5.4 Adrenal glands

The adrenal glands, located atop the kidneys, play a crucial role in the endocrine system. They're composed of two distinct regions: the outer cortex and inner medulla, each producing different hormones that regulate various bodily functions.

The adrenal cortex secretes mineralocorticoids, glucocorticoids, and androgens, while the medulla produces catecholamines. These hormones are vital for stress response, metabolism, and maintaining homeostasis, making the adrenal glands essential for overall health and well-being.

Adrenal Gland Anatomy and Structure

Location and Composition

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• Adrenal glands are paired endocrine glands located superior to each kidney
  • Embedded in the perirenal fat and enclosed within the renal fascia
• Each adrenal gland is composed of two distinct regions: the outer adrenal cortex and the inner adrenal medulla
  • These regions differ in their embryonic origin, structure, and function

Adrenal Cortex Zones and Medulla

• The adrenal cortex is divided into three concentric zones
  • Zona glomerulosa (outermost layer)
  • Zona fasciculata (middle layer)
  • Zona reticularis (innermost layer)
  • Each zone secretes specific hormones
• The adrenal medulla is composed of chromaffin cells
  • Modified postganglionic sympathetic neurons that secrete catecholamines (epinephrine and norepinephrine)

Blood Supply and Venous Drainage

• The adrenal glands are highly vascularized
  • Receive blood supply from the superior, middle, and inferior suprarenal arteries
• Venous drainage occurs through the suprarenal veins

Adrenal Hormone Secretion and Function

Adrenal Cortex Hormones

• The zona glomerulosa secretes mineralocorticoids, primarily aldosterone
  • Regulates sodium and potassium balance
  • Helps maintain blood pressure and volume
• The zona fasciculata and zona reticularis secrete glucocorticoids, mainly cortisol
  • Regulates glucose metabolism, immune function, and the body's response to stress
• The zona reticularis also secretes small amounts of androgens
  • Dehydroepiandrosterone (DHEA) and androstenedione
  • Can be converted to testosterone and estrogens in peripheral tissues

Adrenal Medulla Hormones

• The adrenal medulla secretes catecholamines in response to sympathetic stimulation
  • Primarily epinephrine (adrenaline) and norepinephrine (noradrenaline)
• These hormones mediate the "fight or flight" response
  • Have effects on various organ systems (cardiovascular, respiratory, metabolic)

Regulation of Adrenal Cortex Hormone Secretion

Hypothalamic-Pituitary-Adrenal (HPA) Axis

• The HPA axis is a neuroendocrine system that regulates the secretion of adrenal cortex hormones, particularly cortisol
  • The hypothalamus secretes corticotropin-releasing hormone (CRH)
  • CRH stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH)
  • ACTH binds to receptors on the adrenal cortex, stimulating the synthesis and secretion of glucocorticoids (cortisol), mineralocorticoids, and androgens

Negative Feedback and Circadian Regulation

• Cortisol exerts negative feedback on the hypothalamus and anterior pituitary
  • Inhibits the secretion of CRH and ACTH, respectively, to maintain homeostasis
• The HPA axis is subject to circadian regulation
  • Cortisol levels peak in the morning and gradually decline throughout the day
• Various stressors (physical, emotional, or psychological) can activate the HPA axis
  • Leads to increased cortisol secretion to help the body cope with the stressor

Physiological Effects of Adrenal Medullary Hormones

Adrenergic Receptor Binding and Effects

• Epinephrine and norepinephrine bind to adrenergic receptors (α1, α2, β1, β2, and β3) in target tissues
  • Leads to various physiological effects
• Cardiovascular effects
  • Increased heart rate and contractility (β1 receptors)
  • Vasoconstriction in skin and viscera (α1 receptors)
  • Vasodilation in skeletal muscles (β2 receptors)
• Metabolic effects
  • Increased glycogenolysis and gluconeogenesis in the liver (β2 receptors)
  • Increased lipolysis in adipose tissue (β3 receptors)

Other Physiological Effects

• Respiratory effects
  • Bronchodilation (β2 receptors)
• Pupillary dilation (α1 receptors)
• Decreased gastrointestinal motility (α2 receptors)
• Increased alertness and arousal (central nervous system effects)
• These effects collectively contribute to the "fight or flight" response
  • Prepares the body for immediate action in response to stress or emergencies