Unit 7 Hormonal coordination in animals
Key unit competence: Analyse key endocrine glands and their respective functions in the human body.
Key ideas
- The endocrine system consists of ductless glands that secrete hormones directly into the blood. A hormone is a chemical messenger made in one place and carried to target cells with specific receptors, where it changes their activity. Hormones are proteins/peptides (insulin, ADH, glucagon), amines (adrenaline, thyroxine) or steroids (oestrogen, testosterone, cortisol). Exocrine glands (salivary, sweat) secrete through ducts.
- Mechanisms of action: protein and peptide hormones cannot cross the membrane; they bind to surface receptors and activate a second messenger (e.g. cyclic AMP) that triggers an enzyme cascade (e.g. adrenaline and glucagon in liver cells). Steroid hormones are lipid-soluble, cross the membrane, bind receptors in the cytoplasm or nucleus and act as transcription factors, switching genes on or off.
- Pituitary gland ("master gland") at the base of the brain, controlled by the hypothalamus. Anterior pituitary (controlled by releasing hormones carried in portal blood vessels): growth hormone (GH), TSH (thyroid-stimulating), ACTH (adrenocorticotrophic), FSH and LH (gonadotrophins), prolactin (milk production). Posterior pituitary stores and releases hormones made in the hypothalamus (neurosecretion): ADH (water reabsorption) and oxytocin (uterine contraction, milk ejection).
- Thyroid gland (neck): thyroxine (T₄) and T₃, made using iodine, raise basal metabolic rate, promote growth and brain development. Calcitonin lowers blood calcium. Parathyroid glands (four, behind the thyroid): parathormone (PTH) raises blood Ca²⁺ by releasing it from bones, increasing reabsorption in kidneys and absorption in the gut (with vitamin D).
- Pancreas (islets of Langerhans): insulin (β cells) lowers and glucagon (α cells) raises blood glucose. Adrenal glands (on the kidneys): medulla secretes adrenaline and noradrenaline (fight or flight); cortex secretes cortisol (glucocorticoid — stress, raises glucose, anti-inflammatory) and aldosterone (mineralocorticoid — Na⁺ reabsorption and K⁺ excretion in kidney). Gonads: testes — testosterone (sperm production, male secondary sexual characteristics); ovaries — oestrogen and progesterone (menstrual cycle, pregnancy, female characteristics). Other: pineal (melatonin), thymus (thymosin, T-cell maturation), stomach (gastrin), duodenum (secretin, CCK), kidney (erythropoietin), placenta (hCG, progesterone).
- Negative feedback keeps hormone levels within limits: e.g. thyroxine: hypothalamus releases TRH → anterior pituitary releases TSH → thyroid releases thyroxine; high thyroxine inhibits TRH and TSH. Blood glucose and ADH are also controlled by negative feedback. Positive feedback: oxytocin in childbirth (contractions → more oxytocin), LH surge before ovulation.
- Hormonal balance is necessary because hormones work together (e.g. insulin and glucagon are antagonistic) and small changes have large effects. Imbalances: GH excess in childhood → gigantism; in adults → acromegaly (enlarged hands, feet, jaw); deficiency in childhood → pituitary dwarfism (normal proportions). Thyroid: hypothyroidism → cretinism in children (stunted growth, intellectual disability), myxoedema in adults (low BMR, weight gain, tiredness); goitre from iodine deficiency (TSH stimulates enlargement; prevented by iodised salt); hyperthyroidism (Graves' disease: high BMR, weight loss, bulging eyes).
- Parathyroid: hypoparathyroidism → low Ca²⁺ → tetany (muscle spasms); hyperparathyroidism → bones weaken, kidney stones. Adrenal: Cushing's syndrome (excess cortisol: obesity of trunk, round face, high glucose, thin skin); Addison's disease (deficiency of cortisol and aldosterone: weakness, low blood pressure, skin darkening). Pancreas: diabetes mellitus. ADH deficiency: diabetes insipidus (large volumes of dilute urine).
- Nervous vs hormonal coordination: nervous — electrical impulses (and chemical at synapses), along neurones, very fast, short-lived, localised (precise) responses. Hormonal — chemical messengers, carried in blood, slower, long-lasting, widespread effects on many targets (growth, metabolism, reproduction). The two are linked by the hypothalamus–pituitary axis and the adrenal medulla.
Common misconceptions
- "Hormones act on every cell they reach." No: only target cells with the right receptors respond.
- "The pituitary controls itself." No: the anterior pituitary is controlled by releasing hormones from the hypothalamus and by negative feedback.
