Hormones and the Endocrine System

Whereas the nervous system delivers lightning-fast, pinpoint-accurate electrical signals, the endocrine system delivers slower, longer-lasting chemical messages that wash over the entire body in the bloodstream. The two systems complement each other: together they achieve homeostasis, coordinate development, regulate metabolism and respond to stress. This lesson introduces the endocrine system and the molecular mechanisms of hormone action required by OCR A-Level Biology A specification 5.1.4(a)–(c).

Key Definitions:

• Hormone — a chemical messenger released from an endocrine gland into the blood, travelling to distant target cells where it produces a specific response.
• Endocrine gland — a ductless gland that secretes hormones directly into the bloodstream.
• Exocrine gland — a gland that secretes via a duct onto an epithelial surface (e.g. sweat glands, salivary glands).
• Target cell — a cell bearing specific receptors for a particular hormone.
• Second messenger — an intracellular molecule produced in response to a hormone binding its receptor; it amplifies and spreads the signal.

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Endocrine vs Exocrine Glands

Many students struggle to remember the difference between endocrine and exocrine glands. A simple rule: "exo" means "out" — exocrine glands secrete out through a duct; "endo" means "in" — endocrine glands secrete in to the blood.

Feature	Endocrine	Exocrine
Ducts?	No	Yes
Secretes into	Blood	Epithelial surface / lumen
Example	Pancreas (islets), adrenal, pituitary, thyroid	Sweat glands, salivary glands, pancreas (acini)
Target	Distant cells with matching receptors	Local (the organ containing the duct's opening)
Speed of effect	Seconds to days	Seconds

Note that the pancreas has both endocrine and exocrine components — a classic exam point. Islets of Langerhans (endocrine) secrete insulin and glucagon into the blood; the acini (exocrine) secrete pancreatic juice containing digestive enzymes into the pancreatic duct.

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Major Endocrine Glands and Their Hormones

Gland	Hormone(s)	Main function
Hypothalamus	Releasing factors (e.g. CRH, TRH, GnRH)	Control the pituitary
Anterior pituitary	ACTH, TSH, FSH, LH, GH, prolactin	Control other endocrine glands and growth
Posterior pituitary	ADH, oxytocin	Water balance; uterine contraction / milk ejection
Thyroid	Thyroxine (T3, T4), calcitonin	Metabolism; Ca²⁺ homeostasis
Parathyroid	PTH	Ca²⁺ homeostasis
Adrenal cortex	Cortisol, aldosterone, androgens	Stress response, Na⁺/K⁺ balance
Adrenal medulla	Adrenaline, noradrenaline	Fight or flight response
Pancreas (islets)	Insulin, glucagon	Blood glucose homeostasis
Ovaries	Oestrogen, progesterone	Menstrual cycle, pregnancy
Testes	Testosterone	Spermatogenesis, secondary sexual characteristics
Pineal	Melatonin	Circadian rhythms, sleep

OCR expects detailed knowledge of the adrenal gland and the pancreas, which are covered in the next lesson. Here we focus on the general principles of hormone action.

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Two Classes of Hormone: Steroid and Protein

Hormones fall into two broad molecular categories, and their modes of action differ fundamentally.

Steroid Hormones

• Derived from cholesterol, lipid-soluble.
• Examples: cortisol, aldosterone, testosterone, oestrogen, progesterone.
• Can pass directly through the plasma membrane.
• Bind to intracellular receptors (usually in the cytoplasm or nucleus).
• The hormone-receptor complex acts as a transcription factor, binding to specific DNA sequences and switching genes on or off.
• Effects are slow (hours) but long-lasting, because they involve new protein synthesis.

Protein / Peptide Hormones

• Water-soluble and cannot cross the plasma membrane.
• Examples: insulin, glucagon, adrenaline, ADH, growth hormone.
• Bind to cell-surface receptors (usually G-protein coupled receptors or tyrosine kinase receptors).
• Receptor activation triggers intracellular signalling via a second messenger (e.g. cAMP).
• Effects are fast (seconds to minutes) but generally short-lived.

Feature	Steroid hormones	Protein / peptide hormones
Solubility	Lipid-soluble	Water-soluble
Receptor location	Inside the cell (cytoplasm / nucleus)	Cell-surface membrane
Mode of action	Direct effect on gene transcription	Second messenger cascade
Speed of response	Slow (hours)	Fast (seconds–minutes)
Duration	Long-lasting	Short-lived
Examples	Cortisol, testosterone, oestrogen	Insulin, glucagon, adrenaline

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Second Messengers — The Cyclic AMP System