Hormonal Control: Blood Sugar, Adrenaline, and Metabolism

Hormonal control uses chemical messengers to regulate body processes: insulin and glucagon keep blood sugar steady, adrenaline prepares the body for emergencies, and thyroxine sets the metabolic rate. In brief: after a meal, insulin lowers blood glucose; between meals, glucagon raises it. Adrenaline works within seconds and its effects last minutes, while sex hormones act over months and years. Hormonal responses are slower but longer-lasting than nervous responses.

These hormones are good examples of how the body keeps conditions stable and responds to change. This guide explains each hormone’s action, shows how negative feedback keeps glucose and thyroxine in balance, compares nervous and hormonal communication in a table, works through a glucose tolerance example and corrects the typical mistakes.

How do insulin and glucagon control blood sugar?

The concentration of glucose in the blood is kept around 4 to 6 millimoles per liter (mmol/L) when fasting. It must not fall too low, as the brain depends on glucose, or rise too high, which damages blood vessels and other tissues. The pancreas monitors the level directly and responds with two opposing hormones.

When blood glucose is high

1. Beta cells in the islets of Langerhans detect the rise and release insulin.
2. Insulin makes liver and muscle cells take up glucose and convert it to glycogen (glycogenesis), and it increases the use of glucose in respiration. It also helps fat cells to store energy.
3. Blood glucose falls. This reduces the stimulus, and insulin release decreases (negative feedback).

When blood glucose is low

1. Alpha cells in the islets release glucagon.
2. Glucagon acts on liver cells to break down glycogen to glucose (glycogenolysis) and to make glucose from other substances (gluconeogenesis).
3. The liver releases glucose into the blood, and the level rises.

In type 1 diabetes, the immune system destroys the beta cells, so little or no insulin is made, and treatment requires insulin injections or a pump. In type 2 diabetes, body cells become less responsive to insulin, which is linked to overweight, inactivity and genetics; it is managed with diet, exercise, medicine and sometimes insulin.

What does adrenaline do?

Adrenaline (called epinephrine in some countries) is released from the adrenal medulla when the brain detects danger, excitement or stress. The sympathetic nerves stimulate the adrenal glands directly, so the response is quick. Adrenaline binds to receptors on target cells and has the following effects:

• The heart beats faster and with more force, which raises blood pressure.
• Breathing becomes faster and deeper, and airways widen.
• Blood vessels in the skin and gut narrow, while those in the muscles widen, which redirects blood to the muscles.
• The liver converts glycogen to glucose, and blood glucose rises to provide fuel.
• The pupils widen, and the person becomes more alert.

These changes prepare the body for vigorous action: running away or fighting. Adrenaline also acts as an emergency treatment, since injected adrenaline opens the airways and raises blood pressure in a severe allergic reaction (anaphylaxis).

How is metabolism controlled by thyroxine?

Thyroxine increases the rate of cell respiration and the metabolic rate in most tissues, and it is important for growth and brain development in children. Its control is a negative feedback loop involving three levels.

1. The hypothalamus releases thyrotropin-releasing hormone (TRH).
2. TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
3. TSH stimulates the thyroid gland to release thyroxine.
4. When thyroxine in the blood is high, it inhibits the release of TRH and TSH, so production falls. When it is low, inhibition is lifted, and production rises.

Cold conditions raise thyroxine secretion, which increases heat production. Thyroxine requires iodine, so a lack of dietary iodine means less thyroxine, more TSH and growth of the thyroid, causing goiter.

What do sex hormones do?

The sex hormones, testosterone, estrogen and progesterone, control the development of secondary sexual characteristics at puberty, the production of gametes and the menstrual cycle, as described in the articles on the reproductive systems. They are an example of hormones with long-term effects, acting over years to change the body.

How does hormonal control differ from nervous control?

Worked example: a glucose tolerance test

In a glucose tolerance test, a person fasts, drinks a glucose solution, and blood glucose is measured over two hours. Compare two imaginary people.

1. Person A: the fasting level of 4.8 mmol/L is normal. The peak is 7.5 at 30 min, an increase of 7.5 − 4.8 = 2.7. By 120 min, the level has returned to 5.0, within 0.2 of the start. Insulin is working well, and negative feedback has brought the level back.
2. Person B: the fasting level of 8.0 is already high, the peak is 14.5 (a rise of 6.5), and at 120 min it is still 13.0, which is 5.0 above the start. Glucose is not being cleared, which suggests too little insulin or reduced response to it, as in diabetes.

A clinical diagnosis uses defined cutoffs, such as a fasting value of 7.0 mmol/L or more and a two-hour value of 11.1 mmol/L or more. The values in the table are invented for the exercise. The test shows how the feedback loop normally returns the glucose concentration to its set point within about two hours, and what happens when it fails.

How were these hormones discovered?

In 1889 Oskar Minkowski and Joseph von Mering found that removal of the pancreas in dogs caused diabetes. Banting, Best, Macleod and Collip isolated insulin in 1921 and 1922, and Banting and Macleod received the 1923 Nobel Prize in Physiology or Medicine. Adrenaline was isolated in 1901 by Jokichi Takamine, and thyroxine was isolated by Edward Kendall in 1914. Frederick Sanger determined the amino acid sequence of insulin in the 1950s.

Which mistakes are common?

Mistake 1: Mixing up insulin and glucagon

Insulin lowers blood glucose. Glucagon raises it.

Mistake 2: Saying that glucose is stored as glucagon

Glucose is stored as glycogen. Glucagon is a hormone.

Mistake 3: Writing that adrenaline lowers blood glucose

Adrenaline raises it, by releasing glucose from glycogen stores.

Mistake 4: Describing the thyroxine loop as positive feedback

High thyroxine inhibits further release, which is negative feedback.