1. Endocrine and exocrine glands; where the glands are
Hormones are chemical messengers secreted by endocrine glands directly into the blood, which carries them to target organs. They act slowly compared with nerves, but their effects last longer.
| Endocrine (ductless) glands | Exocrine glands |
|---|---|
| No ducts | Have ducts |
| Secrete hormones into the blood | Secrete enzymes, sweat, tears, saliva onto a surface or into a cavity |
| Thyroid, pituitary, adrenal | Salivary, sweat, lacrimal glands |
The pancreas is a heterocrine (mixed) gland: its acini are exocrine (pancreatic juice through a duct) and its islets of Langerhans are endocrine.
Location and shape
- Pituitary: pea-sized, at the base of the brain, attached to the hypothalamus; anterior and posterior lobes.
- Thyroid: two lobes joined by an isthmus (butterfly or H-shaped), in the neck on either side of the windpipe below the larynx.
- Pancreas: long, leaf-like, lying behind the stomach with its head in the loop of the duodenum.
- Adrenal: a cap (roughly triangular) on top of each kidney; outer cortex, inner medulla.
Key terms
- Hormone
A chemical messenger secreted by a ductless gland and carried by blood to a target organ.
- Endocrine gland
A ductless gland that releases hormones into the blood.
- Heterocrine gland
A gland with both exocrine and endocrine parts, such as the pancreas.
2. Pancreas (insulin, glucagon) and thyroid (thyroxine)
Islets of Langerhans: beta cells secrete insulin; alpha cells secrete glucagon.
- Insulin lowers blood sugar: cells take up glucose; the liver and muscles store extra glucose as glycogen.
- Glucagon raises blood sugar: liver glycogen is changed into glucose.
- Together they keep blood sugar near 80–120 mg per 100 mL of blood.
| Insulin | Effect |
|---|---|
| Hyposecretion | Diabetes mellitus: high blood sugar, glucose in urine (glycosuria), frequent urination, thirst, tiredness, weight loss |
| Hypersecretion (or overdose) | Hypoglycaemia: low blood sugar, shaking, sweating, fainting (insulin shock) |
Thyroxine (contains iodine) controls the basal metabolic rate and is needed for normal physical and mental development.
| Thyroxine | Effect |
|---|---|
| Too little in childhood | Cretinism: stunted, disproportionate growth, delayed mental development, puffy face |
| Too little in adults | Myxoedema: low metabolic rate, puffy skin, sluggishness, weight gain |
| Lack of iodine | Simple goitre: enlarged thyroid |
| Too much | Exophthalmic goitre: protruding eyes, high metabolic rate, fast heartbeat, weight loss, nervousness |
Key terms
- Insulin
Hormone of the beta cells that lowers blood sugar.
- Glucagon
Hormone of the alpha cells that raises blood sugar.
- Diabetes mellitus
Disorder due to too little insulin, with high blood sugar and sugar in urine.
- Thyroxine
Iodine-containing thyroid hormone that controls metabolic rate.
3. Adrenal gland: cortical hormones and adrenaline
Adrenal cortex (outer): secretes cortical hormones (corticoids): they regulate salt and water balance (for example aldosterone), help in carbohydrate metabolism (for example cortisol), and include some sex hormones. They are needed for life.
Adrenal medulla (inner): secretes adrenaline, the emergency or "fight or flight" hormone. In danger or anger it:
- increases the heartbeat and blood pressure;
- increases the breathing rate;
- raises blood sugar (liver glycogen → glucose);
- dilates the pupils; makes the hair stand on end;
- sends more blood to the skeletal muscles and less to the gut and skin.
| Region | Hormone | Role |
|---|---|---|
| Cortex | Cortical hormones | Salt, water and sugar balance |
| Medulla | Adrenaline | Emergency response |
Key terms
- Adrenaline
Hormone of the adrenal medulla that prepares the body for an emergency.
- Cortical hormones
Hormones of the adrenal cortex that regulate salts, water and carbohydrate metabolism.
4. Pituitary gland: the master gland
The pituitary is called the master gland because its tropic hormones control other endocrine glands.
| Lobe | Hormone | Action |
|---|---|---|
| Anterior | Growth hormone (GH) | Growth of bones and muscles |
| Anterior | TSH | Stimulates the thyroid to release thyroxine |
| Anterior | ACTH | Stimulates the adrenal cortex |
| Anterior | FSH, LH | Act on the gonads (testes, ovaries) |
| Posterior | ADH (vasopressin) | Increases reabsorption of water in the kidney tubules |
| Posterior | Oxytocin | Contracts the uterus at childbirth; milk release |
Growth hormone: too little in childhood → dwarfism (short but proportionate body, normal intelligence); too much in childhood → gigantism; too much in adults → acromegaly (enlarged jaw, hands and feet).
ADH: too little → diabetes insipidus (large volumes of dilute urine, thirst, no sugar in urine); too much → little, concentrated urine and water retention.
| Hormone | Too little | Too much |
|---|---|---|
| Growth hormone | Dwarfism | Gigantism; acromegaly |
| ADH | Diabetes insipidus | Concentrated urine |
Key terms
- Tropic hormone
A hormone that stimulates another endocrine gland.
- ADH
Antidiuretic hormone; increases water reabsorption in the kidneys.
- Acromegaly
Enlarged jaw, hands and feet due to excess growth hormone in adults.
5. Feedback: TSH and thyroxine
The secretion of thyroxine is controlled by negative feedback:
- Low thyroxine in the blood → anterior pituitary releases more TSH → thyroid releases more thyroxine.
- High thyroxine → pituitary releases less TSH → thyroid releases less thyroxine.
So the level of the hormone itself controls the signal that releases it, keeping it steady.
Simple goitre and feedback: without iodine, thyroxine stays low, TSH stays high, and the overstimulated thyroid enlarges.
Key terms
- Negative feedback
Control in which a rise in a hormone reduces the signal that causes its release.
- TSH
Thyroid-stimulating hormone of the anterior pituitary.
Exam tips
Common mistakes
- Swapping alpha and beta cells (beta → insulin, alpha → glucagon).
- Confusing diabetes mellitus (insulin, sugar in urine) with diabetes insipidus (ADH, no sugar).
- Calling the thyroid the master gland, or placing the adrenal glands below the kidneys.
- Saying a pituitary dwarf is mentally slow (that is a cretin).
- Describing feedback in one direction only.
How marks are usually awarded
- 1 mark: a term, a gland, a hormone, an odd one out with its category.
- 2 marks: two differences point for point, or a hormone and its effect.
- 3 marks: a labelled diagram (gland positions, feedback loop) or a three-step explanation.