Medulla Physiology
The adrenal glands are made up of two different parts: the medulla (inner portion) and the cortex (outer portion). The adrenal medulla is responsible for making the two catecholamine hormones, which are epinephrine and norepinephrine. These two hormones are more commonly known as adrenaline. Epinephrine and norepinephrine work to activate the body's "fight or flight" response. They speed up the heart rate and increase blood flow to the muscles while simultaneously taking blood away from the skin and certain internal organs, such as the intestines and kidneys. These hormones are stored within the adrenal medulla and are released in response to stresses, including physical trauma, low blood sugar and exercise.
Zona Reticularis and Fasciculata
The adrenal cortex is divided into three sections. The two innermost sections are called the zona reticularis and the zona fasciculata. These two layers are responsible for making the sex hormones as well as the glucocorticoid class of hormones (primarily cortisol). The sex hormones are estrogens and androgens (which includes testosterone). Sex hormones are responsible for the development of secondary sexual characteristics, such as body hair and (in women) the menstrual cycle. The amount of each sex hormone that is made by the gland depends on the person's gender; the adrenal glands of women produce more estrogen, whereas those of men make more androgens. Glucocorticoids are hormones that have a variety of functions within the body. For example, they affect metabolism by causing the liver to make more glucose and by inhibiting the uptake of glucose by muscle tissue. They also work to inhibit the immune system and can block inflammation. Glucocorticoids are secreted by the adrenal cortex in response to adrenocorticotropic hormone, which is made by the pituitary.
Zona Glomerulosa
The outermost layer of the adrenal cortex is called the zona glomerulosa, which makes a type of hormone called mineralocorticoids. The major mineralocorticoid made by the adrenal glands is called aldosterone. Aldosterone works on the kidneys to helps maintain the body's electrolyte balance. Aldosterone causes the kidneys to absorb more sodium and water and also to secrete more potassium. Aldosterone production is a critical part of the adrenal gland's function, because without it the levels of sodium and potassium in the blood would become unbalanced, which is ultimately fatal. Aldosterone is secreted by the zona glomerulosa in response to elevated potassium levels. It is also made in response to the hormone angiotensin, which is made when the kidneys experience decreased blood flow.
