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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.
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:
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 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.
Glucocorticoids (chiefly cortisol)
Mineralocorticoids (chiefly aldosterone)
Androgens (small amounts of sex steroids such as DHEA)
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:
Both are catecholamines derived from the amino acid tyrosine. They act within seconds to produce the classic fight-or-flight response:
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.
| 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 |
The pancreas lies in the abdomen behind the stomach. It has both endocrine and exocrine tissue, arranged intimately together.
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