2. How hormones work

What is a hormone? ‘How hormones work’ defines a hormone as a chemical signal which enables an event in one part of the body to have an effect somewhere else. Hormones make up one of the two great physiological control systems—the other being the nervous system—which keep the functions of the body working together. There are strong, multifunctional connections between the nervous and endocrine systems. As well as only operating inside the body, hormones are affected by internal stimuli and by external events detected by the senses. The endocrine system comprises several interlinked sub-systems, including five main axes centred on the hypothalamic and anterior pituitary gland. The processes of hormone transport and action through these systems are described.

9. Changing perspectives

It is now more realistic to see hormones as a collection of biologically active chemicals, each of which just happens to cause one cell to change its behaviour in response to another, rather than individually identifiable chemical signals, secreted in one place in the body and acting in another. The endocrine system and hormones have evolved to become extremely complex. ‘Changing perspectives’ shows that there are abundant examples of how contemporary understanding of hormones has been exploited to detect, prevent, treat, and cure disease. Hormones have also been used to improve human and animal performance, and, in the case of sport, sometimes abused. Future hormone research and potential future applications may challenge our ethical views.

7. The thyroid gland

‘The thyroid gland’ illustrates how the thyroid takes up iodine from the blood and stores it in a concentrated form attached to a protein, thyroglobulin. When stimulated by a pituitary gland hormone, thyroid cells build up thyroglobulin but also turn some of it into the hormones thyroxine and triiodothyronine. These hormones speed up a cell’s use of oxygen and so accelerate metabolism in most tissues and organs. This increased metabolism extends to processes like immunity, growth, and reproduction, which is why the right level of thyroid activity is important for development and health. Thyroid diseases such as hyperthyroidism and goitre, are relatively common medical conditions, but are also easily detected and treated.

5. The hard stuff: calcium, cells, bones, and cancer

‘The hard stuff: calcium, cells, bones, and cancer’ considers calcium and the hormone systems that regulate it. These hormones keep the correct balance between the highly reactive Ca2+ ions, which are essential for many life processes, and the precipitating salts like carbonates and phosphates. Controlled precipitation builds bones that defy the pull of gravity and hard materials that protect against predators. Bone formation and recycling take energy, as well as calcium and phosphorus, and all aspects of skeletal development are affected by the hormones of growth and metabolism. There are connections between bone development and appetite which work through serotonin, leptin, insulin, and hormones produced by the gut, in addition to calcitonin.

3. The mysteries of reproduction

‘The mysteries of reproduction’ begins with Pergonal (human menopausal gonadotrophin), a hormone preparation extracted from urine which contains pituitary hormones crucial to the reproductive cycle. Prolactin, another anterior pituitary hormone, affects fertility in women, but is also found in many different kinds of animal and has more than 300 different actions. Prolactin has the general effect of controlling the movement of salts and water across membranes, especially in tissues related to the skin. The important roles of hormones—progesterone, chorionic gonadotrophin, prostaglandin, oxytocin, oestrogen, and testosterone—in the reproductive cycle, human and gender development, the menopause, and the ageing process are also described.

6. Appetite, fat, and obesity

‘Appetite, fat, and obesity’ considers the role of insulin in the body’s fat storage process. In the condition diabetes mellitus, the sugar level in the blood rises uncontrollably so the kidneys cannot stop it leaking into the urine. Type 1 diabetes is an autoimmune disease in which a person’s immune system attacks the insulin-secreting β-cells of the pancreas. The only effective treatment is to inject the missing insulin. Type 2 diabetes is considered more of a lifestyle disease. But what exactly is the connection between obesity and poorly regulated blood glucose? The body has more hormones capable of responding to energy shortage than to energy abundance, which may be why keeping control of body weight can be difficult. The hormones affecting appetite are also discussed.

8. Light and dark

Many of the body’s rhythms reflect the circadian cycle, but how do they interact? ‘Light and dark’ describes a network of systems involving the senses, parts of the brain, the activation and inactivation of genes, biochemical reactions, and cellular responses, all coordinated by complex hormonal cycles. Melatonin, a hormone from the pineal gland, tracks darkness: concentrations are high at night and low during the day. Melatonin cycles affect reproduction and many other physiological changes in animals and it can also slow the division of cancer cells. The circadian connection to the important hypothalamic–pituitary–adrenal axis and the hormone cortisol is also explained.

4. Water, salt, and blood pressure

‘Water, salt, and blood pressure’ describes how the balance of water in the body is controlled by several hormones, including vasopressin (arginine vasopressin (AVP)). AVP reduces urine production by the kidneys and also causes small blood vessels to contract, raising blood pressure. Blood volume and pressure are also adjusted by changing the amount of sodium ions reclaimed by kidney nephrons. The renin–Ang-II–aldosterone hormone system balances blood volume and circulatory space to keep pressure stable when the volume and dilution of the blood change. But what happens if the concentration of salt (sodium and other ions) in the blood starts to rise? Is there a direct way to get rid of excess salt? A hormone secreted by the heart, called ANP, does exactly this.

1. Hormones, history, and the shoulders of giants

‘Hormones, history, and the shoulders of giants’ begins with accounts of extreme human growth: William Rice, a 7 feet 4 inches tall ‘giant’ who died in 1773, and the Efe people of Northeastern Democratic Republic of Congo who are the shortest of any human population. These accounts illustrate how an understanding of hormones permeates our view of how the body works. Hormones are chemicals that coordinate bodily functions. They are secreted in one location and act in another. They move around in the blood and other fluids, and their effects can last for a few seconds or several weeks. Hormone science began with the discovery of secretin by Ernest Starling and William Bayliss in 1902 and has since advanced rapidly.