Adrenal Glands

The adrenal gland (also known as suprarenal gland) is an endocrine organ located atop each kidney that responds to stress and low blood glucose levels. Each adrenal gland consists of an outer cortex and inner medulla. In an adult, an average adrenal gland weighs approximately 4 grams, but this can increase or decrease over a person's lifetime, influenced by factors such as chronic illness or prolonged stress.

The outer adrenal cortex is responsible for the production of corticosteroids and androgen hormones. It contains three zones that appear distinct when viewed under a light microscope. Each zone has its own function. The zona glomerulosa is the outermost layer of the adrenal cortex. It contains cells clustered into oval groups along with wide capillaries. Cells in this zone produce the corticosteroid aldosterone, an adrenal hormone that acts on the kidneys to increase sodium and water retention and blood volume in order to regulate blood pressure. It accomplishes this by causing an increase in sodium reabsorption and potassium excretion. This increases water retention and blood volume, which in turn regulates blood pressure.

The next layer, the zona fasciculata, is the largest layer and accounts for nearly 80 percent of the adrenal cortex. Cells in this layer are arranged into columns. This layer produces several different corticosteroids, including cortisol. Cortisol is a hormone produced by the adrenal cortex in response to stress and low blood sugar that increases blood sugar levels and suppresses the inflammatory response. Its release is triggered by secretion of ACTH from the anterior pituitary. Cortisol increases blood glucose levels by triggering the breakdown of protein and lipids into glucose and by inhibiting glucose uptake by muscle and fat cells. Cortisol also acts on the immune system to decrease the inflammatory response.

The innermost layer of the adrenal cortex is the zona reticularis. Cells in this layer form irregular clusters separated by capillaries and connective tissue. These cells secrete androgen hormones. An androgen is a hormone responsible for the development of male sex organs and the maintenance of secondary male characteristics such as body hair and musculature. Androgens produced by the adrenal cortex mainly act as precursors for the synthesis of other more potent sex hormones in both men and women. The adrenal medulla is the inner part of the adrenal gland, surrounded by the adrenal cortex. The adrenal medulla produces hormones called catecholamines. A catecholamine is a class of water-soluble hormone produced by the adrenal medulla in response to stressful situations. The main catecholamines include epinephrine, norepinephrine, and dopamine. They are part of the sympathetic nervous system response to acutely stressful situations such as exercise or danger. Chromaffin cells in the adrenal medulla receive input directly from sympathetic nervous system neurons located in the thoracic spinal cord. When stress triggers the sympathetic nervous system, these neural inputs stimulate chromaffin cells, which secrete epinephrine (also called adrenaline) and norepinephrine (also called noradrenaline), along with a small amount of dopamine. Epinephrine and norepinephrine are catecholamine hormones produced in response to stress that triggers the body's fight-or-flight response. This direct neural control of the chromaffin cells allows a rapid release of hormones in response to a stressful trigger.

Different organs in the body have unique receptors to which the catecholamine hormones attach. Catecholamine receptors are present throughout the body, though their mechanism of action depends on the specific organ and location in the body. Norepinephrine is a catecholamine hormone produced by the adrenal medulla that acts mainly on alpha receptors. These receptors are located on the pancreas and liver. Stimulation of these receptors leads to increased insulin secretion, increased glycogen breakdown, and promotion of glycolysis. Epinephrine is a catecholamine hormone produced by the adrenal medulla that acts on alpha, beta-1, beta-2, and beta-3 receptors. This means that in addition to stimulating and producing the same responses as norepinephrine, epinephrine also increases adrenocorticotropic hormone (ACTH) from the pituitary, increases the breakdown of fat in the body, and increases secretion of glucagon by the pancreas. Both epinephrine and norepinephrine act on the cardiovascular system to increase heart rate and blood pressure. They increase bronchodilation, or widening of the air conduits, or bronchioles, in the lungs and increase the respiratory rate. They constrict blood vessels in the skin and the gastrointestinal tract, diverting blood flow to the muscles and the brain.