- "Goitre is caused by too much thyroxine." No: the common goitre is caused by iodine deficiency; low thyroxine leads to high TSH, which enlarges the gland.
- "Adrenaline is produced by the kidneys." No: it is made by the adrenal medulla, which sits on top of the kidneys.
Worked examples
Example 1. A child is very tall for her age and still growing quickly. A scan shows a pituitary tumour. Explain.
Solution
The tumour secretes too much growth hormone before the growth plates (epiphyses) of long bones have closed. GH stimulates protein synthesis and bone growth, causing gigantism. Treatment: surgery or drugs that block GH.
Example 2. Explain why iodine deficiency causes the thyroid gland to swell.
Solution
Without iodine, the thyroid cannot make enough thyroxine. Low thyroxine removes the negative feedback on the hypothalamus and pituitary, so TSH stays high. TSH continually stimulates the thyroid to grow, forming a goitre. Iodised salt prevents it.
Practice questions
Define hormone and distinguish endocrine from exocrine glands.
Show answer
Hormone: chemical messenger secreted into blood, acting on target cells. Endocrine: ductless, secretes into blood. Exocrine: secretes through ducts, e.g. salivary glands.
Common mistake: Calling the pancreas only exocrine.
Explain how peptide hormones act on target cells.
Show answer
Bind specific receptors on cell surface membrane; activate enzyme (adenylyl cyclase); second messenger cAMP formed; activates enzyme cascade; response such as glycogen breakdown.
Common mistake: Saying peptide hormones enter the nucleus.
Name four hormones from the anterior pituitary and their targets.
Show answer
GH – bones and muscles; TSH – thyroid; ACTH – adrenal cortex; FSH and LH – gonads; prolactin – mammary glands.
Common mistake: Listing ADH as anterior pituitary.
Explain the negative feedback control of thyroxine.
Show answer
Hypothalamus releases TRH; anterior pituitary releases TSH; thyroid secretes thyroxine; high thyroxine inhibits TRH and TSH; low thyroxine removes inhibition.
Common mistake: Describing it as positive feedback.
Describe the functions of the adrenal cortex and medulla hormones.
Show answer
Medulla: adrenaline/noradrenaline, fight or flight, raise heart rate and glucose. Cortex: cortisol for stress and glucose; aldosterone for sodium and water balance.
Common mistake: Swapping cortex and medulla.
Describe three disorders of the thyroid gland.
Show answer
Goitre: enlarged thyroid from iodine deficiency; cretinism: low thyroxine in childhood; myxoedema: low thyroxine in adults; Graves' disease: excess thyroxine, high BMR, weight loss.
Common mistake: Confusing cretinism with dwarfism.
Explain why hormonal balance is important.
Show answer
Hormones work together and antagonistically; small changes have large effects; imbalance disturbs growth, metabolism, reproduction and homeostasis, causing disease.
Common mistake: Answering only "for health".
Compare nervous and hormonal coordination.
Show answer
Nervous: electrical, neurones, fast, short-lived, localised. Hormonal: chemical, blood, slow, long-lasting, widespread.
Common mistake: Saying both use the same pathway.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. ADH and oxytocin are released from the: a. thyroid gland 2. TSH stimulates the: b. pituitary gland 3. The "master gland" is the: c. posterior pituitary 4. Excess growth hormone in adults causes: d. acromegaly Show answer
1 → c (posterior pituitary) · 2 → a (thyroid gland) · 3 → b (pituitary gland) · 4 → d (acromegaly)
- Made in hypothalamus.
- To release thyroxine.
- Controls other glands.
- Bones thicken.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
calcium · gigantism · entering cells and affecting gene expression · cretinism- Steroid hormones act by .
- Parathormone raises blood levels of .
- Low thyroxine in children causes .
Show answer
1: entering cells and affecting gene expression · 2: calcium · 3: cretinism
- Lipid-soluble.
- From bones.
- Stunted growth.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Insulin and glucagon are described as: a. antagonistic hormones 2. Low blood calcium due to hypoparathyroidism causes: b. iodine 3. Milk production is stimulated by: c. tetany 4. Thyroxine requires the element: d. prolactin Show answer
1 → a (antagonistic hormones) · 2 → c (tetany) · 3 → d (prolactin) · 4 → b (iodine)
- Opposite effects.
- Muscle spasms.
- Anterior pituitary.
- Iodised salt.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
directly into the blood · into the gut only · target cells · testosterone- Cells that respond to a hormone are .
- Endocrine glands secrete hormones .
- The hormone made by the testes is .
Show answer
1: target cells · 2: directly into the blood · 3: testosterone
- They have receptors.
- They are ductless.
- Male characteristics.