The Adrenal Glands

Anatomy of the adrenal glands:

Adrenal glands, which are also called suprarenal glands, are small, triangular paired organs located in the back of the abdomen above each kidney. The adrenal gland is really two glands in one.  Each gland has an outer layer called the cortex and an inner layer called the medulla.

The outer cortex gives the gland a characteristic yellow orange color. The cortex is divided into three zones. The outer zone makes and secretes a hormone called aldosterone. The central zone makes up most of the cortex and makes and secretes adrenocortical steroids or cortisol. The inner zone makes and secretes small amounts of sex hormones including testosterone, estradiol and dihydroepiandrosterone. The inner layer or the medulla synthesizes and secretes norepinephrine or noradrenaline and epinephrine or adrenaline.

Function of the adrenal glands:

The adrenal glands work interactively with the hypothalamus and pituitary gland in the following process:

The hypothalamus, a part of the central nervous system, releases hormones, which stimulate the pituitary gland.  One of these is corticotropin releasing hormone (CRF).

The pituitary gland, in response to CRF, produces adrenocortical trophic hormone (ACTH). ACTH stimulates the adrenal glands to produce corticosteroid hormones (cortisol).  

Both parts of the adrenal glands - the adrenal cortex and the adrenal medulla - perform very separate functions.

What is the adrenal cortex?

The adrenal cortex, the outer portion of the adrenal gland, is essential to life by secreting hormones that have an effect on the body's metabolism, on chemicals in the blood, and on certain body characteristics. Some of the hormones produced by the adrenal cortex include:

glucocorticoids

Glucocorticoids are essential for human life. The principle glucocorticoid is cortisol, which controls the body’s use of fats, proteins, and carbohydrates.   Cortisol is produced by the middle part of the adrenal cortex. Cortisol secretion is at its highest in the early morning and at its lowest at night. Serum cortisol levels are regulated by a feedback loop mechanism involving the pituitary gland. The release of ACTH or adrenocorticotrophic hormone from the pituitary is the main stimulus for cortisol secretion. Normally the gonads (the ovaries in women and the testes in men) are the principle source of sex hormones in men and women. However the adrenal gland can produce some of these from the inner part of the adrenal cortex.

mineralocorticoids                                                                                         

Aldosterone is the major mineralocorticoid. It plays a key role in regulating the amount of fluid and the balance of electrolytes in our body. It stimulates the re-absorption of sodium and the loss of potassium in the kidney. By causing the kidney and other tissues to retain sodium, aldosterone increases the amount of fluid in our body.

Aldosterone is regulated by a system called the renin angiotensin system. When there is a decrease in blood flow to the kidney, renin is secreted into the blood stream. This then activates angiotensin in the liver and another form of angiotensin in the lung. This stimulates aldosterone synthesis and secretion from the adrenal glands,

What is the adrenal medulla?

The adrenal medulla, the inner part of the adrenal gland, is not essential for life, but helps a person in coping with physical and emotional stress. The adrenal medulla secretes catecholamines. These hormones have wide ranging effects on almost all tissues and organs in the body. Catecholamines increase oxygen consumption and heat production by the cells in the body. They stimulate the breakdown of sugars in the liver and inhibit the secretion of insulin. This causes elevation in blood sugar.

epinephrine (also called adrenaline)
This hormone increases the heart rate and force of heart contractions, facilitates blood flow to the muscles and brain, causes relaxation of smooth muscles, helps with conversion of glycogen to glucose in the liver, and other activities.  These are known as beta effects.

norepinephrine (also called noradrenaline)
This hormone has little effect on smooth muscle, metabolic processes, and cardiac output, but has strong vasoconstrictive effects, thus increasing blood pressure. The latter is known as an alpha effect.

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