Summary

The body is coordinated not only by the nervous system but also by a set of special chemicals called hormones, which provide chemical coordination, integration and regulation. Hormones oversee the metabolism, growth and development of organs, and they are produced by the endocrine glands or by certain specialised cells. Together these ductless glands make up the endocrine system, which works alongside the neural system to keep the body's activities in step.

The organised endocrine system is made up of the hypothalamus, the pituitary and pineal glands, the thyroid, the parathyroids, the thymus, the adrenal glands, the pancreas and the gonads (the testis and the ovary). The pituitary gland has three parts. Its pars distalis produces six trophic hormones, the pars intermedia secretes a single hormone, and the pars nervosa releases two hormones; together the pituitary hormones govern the growth and development of body tissues and control the activity of the other, peripheral endocrine glands. The pineal gland secretes melatonin, which regulates the twenty-four-hour daily rhythms of the body, such as the cycle of sleep and wakefulness and changes in body temperature.

The thyroid gland's hormones help regulate the basal metabolic rate, the development and maturation of the central neural system, the formation of red blood cells, the metabolism of carbohydrates, proteins and fats, and the menstrual cycle. A further thyroid hormone, thyrocalcitonin, lowers the level of calcium in the blood. The parathyroid glands secrete parathyroid hormone, which raises blood calcium and is central to calcium balance in the body, so the thyroid and parathyroid together fine-tune calcium levels in opposite directions. The thymus secretes thymosins, which drive the differentiation of T-lymphocytes for cell-mediated immunity and also boost antibody production for humoral immunity.

The adrenal gland has two regions: the inner adrenal medulla and the outer adrenal cortex. The medulla secretes epinephrine and norepinephrine, which sharpen alertness, widen the pupils, raise the heartbeat and the force of heart contraction, quicken breathing, cause piloerection and sweating, and break down glycogen, fats and proteins. The cortex secretes glucocorticoids and mineralocorticoids: glucocorticoids promote the making of new glucose, the breakdown of fats and proteins, red-blood-cell formation, and support the cardiovascular system, blood pressure and filtration in the kidney while damping down inflammatory and immune responses, whereas mineralocorticoids manage the water and electrolyte content of the body.

The pancreas, acting as an endocrine gland, secretes glucagon and insulin, which pull blood sugar in opposite directions. Glucagon breaks down glycogen and forms new glucose, raising blood sugar, while insulin promotes the uptake and use of glucose by cells and its storage as glycogen, lowering blood sugar. A shortage of insulin, or the body's resistance to it, leads to diabetes mellitus. The gonads add their own hormones: the testis secretes androgens that develop and maintain the male accessory sex organs and secondary sex characters, drive sperm formation and male sexual behaviour, and support anabolic pathways and red-blood-cell formation; the ovary secretes estrogen, which develops the female accessory sex organs and secondary sex characters, and progesterone, which maintains pregnancy and promotes mammary-gland development and milk production. …