Adrenal (Suprarenal) Glands: Structure, Function, and Hormones

Adrenal (Suprarenal) Glands

Overview and Location

The adrenal glands, also known as suprarenal glands, are paired endocrine organs located superior to each kidney. They play a crucial role in the body's response to stress and in maintaining homeostasis through hormone secretion.

• Adrenal medulla: Composed primarily of neural tissue.

• Adrenal cortex: Composed of glandular tissue.

• Secretes steroid hormones: epinephrine and norepinephrine (from the medulla).

The adrenal cortex is organized into three distinct layers, each responsible for producing specific classes of hormones:

• Zona glomerulosa: Produces mineralocorticoids (chiefly aldosterone) – regulates SALT.

• Zona fasciculata: Produces glucocorticoids (chiefly cortisol) – regulates SUGAR.

• Zona reticularis: Produces gonadocorticoids (chiefly androgens) – regulates SEX hormones.

Adrenal Cortex Structure

The adrenal cortex consists of three layers, each with distinct histological and functional characteristics.

Layer	Main Hormone(s)	Main Function
Zona glomerulosa	Aldosterone (mineralocorticoid)	Regulates sodium and potassium balance
Zona fasciculata	Cortisol (glucocorticoid)	Regulates metabolism and stress response
Zona reticularis	Androgens (gonadocorticoids)	Influences sexual development and characteristics

Zona Glomerulosa: Mineralocorticoids

Aldosterone: Function and Regulation

Aldosterone is the most important mineralocorticoid produced by the zona glomerulosa. It plays a vital role in regulating electrolyte balance and blood pressure.

• Promotes potassium loss at the kidneys.

• Elevates sodium levels in the blood by reducing excretion.

• Stimulates reabsorption of Na+ by the kidneys.

Aldosterone secretion is stimulated by:

• Rising blood levels of potassium.

• Low blood levels of sodium.

• Decreasing blood volume or pressure.

Major Mechanisms of Aldosterone Secretion

Aldosterone release is regulated by several mechanisms, including the renin-angiotensin-aldosterone system (RAAS), adrenocorticotropic hormone (ACTH), and atrial natriuretic peptide (ANP).

• Renin-angiotensin-aldosterone system (RAAS): Activated by decreased blood pressure or volume, leading to renin release from the kidneys, production of angiotensin II, and stimulation of aldosterone secretion.

• ACTH: Released from the anterior pituitary in response to stress, can stimulate aldosterone secretion.

• ANP: Produced by the heart, inhibits aldosterone secretion to reduce blood pressure and volume.

Equation:

$\text{Renin} \xrightarrow{\text{Kidney}} \text{Angiotensinogen} \rightarrow \text{Angiotensin I} \rightarrow \text{Angiotensin II} \rightarrow \text{Aldosterone}$

Zona Fasciculata: Glucocorticoids

Cortisol: Function and Effects

Cortisol is the principal glucocorticoid produced by the zona fasciculata. It helps the body resist stress and maintain metabolic homeostasis.

• Helps maintain blood glucose levels.

• Stimulates gluconeogenesis – the synthesis of glucose from non-carbohydrate sources.

• Increases blood levels of glucose, fatty acids, and amino acids.

• Long-term stressors cause prolonged cortisol release.

Excessive levels of glucocorticoids can:

• Depress cartilage and bone formation.

• Inhibit inflammation.

• Depress the immune system.

• Promote changes in cardiovascular, neural, and gastrointestinal function.

Equation:

$\text{Gluconeogenesis:} \quad \text{Amino acids} + \text{Fatty acids} \rightarrow \text{Glucose}$

Zona Reticularis: Gonadocorticoids (Sex Hormones)

Androgens: Function and Importance

The zona reticularis produces gonadocorticoids, mainly androgens (male sex hormones), with testosterone being the most important.

• Produced in both males and females.

• Contribute to the onset of puberty.

• Responsible for the appearance of secondary sex characteristics.

• Androgens can be converted into estrogens after menopause.

Adrenal Medulla

Medullary Hormones: Epinephrine and Norepinephrine

The adrenal medulla secretes epinephrine and norepinephrine, which are catecholamines involved in the body's acute stress response (fight-or-flight).

• Increase blood glucose levels.

• Cause vasoconstriction in most regions of the body (with some exceptions).

• Increase heart rate and force of contraction.

• Divert blood to the brain, heart, and skeletal muscle.

• Epinephrine is a more potent stimulator of the heart and metabolic activities.

• Norepinephrine is more influential on peripheral vasoconstriction and blood pressure regulation.

Other Hormone-Producing Structures

Overview of Additional Endocrine Organs

Several other organs and tissues produce hormones that influence physiological processes.

• Heart: Produces atrial natriuretic peptide (ANP), which reduces blood pressure, blood volume, and blood sodium concentration.

• Kidneys: Secrete erythropoietin, which signals the production of red blood cells. Target cells: Erythroid progenitor cells in bone marrow.

• Gastrointestinal tract: Certain cells release local-acting digestive hormones.

• Adipose tissue: Releases leptin, involved in the regulation of satiety and stimulation of increased energy expenditure.

Thymus

Thymic Hormones and Immune Function

The thymus is a gland located deep to the sternum. Its major hormonal products are thymopoietins and thymosins, which are essential for the development of T lymphocytes (T cells) of the immune system.

• Promotes maturation of T cells.

• Critical for adaptive immunity.

Summary Table: Adrenal Cortex Layers and Hormones

Layer	Hormone Class	Main Hormone	Main Function
Zona glomerulosa	Mineralocorticoids	Aldosterone	Regulates Na+ and K+ balance
Zona fasciculata	Glucocorticoids	Cortisol	Regulates metabolism, stress response
Zona reticularis	Gonadocorticoids	Androgens	Sexual development, secondary sex characteristics

Example: During periods of stress, the adrenal cortex releases cortisol, which increases blood glucose levels to provide energy for the body to cope with the stressor.

Additional info: The adrenal glands are essential for survival; removal or dysfunction can lead to life-threatening imbalances in fluid, electrolytes, and metabolism.