Introduction
Adrenal glands (or suprarenal glands) are star-shaped endocrine glands positioned on the anterior portion of kidneys. The term adrenal indicates its position (ad means near or at; - renes means kidneys; and supra- means above).
These glands mainly regulate the stress response by producing corticosteroids and catecholamines, including cortisol and adrenaline (epinephrine), respectively.
Structure
Adrenal gland is divided into outer and inner zones, i.e., adrenal cortex and adrenal medulla, respectively. These two zones differ in structure, functions, and origin. The adrenal insufficiency glands are highly vascularized.
While an embryo develops, the adrenal glands disorder separate into two structurally and functionally dissimilar regions:
1) Adrenal Cortex: This region is present on the outer side and is large (since comprises 80-90% of the gland), and
2) Adrenal Medulla: This region is present in the center and is small (since comprises 10-20% of the gland).
The outer adrenal cortex is itself made up of three distinct zones each secreting different hormones:
1) Zona Glomerulosa: This is the outer zone lying just beneath the connective tissue capsule. It is made up of tightly packed cells arranged in spherical clusters and arched columns. These cells secrete congenital adrenal hyperplasia mineralocorticoids (named so because they affect mineral homeostasis).
2) Zona Fasciculata: This is the middle zone (the widest zone) made up of cells arranged in long and straight columns. These cells secrete glucocorticoids (named so because they affect glucose homeostasis).
3) Zona Reticularis: This is the inner zone made up of cells arranged in branching cords. These cells produce weak androgens (steroid hormones with masculine effects) in small amounts.
Adrenal Medulla
The inner adrenal medulla is a modified sympathetic ganglion of the autonomic nervous system. This is because it originates from the embryonic tissue from which other sympathetic ganglia has originated. However, its cells lack axons and thyroid hormone clusters around large blood vessels. These cells secrete high blood pressure hormones (epinephrine and nor-epinephrine) and are termed chromaffin cells. ANS directly controls these chromaffin cells, thus releasing hormone very rapidly.
Hormones
The hormones secreted by the adrenal cortex are collectively known as corticoids and are of the following types:
1) Glucocorticoids (e.g., cortisol) are secreted by the zona fasciculata part of the adrenal cortex. These hormones stimulate gluconeogenesis, lipolysis, and proteolysis; and inhibit cellular uptake and utilization of amino acids.
2) Mineralocorticoids (e.g., aldosterone and deoxycorticosterone) are secreted by the zona glomerulosa part of the adrenal cortex. These hormones act on the renal tubules and stimulate the reabsorption of Na+ ions and water, and excretion of K+ and phosphate ions. Thus, growth and development of aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure, and blood pressure.
3) Small amounts of androgenic steroids (e.g., androgens) are secret e part of the adrenal cortex. These produce hormones are responsible for the development of symptoms of puberty-like growth of axial hair, pubic hair, and facial hair.
Adrenal medulla produces three catecholamine hormones:
1) Nor-epinephrine (or nor-adrenaline)
2) Epinephrine (or adrenaline), and
3) Dopamine (in small amounts).
Glucocorticoids
Glucocorticoids are steroid hormones bound to the Glucocorticoid Receptor (GR, found in the cells of most of the vertebrate animals). The hormone is named as glucocorticoid (glucose + cortex + steroid) since it regulates glucose metabolism, their synthesis in adrenal cortex, and their steroidal structure.
Functions
1) Intermediary Metabolism: Glucocorticoids aid in gluconeogenesis by increasing the uptake of amino acid by the liver and kidneys, and by elevating the activities of gluconeogenic enzymes. It also stimulates protein catabolism (except in the liver) and lipolysis to fulfil the demands of building blocks and energy for glucose synthesis.
2) Stress: It increases the levels of glucose in plasma to fulfil the demands of energy required for combatting stress related to trauma, fright, infection, bleeding, or disease.
3) Blood Cells: It decreases the number of eosinophils, basophils, monocytes, and lymphocytes by redistributing them to lymphoid tissues from blood. It also increases hemoglobin levels, erythrocytes, platelets, and polymorphonuclear leukocytes.
4) Anti-Inflammatory Action: It reduces inflammatory response and suppresses immunity.
5) Endocrine System: It increases the synthesis of growth hormone.
6) Immune: It interacts with the glucocorticoid receptor to:
i) Up-regulate the expression of anti-inflammatory proteins, and
ii) Down-regulate the expression of pro-inflammatory proteins.
Mineralocorticoids
The natural mineralocorticoids are aldosterone and deoxycorticosterone, exerting similar effects on electrolyte and water metabolism, and on the cardiovascular system. The only difference is that the potency of aldosterone is 30 times more than that of deoxycorticosterone.
1) Electrolyte and Water Metabolism: Aldosterone stimulates sodium reabsorption and increases potassium excretion by the kidneys, intestines, salivary glands, and sweat.
2) Cardiovascular Effects: Aldosterone along with angiotensin 11 aids in blood coagulation, contraction of systemic arterioles, Cushing syndrome and stimulation of thirst. They also regulate the inflammatory and reparative processes occurring after a tissue injury.
Epinephrine
Epinephrine exerts biological effects similar to sympathetic nervous systems, thus is a sympathomimetic hormone. The sympathetic nervous system and adrenaline are together called sympathetic-adrenal systems. This system acts in a similar way on similar organs.
Functions
1) Epinephrine increases blood pressure by increasing the heart beat rate and cardiac output.
2) It increases BMR (20% more than the normal rate) by increasing the tissue oxidation rate.
3) It increases blood sugar level by stimulating liver glycogenolysis.
4) It increases respiration rate by dilating trachea.
5) It increases fatty acids level in blood by increasing lipolysis in adipose tissues.
6) It dilates the pupil of the eye.
7) It acts as a vasodilator since it increases the blood supply to heart, skeletal muscles, and brain.
8) It stimulates the release of blood stored in the spleen stress situations by its contraction.
Nor-Epinephrine
Nor-epinephrine exerts almost the same biological effects as adrenaline. The only difference is that it operates during normal state, epinephrine and norepinephrine produces lesser effect on cardiac activity, and greater constriction of blood vessels in the muscles.
The adrenal gland performs the following functions:
1) It allows the body to deal with stress related to injury, Addison disease, work, or personal life.
2) It determines the energy released when the body responds to the changes in internal and external environment.
3) It secretes hormones which allow the body excessive amounts to mobilize its resources to escape or fight off danger (stress) and survive.
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Thyroid gland and parathyroid gland.
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