period of stress, the suprarenal glands can overproduce hormones that suppress the immune system and create an imbalance in the body’s normal blood sugar levels.