Adrenal Glands Anatomy Quiz

Inferior Suprarenal Arteries

• Arise from the renal arteries, which also supply the kidneys.
• Blood flows into the adrenal glands through small vessels, exiting via a central vein.
• Right adrenal gland's vein drains into inferior vena cava; left adapts to renal vein.

Adrenal Glands - Embryology

• Adrenal medulla originates from neural crest cells during embryonic development.
• Chromaffin cells in the medulla produce catecholamines: adrenaline and noradrenaline.
• Medulla acts as an "overgrown sympathetic ganglion,” closely linked to the sympathetic nervous system (SNS).

Medulla Function and Hormonal Release

• Activated by stress signals from the SNS, the adrenal medulla releases hormones directly into the bloodstream.
• Zona fasciculata produces cortisol, regulating metabolism and stress responses.
• Gonads convert cholesterol into testosterone or estrogen, regulating reproductive functions.

Regulation of Glucocorticoid Secretion

• Corticotrophin-releasing hormone (CRH) is released by the hypothalamus.
• CRH stimulates adrenocorticotropic hormone (ACTH) release from the anterior pituitary.
• ACTH increases cortisol synthesis in the zona fasciculata.

Mechanism of ACTH Action

• ACTH binds to G-protein coupled receptors, activating a G-protein with α-subunit (Gs).
• Activation leads to increased cAMP and subsequent activation of enzymes for cortisol production.
• Protein kinase A (PKA) further phosphorylates target proteins, regulating steroid synthesis.

Cholesterol Mobilization for Steroidogenesis

• Most cholesterol for hormones is obtained from LDL, involving the upregulation of LDL receptors.
• Cholesterol is stored in lipid droplets and released by cholesteryl ester hydrolase (CEH).
• PKA enhances the transport of cholesterol into mitochondria via steroidogenic acute regulatory protein (StAR).

Steroid Hormone Synthesis Pathway

• Begins with the cleavage of cholesterol side chains at the inner mitochondrial membrane, forming pregnenolone.
• A series of enzymes, including side-chain cleavage enzyme (SCC), convert pregnenolone into cortisol.
• SCC is the rate-limiting step in glucocorticoid synthesis.

Effects of Glucocorticoids on Various Tissues

• Crucial for fetal development of lungs and liver.
• Physiological glucocorticoid levels do not harm bone; excess impairs bone formation through various mechanisms.
• Beneficial for tissue healing, but high levels can inhibit fibroblast activity, affecting recovery.

Cardiovascular and CNS Effects

• Increase sympathy activity in peripheral vessels and heart, leading to hypertension.
• Enhances cardiac output and salt/water retention independently of mineralocorticoids.
• Physiological levels influence mood; excess glucocorticoids cause emotional disturbances.

Immune System Impact

• Short-term glucocorticoid elevations enhance neutrophil activity and lymphocyte response.
• Chronic high glucocorticoid levels impair macrophage function and lymphocyte production, hindering healing.

Catecholamine Synthesis Regulation

• Sympathetic stimulation, ACTH, and cortisol increase catecholamine synthesis.
• Stress activates the hypothalamus, releasing CRH to stimulate ACTH and cortisol production.

Stress Response and Acute Regulation

• Acute sympathetic stimulation releases acetylcholine, promoting catecholamine exocytosis in response to stress.
• Chronic regulation involves sustained ACTH increasing catecholamine levels over time.

Catecholamine Biosynthesis Pathway

• Begins with tyrosine that is converted to L-DOPA, then to dopamine, and finally into norepinephrine via key enzymes.
• This pathway occurs inside chromaffin cells in response to neuronal signals and stress.

Adrenal Glands Overview

• Adrenal glands respond to stress by increasing heart rate, blood pressure, and energy through chemical signals.
• The adrenal gland consists of two main parts: the adrenal cortex and the adrenal medulla, which have different developmental origins and functions.

Adrenal Cortex

• Derived from mesoderm, forming the outer layer of the adrenal gland.
• Produces essential steroid hormones with three distinct zones:
  • Zona Glomerulosa: Smallest zone, secretes aldosterone for blood pressure regulation via sodium and water balance.
  • Zona Fasciculata: Largest zone, produces cortisol for metabolism regulation, immune response, and stress response.
  • Zona Reticularis: Secretes androgens, precursors for sex steroids like testosterone and estrogen.

Adrenal Medulla

• Developed from neural crest cells, functions as an “overgrown sympathetic ganglion”.
• Responsible for catecholamine secretion, particularly epinephrine during stress.

Hypothalamic-Pituitary Axis (HPA)

• The hypothalamus releases hormones (e.g., CRH, TRH, GnRH), which regulate the anterior pituitary hormones.
• Anterior pituitary hormones influence various endocrine glands’ activities.

Regulation of Glucocorticoids

• Corticotrophin-releasing hormone (CRH) stimulates adrenocorticotropic hormone (ACTH) release from the anterior pituitary.
• ACTH promotes cortisol synthesis in the adrenal cortex, specifically in zona fasciculata.
• Involves activation of G-protein coupled receptors, increasing cAMP levels, which activates protein kinase A (PKA) leading to cortisol production.

Synthesis of Adrenal Steroids

• Cholesterol is a precursor for all steroid hormone synthesis, primarily sourced from LDL through an exogenous pathway.
• Cholesterol mobilization into mitochondria is facilitated by PKA and steroidogenic acute regulatory protein (StAR).
• Initial step in steroid hormone synthesis involves converting cholesterol into pregnenolone, followed by a cascade of enzyme-driven reactions producing final hormones, such as cortisol.

Mineralocorticoid Regulation

• Aldosterone production depends on aldosterone synthase, expressed exclusively in zona glomerulosa cells.
• Major regulators include:
  • Angiotensin II from the renin-angiotensin-aldosterone system (RAAS)
  • Serum potassium levels (K+)

Renin-Angiotensin-Aldosterone System (RAAS)

• Renin release from the kidney in response to reduced perfusion converts angiotensin to angiotensin I.
• Angiotensin II functions include vasoconstriction, stimulating aldosterone secretion, and promoting sodium reabsorption in kidneys.

Catecholamine Synthesis

• Stress activates hypothalamus, releasing CRH, which triggers ACTH production.
• ACTH enhances catecholamine synthesis and secretion (norepinephrine and epinephrine) in the adrenal medulla.
• Two pathways regulate catecholamine release:
  • Chronic Regulation: Involvement of cortisol and ACTH for long-term stress responses, leading to increased production of epinephrine.
  • Acute Regulation: Neuronal stimulation through acetylcholine rapidly induces catecholamine exocytosis from chromaffin cells.

Catecholamine Biosynthesis Pathway

• Begins with tyrosine, which is converted to catecholamines through enzymatic reactions involving:
  • Tyrosine Hydroxylase (TH): Converts tyrosine to L-DOPA.
  • DOPA Decarboxylase (DD): Converts L-DOPA to dopamine.
  • Dopamine β-hydroxylase (DH): Transforms dopamine into norepinephrine.