Adrenal Glands
Adrenal glands are a pair of ductless glands located above the kidneys . Through hormonal secretions, the adrenal glands regulate many essential functions in the body, including biochemical balances that influence athletic training and general stress response. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla, which secrete different hormones.
Adrenal glands
Adrenal glands are located on top of the kidneys. These glands are composed of the adrenal cortex and the adrenal medulla.
Adrenal Cortex
The adrenal cortex is made up of layers of epithelial cells and associated capillary networks. These layers form three distinct regions: an outer zona glomerulosa that produces mineralocorticoids; a middle zona fasciculata that produces glucocorticoids; and an inner zona reticularis that produces androgens, which are sex hormones that promote masculinity. Androgens are produced in small amounts by the adrenal cortex in both males and females. They do not affect sexual characteristics and may supplement sex hormones released from the gonads.
The hormones made by the adrenal cortex supply long-term responses to stress. The two major hormones produced are the mineralocorticoids and the glucocorticoids. The mineralocorticoids regulate the salt and water balance, leading to the increase of blood volume and blood pressure. The main mineralocorticoid is aldosterone, which regulates the concentration of sodium ions in urine, sweat, pancreas, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The glucocorticoids regulate increases in blood glucose and also reduce the body's inflammatory response. The three main glucocorticoid hormones are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate the synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells. They also promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to maintain levels within a normal range between meals. Cortisol is one of the most active glucocorticoids. It usually reduces the effects of inflammation or swelling throughout the body. It also stimulates the production of glucose from fats and proteins, which is a process referred to as gluconeogenesis. Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the kidney secretes more renin, an enzyme that stimulates the formation of angiotensin from a molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium is reabsorbed as it enters the blood. Aldosterone, the major mineralcorticoid, stimulates the cells of the distal convoluted tubules of the kidneys to decrease re-absorption of potassium and increase re-absorption of sodium. This in turn leads to an increased re-absorption of chloride and water. These hormones, together with such hormones as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.
Adrenal Medulla
The adrenal medulla contains large, irregularly-shaped cells that are closely associated with blood vessels. These cells are innervated by pre-ganglionic autonomic nerve fibers from the central nervous system.
The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and another that produces norepinephrine (noradrenaline). Epinephrine is the primary adrenal medulla hormone, accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system. Secretion of these hormones is stimulated by acetylcholine release from pre-ganglionic sympathetic fibers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in response to stress to prepare the body for the fight-or-flight response.