Adrenal Gland
The adrenal gland also called suprarenal glands is composed of an outer portion, the cortex (80 to 90% of gland) which secretes corticosteroids and the inner portion, the medulla (10 to 20% of gland) which secretes catecholamines.
Adrenal Cortex
In mammals the adrenal cortex is organized into three zones-
- Outer zone – zona glomerulosa secrete the hormones mineralocorticoids
- Middle zone – zona fasciculata secrete the hormones glucocorticoids
- Innermost zone – zona reticularis secrete androgens
The mineralocorticoids play an important role in electrolyte balance and also in the regulation of blood pressure. The major mineralocorticoid is aldosterone.
The glucocorticoids are important in the regulation of metabolism either directly or through an interaction with other hormones. The major glucocorticoid is cortisol.
The adrenal sex steroids are secreted from the inner layers.
Adrenal Medulla
The cells of the adrenal medulla that secrete the catecholamine hormones, are called chromaffin cells and they are innervated by preganglionic cholinergic nerve fibers.
Structure and regulation of hormone release–
The central, medullary region of the adrenal gland acts as an endocrine extension of the sympathetic nervous system. It contains cells (chromaffin cells) which lack axons but possess many of the other properties of postganglionic sympathetic neurones.
These chromaffin cells release the contents of their intracellular vesicles into the bloodstream in response to stimulation by the preganglionic sympathetic nerves which innervate the adrenal medulla.
As a result, activation of sympathetic nerves leads to an increase in the circulating levels of the Catecholamines, i.e. noradrenaline (norepinephrine) and adrenaline (epinephrine).
The catecholamine hormones are manufactured from the amino acid tyrosine in the chromaffin cells.
However, in contrast with sympathetic neurones which release noradrenaline (norepinephrine), the adrenal medulla secretes about 80% adrenaline (epinephrine) to 20% noradrenaline (norepinephrine). However, birds, whales and cat mainly secrete noradrenaline
After release, they are broken down again within minutes in the liver and kidney in reactions involving catechol-O-methyl transferase and monoamine oxidase, so their actions can be switched off rapidly.
Catecholamine metabolites, such as vanillylmandelic acid, and small quantities of free hormone are excreted in the urine.