The Adrenal Glands

The Adrenal Glands

The adrenal glands are two small structures situated one at top each kidney. Both in anatomy and in function, they consist of two distinct regions:


an outer layer, the adrenal cortex, which surrounds
the adrenal medulla.


The Adrenal Cortex

cells of the adrenal cortex secrete a variety of steroid hormones.


glucocorticoids (e.g., cortisol)
mineralocorticoids (e.g., aldosterone)
androgens (e.g., testosterone)



Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.


Glucocorticoids

They Effect by raising the level of blood sugar (glucose). One way they do this is by stimulating gluconeogenesis in the liver: the conversion of fat and protein into intermediate metabolites that are ultimately converted into glucose.

The most abundant glucocorticoid is cortisol (also called hydrocortisone).

Cortisol and the other glucocorticoids also have a potent anti-inflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses. 

Mineralocorticoids

The most important of them is the steroid aldosterone. Aldosterone acts on the kidney promoting the reabsorption of sodium ions (Na+) into the blood. Water follows the salt and this helps maintain normal blood pressure.

Aldosterone also


acts on sweat glands to reduce the loss of sodium in perspiration;
acts on taste cells to increase the sensitivity of the taste buds to sources of sodium.


The secretion of aldosterone is stimulated by:


a drop in the level of sodium ions in the blood;
a rise in the level of potassium ions in the blood;
angiotensin II
ACTH (as is that of cortisol)


Androgens

The adrenal cortex secretes precursors to androgens such as testosterone.

Excessive production of adrenal androgens can cause premature puberty in young boys.

In females, the adrenal cortex is a major source of androgens. Their hypersecretion may produce a masculine pattern of body hair and cessation of menstruation.

Addison's Disease: Hyposecretion of the adrenal cortices

Addison's disease has many causes, such as


destruction of the adrenal glands by infection;
their destruction by an autoimmune attack;
an inherited mutation in the ACTH receptor on adrenal cells.


Cushing's Syndrome: Excessive levels of glucocorticoids

In Cushing's syndrome, the level of adrenal hormones, especially of the glucocorticoids, is too high.It can be caused by:


excessive production of ACTH by the anterior lobe of the pituitary;
excessive production of adrenal hormones themselves (e.g., because of a tumor), or (quite commonly)
as a result of glucocorticoid therapy for some other disorder such as

rheumatoid arthritis or
preventing the rejection of an organ transplant.




The Adrenal Medulla

The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.The adrenal medulla releases:


adrenaline (also called epinephrine) and
noradrenaline (also called norepinephrine)


Both are derived from the amino acid tyrosine.

Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors  transmembrane proteins in the plasma membrane of many cell types.

Some of the effects are:


increase in the rate and strength of the heartbeat resulting in increased blood pressure;
blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain;
rise in blood sugar;
increased metabolic rate;
bronchi dilate;
pupils dilate;
hair stands on end (gooseflesh in humans);
clotting time of the blood is reduced;
increased ACTH secretion from the anterior lobe of the pituitary.


All of these effects prepare the body to take immediate and vigorous action.