The adrenal glands are two endocrine glands on the anterior aspects of the superior portions of both kidneys, each weighing 4 grams. An adrenal gland of mammals consists of two main parts: the adrenal cortex and the adrenal medulla.
Though the adrenal gland is organized as a single endocrine gland in mammals, the medulla and cortex have different embryological origins, the medulla developing from ectoderm while the cortex develops from mesoderm. In amphibians and in some fish, the medulla and cortex are two separate organs.
The adrenal medulla is innervated by the sympathetic nervous system and takes up the core 20% of the adrenal gland. It secretes two hormones (both catecholamines) into the blood: epinephrine (adrenaline) and norepinephrine (noradrenaline). The adrenal medulla produces about 80% adrenaline and 20% noradrenaline. These water-soluble hormones are part of the fight-or-flight response, initiated by the sympathetic nervous system.
The adrenal cortex takes up the remaining 80% of the adrenal gland, and wraps circumferentially around the adrenal medulla. The secretion activity of the adrenal cortex is regulated by adrenocorticotropic hormone produced in the anterior lobe of the pituitary gland (the hypothalamic-pituitary axis). It secretes steroid hormones, including glucocorticoids, mineralocorticoids and androgens, which are produced from the steroid cholesterol. The androgens play a minor role as a sexual hormone that regulates some traits of masculinity (much like testosterone). The glucocorticoids play a part in the regulation of glucose levels in the blood, protein metabolism, and fat metabolism as well as in the control of inflammation processes. The mineralocorticoids help regulate the extracellular concentrations of electrolytes (such as potassium and sodium). Although the adrenal cortex has been found to produce over 30 steroids of the aforementioned types, the only two steroids that have a major bearing on the normal function of the endocrine system are aldosterone (the major mineralocorticoid), and cortisol (the main glucocorticoid).