Adrenal Glands and Pancreas

Two endocrine organs are particularly important for OCR A-Level Biology A: the adrenal glands, which mount the fight-or-flight response and set long-term metabolic rate, and the pancreas, which together with the liver controls blood glucose. This lesson explores the histology, cell types and hormones of both glands, in line with specification 5.1.4(d)–(f).

Key Definitions:

• Adrenal gland — a paired endocrine gland sitting on top of each kidney; has two distinct regions (cortex and medulla).
• Pancreas — a mixed exocrine–endocrine gland behind the stomach; endocrine cells cluster in islets of Langerhans.
• Islets of Langerhans — clusters of endocrine cells within the pancreas.
• α cells — cells in the islets that secrete glucagon when blood glucose falls.
• β cells — cells in the islets that secrete insulin when blood glucose rises.

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The Adrenal Glands

Each adrenal gland is a pyramid-shaped organ about 5 cm across, sitting on top of a kidney. In cross-section you can see two distinct regions:

• Outer cortex — pale, derived from mesoderm.
• Inner medulla — dark brown, derived from neural crest.

The two regions have entirely different embryological origins, entirely different hormones and entirely different control mechanisms. Treating them as "one gland" is a common mistake.

The Adrenal Cortex

The cortex makes steroid hormones from cholesterol. It has three histological zones, each producing different hormones. OCR does not require the zone names but expects you to know the three hormone classes produced.

1. Glucocorticoids (chiefly cortisol)

   • Help regulate metabolism; promote gluconeogenesis and lipolysis.
   • Raise blood glucose over the longer term (hours).
   • Suppress inflammation (which is why synthetic cortisol derivatives are used as anti-inflammatory drugs).
   • Secreted under the control of ACTH from the anterior pituitary, which itself is stimulated by CRH from the hypothalamus — the HPA (hypothalamic-pituitary-adrenal) axis.

2. Mineralocorticoids (chiefly aldosterone)

   • Act on the distal convoluted tubule and collecting duct of the kidney.
   • Promote Na⁺ reabsorption and K⁺ excretion.
   • Water follows Na⁺ osmotically, so aldosterone raises blood volume and pressure.
   • Secretion is controlled by the renin-angiotensin system (triggered by low blood pressure) and by plasma K⁺ concentration.

3. Androgens (small amounts of sex steroids such as DHEA)

   • Precursors of testosterone and oestrogen.
   • Of minor importance in healthy adults, but significant in some pathological states.

The Adrenal Medulla

The medulla is really modified sympathetic nervous tissue. It contains chromaffin cells, which are essentially specialised postganglionic sympathetic neurones without axons. When the sympathetic nervous system is activated (fight or flight), it directly stimulates the chromaffin cells, which release their hormones into the blood:

1. Adrenaline (~80% of medullary secretion)
2. Noradrenaline (~20%)

Both are catecholamines derived from the amino acid tyrosine. They act within seconds to produce the classic fight-or-flight response:

• Heart rate and stroke volume increase.
• Vasoconstriction in the skin and gut; vasodilation in skeletal muscle and cardiac muscle.
• Bronchioles dilate (easier breathing).
• Pupils dilate.
• Liver glycogenolysis is activated (via cAMP) raising blood glucose.
• Piloerection (hairs stand up).
• Energy is mobilised from fat stores.

Because adrenaline is a hormone in the blood rather than a neurotransmitter at a synapse, its effects last longer than a direct sympathetic nerve stimulation — minutes rather than seconds.

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Summary of Adrenal Hormones

Region	Hormone	Class	Target	Main effect
Cortex	Cortisol	Steroid	Many	Raises blood glucose; anti-inflammatory
Cortex	Aldosterone	Steroid	Kidney	Na⁺ reabsorption, water retention
Cortex	Androgens	Steroid	Many	Precursors of sex steroids
Medulla	Adrenaline	Catecholamine	Liver, heart, vessels, airways	Fight or flight
Medulla	Noradrenaline	Catecholamine	Heart, vessels	Fight or flight

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The Pancreas

The pancreas lies in the abdomen behind the stomach. It has both endocrine and exocrine tissue, arranged intimately together.

Exocrine Function