Chemical coordination

MDCAT Biology · Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Endocrine glands have no ducts. They release hormones straight into the blood, which carries them to every part of the body, but only target cells with the right receptors respond. Compared with nerves, hormonal control is slower, lasts longer and reaches many organs at once.

Most hormones are controlled by negative feedback: when the level of a hormone, or the effect it produces, rises above normal, its secretion is cut down, and when it falls, secretion rises. This keeps conditions steady. The hypothalamus and the pituitary gland sit at the top of this system and control several other glands.

Too much or too little of a hormone gives a recognisable disorder.

Rules to remember

Anterior pituitary: growth hormone, TSH (thyroid), ACTH (adrenal cortex), FSH, LH, prolactin (milk production). Posterior pituitary releases ADH (water reabsorption in the kidney) and oxytocin (labour, milk ejection), both made in the hypothalamus
Thyroid: thyroxine raises metabolic rate and needs iodine; calcitonin lowers blood Ca²⁺. Parathyroid: parathormone raises blood Ca²⁺
Pancreas (islets of Langerhans): β cells, insulin lowers blood glucose (glucose → glycogen); α cells, glucagon raises it (glycogen → glucose)
Adrenal medulla: adrenaline and noradrenaline (emergency). Adrenal cortex: cortisol (raises blood glucose in stress), aldosterone (Na⁺ reabsorption)
Gonads: testosterone (testes); oestrogen and progesterone (ovaries)
Growth hormone: too little in childhood = dwarfism; too much in childhood = gigantism; too much in adults = acromegaly
Thyroxine: too little = cretinism (child), myxoedema (adult); too much = Graves' disease. Insulin lack = diabetes mellitus; ADH lack = diabetes insipidus; cortex hormones low = Addison's, high = Cushing's

Common mistake

Confusing the two kinds of diabetes. Diabetes mellitus is lack of insulin, with glucose in the urine; diabetes insipidus is lack of ADH, with large volumes of dilute urine and no glucose.

Worked example

A patient passes very large volumes of dilute urine and is always thirsty. Tests show normal blood glucose and no glucose in the urine. Which hormone is lacking?

Large volumes of dilute urine mean too little water is reabsorbed in the kidney.
Normal blood glucose and no glucose in the urine rule out lack of insulin (diabetes mellitus).
Water reabsorption in the collecting ducts is controlled by ADH from the posterior pituitary.

Answer: ADH is lacking: the condition is diabetes insipidus.

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