41. Adrenal Gland Flashcards
Describe the anatomical organisation of the adrenal
gland.
The adrenal or suprarenal glands lie on top of the upper poles of the kidneys and play a key role in the synthesis of corticosteroids and catecholamines.
The adrenal glands are at the
level of the twelfth thoracic vertebra.
Anatomically, the adrenal gland is
divided into two distinct areas: an outer
cortex and inner medulla.
Adrenal cortex:
Adrenal cortex:
> Site of synthesis of
corticosteroid hormones
(glucocorticoids and mineralocorticoids)
and androgens.
> Under neuroendocrine
control via the
hypothalamic–pituitary–adrenal
axis.
> Part of the renin–angiotensin–aldosterone pathway.
> Divided into three functional zones from outside to inside: zona glomerulosa, zona fasciculata and zona reticularis (an easy way to remember this is ‘GFR’).
Adrenal medulla:
Adrenal medulla:
> Composed of chromaffin cells.
> Main site of synthesis of adrenaline
and noradrenaline.
> Hormone secretion occurs in response to stimulation by pre-ganglionic (cholinergic) nerve fibres from the sympathetic nervous system (via splanchnic nerves).
What are the main hormones
secreted from each of the three
zones of the adrenal cortex?
> Zona glomerulosa:
mineralocorticoids (aldosterone).
> Zona fasciculata:
glucocorticoids (cortisol).
> Zona reticularis:
androgens (dehydroepiandrosterone).
Describe the cortisol negative feedback pathway
The hypothalamus secretes
corticotrophin-releasing hormone (CRH),
which stimulates release of adrenocorticotropic hormone (ACTH)
from the anterior pituitary.
ACTH stimulates cortisol
secretion from the zona fasciculata
of the adrenal cortex.
Cortisol exerts negative feedback on
both CRH and ACTH release.
Describe the control of aldosterone secretion.
> Renin–angiotensin–aldosterone (RAA) system:
Reduced circulating volume is
detected by the reduction
in renal afferent arteriolar pressure
causing renin secretion
from the juxtaglomerular cells.
Renin cleaves
angiotensinogen to
produce angiotensin I,
which is then converted to
angiotensin II in the
pulmonary vasculature by
angiotensin-converting enzyme (ACE).
Angiotensin II promotes aldosterone secretion.
> Fall in plasma sodium concentration:
Reduced serum sodium is
detected by the macula densa
in the distal convoluted tubule of the
kidney and stimulates the
secretion of aldosterone
in order to increase
sodium retention.
> Rise in plasma ACTH:
Also exerts a direct effect in increasing
aldosterone secretion from the zona glomerulosa.
What are major actions of
cortisol?
Cortisol exerts its effects by
binding to glucocorticoid receptors and
promoting specific enzyme synthesis.
The glucocorticoids works in many
ways to provide resistance to
‘stress’, having effects on metabolism,
immune function and vascular reactivity.
> Metabolism:
• Increased protein catabolism
• Increased hepatic gluconeogenesis
and increased plasma glucose levels
• Increased lipolysis
> Vascular:
• Cortisol is essential in maintaining vascular
reactivity to noradrenaline.
> Immune: • Suppresses the immune system, impairs wound healing and has antiinflammatory effects.
What are the main actions of
aldosterone?
Aldosterone increases the
reabsorption of sodium from
the distal convoluted
tubules of the kidney,
resulting in sodium retention
and plasma expansion.
It also increases urinary potassium excretion.
Describe how catecholamines
are synthesised.
Catecholamines are synthesised in the
chromaffin cells of the adrenal medulla:
L-TYROSINE
↓ Tyrosine hydroxylase
L-DOPA
↓ Dopa decarboxylase
DOPAMINE
↓ Dopamine hydroxylase
NORADRENALINE
↓ Phenylethanolamine N-methyltransferase
ADRENALINE
