Hormonal Coordination in Animals
Senior 6 · Biology
Hormonal Coordination in Animals
Lesson 4 of 4
30 min
According to the World Health Organization, the number of people living with diabetes rose from 200 million in 1990 to 830 million in 2022, and it has been rising fastest in low- and middle-income countries. Diabetes is the commonest of many conditions caused by a hormonal imbalance: a gland that secretes too little or too much of its hormone, or target cells that stop responding. This lesson uses what you know about feedback to explain the main disorders, and ends by comparing hormonal coordination with the nervous coordination of unit 6.
Definition
Hyposecretion: secretion of too little of a hormone. Hypersecretion: secretion of too much.
Diabetes mellitus: a long-lasting disease in which the blood glucose concentration is too high (hyperglycaemia), because the pancreas makes too little insulin or the body's cells do not respond to insulin properly.
Hypoglycaemia: a blood glucose concentration that is too low.
Goitre: an enlargement of the thyroid gland, seen as a swelling at the front of the neck.
| Type 1 diabetes | Type 2 diabetes | |
|---|---|---|
| Cause | The body's own immune system destroys the β cells of the islets, so little or no insulin is made | Body cells become less responsive to insulin (insulin resistance), and later the β cells may secrete less |
| Usual onset | Often in childhood or youth, quite suddenly | Usually in adults, slowly; increasingly seen in young people |
| Risk factors | Inherited tendency; not caused by lifestyle | Excess body weight, physical inactivity, unhealthy diet, family history |
| Treatment | Insulin injections for life, matched to meals and exercise, with regular glucose checks | Healthy diet, physical activity and weight control; tablets; sometimes insulin |
Why the symptoms occur. Without effective insulin, cells take up little glucose and the liver keeps releasing it, so blood glucose stays high. The kidney tubules can reabsorb glucose only up to a limit, because their carrier proteins become saturated, so glucose appears in the urine. Glucose in the filtrate lowers its water potential, so less water is reabsorbed: the person passes large volumes of urine and feels very thirsty. Because cells cannot use glucose, fat and protein are broken down instead, causing tiredness and weight loss. Over years, high blood glucose damages blood vessels and nerves; the WHO lists blindness, kidney failure, heart attacks, stroke and lower-limb amputation among its effects.
Prevention. The WHO states that a healthy diet, regular physical activity, keeping a normal body weight and avoiding tobacco prevent or delay type 2 diabetes. Type 1 cannot be prevented this way, but good control of blood glucose prevents most of its complications.
| Hormone (gland) | Too little | Too much |
|---|---|---|
| Thyroxine (thyroid) | Hypothyroidism: low metabolic rate, tiredness, weight gain, feeling cold; in babies, poor growth and brain development unless treated early. Often with a goitre when caused by lack of iodine | Hyperthyroidism (for example Graves' disease): high metabolic rate, weight loss, fast heartbeat, feeling hot, anxiety; sometimes bulging eyes and a goitre |
| Growth hormone (anterior pituitary) | Pituitary dwarfism in children: short but well-proportioned body | Gigantism if before the bones stop growing; acromegaly in adults: thickened bones of the hands, feet and jaw |
| ADH (posterior pituitary) | Diabetes insipidus: large volumes of dilute urine and great thirst (blood glucose is normal) | Water retained; the blood becomes too dilute |
| Cortisol (adrenal cortex) | Addison's disease: tiredness, weakness, low blood pressure, low blood glucose | Cushing's syndrome: high blood glucose, fat on the face and trunk, thin skin, high blood pressure |
| Insulin (pancreas) | Diabetes mellitus (above) | Hypoglycaemia, for example after too large an insulin dose: shaking, sweating, confusion, and if severe, unconsciousness |
The thyroid needs iodine to make thyroxine. If the diet lacks iodine, little thyroxine is made. With little thyroxine there is little negative feedback, so the hypothalamus and pituitary secrete more TRH and TSH. TSH stimulates the thyroid to grow, and it swells into a goitre while still failing to make enough thyroxine. Iodine is found in seafood and in iodised salt; a WHO guideline (2014) recommends that all food-grade salt, used in homes and in food processing, be fortified with iodine to prevent iodine deficiency disorders.
Worked example 1: three glucose tolerance tests
Three adults fast overnight, drink 75 g of glucose, and have their blood glucose and insulin measured (illustrative data).
| Person | Glucose at 0 min / mmol dm⁻³ | Glucose at 120 min / mmol dm⁻³ | Insulin after the drink |
|---|---|---|---|
| A | 4.9 | 5.2 | Rises, then falls |
| B | 8.6 | 16.8 | Stays very low |
| C | 7.4 | 13.1 | Rises higher than in A, and stays high |
Worked example 2: how much of thyroxine is iodine?
Thyroxine has the formula C₁₅H₁₁I₄NO₄ and a relative molecular mass of 776.9. ( of I = 126.9.) (a) Calculate the percentage by mass of iodine. (b) Use your answer and feedback to explain why a lack of iodine causes a goitre.
| Feature | Nervous system | Endocrine (hormonal) system |
|---|---|---|
| Nature of the signal | Electrical impulses along neurones, with chemical transmitters at synapses | Chemical (hormones) |
| Transmitted by | Neurones | Blood |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Duration of the effect | Short; stops when impulses stop | Often long-lasting (minutes to years) |
| Where it acts | Precisely on particular muscle fibres or gland cells | Widespread: on all target cells with receptors, often in many organs |
| Kinds of response | Muscle contraction, gland secretion | Changes in metabolism, growth, development and reproduction |
| Examples | Reflexes, voluntary movement | Blood glucose control, growth, puberty |
The two systems work together. The hypothalamus is a nervous tissue that controls the pituitary; the adrenal medulla is stimulated by sympathetic nerves; and hormones such as adrenaline change the activity of neurones and of the organs they control.
Warning
Common mistakes
Alila Medical Media: "Diabetes Type 1 and Type 2, Animation". Watch for the difference between a lack of insulin and resistance to insulin.
Practice: hormonal imbalance
Key points
Further reading for this unit
Sources and credits
The tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.
