The Endocrine System

Your endocrine system is a network of glands that produce hormones: chemical messengers that travel around in your blood and tell different parts of your body what to do. While your nervous system sends fast electrical signals, the endocrine system sends slower chemical signals that can affect many parts of the body at once and can last for hours or days. Hormones control your growth, sleep, mood, blood sugar, body temperature, response to stress and many other things. You have around 50 known hormones in your body at any time, all working together.

• Main glandsapprox. 10Pituitary, thyroid, adrenals, pancreas, others
• Hormones knownapprox. 50+And more being discovered
• Master glandPituitaryTiny gland at the base of the brain
• Stress hormoneAdrenaline (epinephrine)Triggers "fight or flight"
• Sleep hormoneMelatoninMade in the pineal gland
• Speed of actionMinutes to hoursSlower than nerves but longer-lasting

What is a hormone?

A hormone is a chemical messenger. It is produced by a particular gland, then released into the bloodstream, where it travels around the body. Only cells with the right receptor for that hormone will respond to it: the hormone fits into the receptor like a key into a lock, and triggers a specific change inside the cell. Different hormones affect different cells, even though they all travel in the same blood.

The main endocrine glands

• Pituitary gland: a tiny pea-sized gland at the base of the brain. Often called the "master gland" because it controls many of the other glands.
• Hypothalamus: a part of the brain just above the pituitary. Links the nervous system to the endocrine system.
• Thyroid gland: in the front of your neck. Controls your metabolism (the rate at which you burn energy).
• Parathyroid glands: four tiny glands behind the thyroid. Control calcium levels in your blood.
• Adrenal glands: one sits on top of each kidney. Produce adrenaline (for stress) and cortisol (which regulates many longer-term responses).
• Pancreas: produces insulin and glucagon, which control your blood sugar.
• Pineal gland: a small gland in the brain. Produces melatonin, which controls sleep.
• Reproductive organs: the ovaries (in females) produce oestrogen and progesterone; the testes (in males) produce testosterone.
• Thymus: in the upper chest. Active in childhood; trains immune cells.

Famous hormones and what they do

• Adrenaline (epinephrine): the "fight or flight" hormone, released when you are scared or excited. Speeds up your heart, dilates your pupils, sends blood to your muscles.
• Insulin: lowers your blood sugar by telling cells to take in glucose. Without it (as in type 1 diabetes), blood sugar climbs to dangerous levels.
• Glucagon: opposite of insulin, raises blood sugar when it is too low by telling the liver to release stored glucose.
• Growth hormone: tells your body to grow during childhood and helps heal injuries throughout life.
• Thyroid hormones: speed up or slow down your metabolism. Too little makes you sluggish and cold; too much makes you nervous and over-heated.
• Cortisol: a longer-acting stress hormone. Helpful in short bursts; harmful if elevated for long periods.
• Melatonin: makes you sleepy in the evening. Levels rise after dark and fall in the morning.
• Sex hormones: testosterone, oestrogen and progesterone control puberty, reproduction and many other features of male and female bodies.

The pituitary: master controller

The pituitary gland is the size of a pea and sits at the base of your brain. Despite its small size, it controls a huge proportion of the endocrine system. The pituitary produces hormones that tell other glands (thyroid, adrenals, reproductive organs) when to release their own hormones. It is itself controlled by the hypothalamus, a region of the brain just above it. Together these two form a kind of master command centre for the whole body.

Fact The "fight or flight" response is one of the fastest, most dramatic responses your body can produce. When you are suddenly frightened, your adrenal glands flood your blood with adrenaline within seconds. Your heart rate jumps from around 70 to over 150 beats per minute. Your pupils dilate to let in more light. Blood is redirected away from your gut and skin into your muscles. Your liver dumps stored sugar into your blood. Your senses sharpen. The whole transformation happens in 10 to 30 seconds and prepares your body to either fight whatever is threatening you or to run away from it.

Hormones and growing up

The endocrine system is what causes puberty: the years (usually between 9 and 16) when a child's body transforms into an adult body. The trigger is a signal from the hypothalamus to the pituitary, which then sends hormones to the gonads (testes or ovaries), which respond by producing huge amounts of sex hormones (mainly testosterone in males, oestrogen and progesterone in females).

These sex hormones produce all the changes of puberty: growth spurts, changes in body shape, deeper voices, body hair, sexual maturity. The same hormones continue to play important roles throughout adult life and gradually decrease as people get older.

Did you know? Hormones can affect your mood. Cortisol (chronic stress hormone) can make you feel anxious or down. Serotonin (often called the "happiness chemical") affects feelings of well-being. Oxytocin (sometimes called the "love hormone") helps form social bonds; it surges when mothers hold their babies and when friends hug. Many psychiatric medications work by changing hormone or hormone-like chemical levels in the brain.

Deeper dive: how scientists discovered hormones

The idea that the body has chemical messengers carried in the blood was not obvious. For most of medical history, doctors thought all signals in the body had to travel through nerves. The word "hormone" (from the Greek for "I urge on") was only coined in 1905, by British scientists Ernest Starling and William Bayliss, after a series of clever experiments.

Starling and Bayliss were studying digestion in dogs. They noticed that when food entered the small intestine, the pancreas started releasing digestive juices, even when all the nerves between the intestine and the pancreas had been cut. There had to be some other signal. They extracted a substance from the intestinal lining, injected it into the blood of another dog, and watched its pancreas start producing juice. They named the substance secretin: the first known hormone. They proposed that the body must use many such "chemical messengers", carried in the blood from one organ to another.

The next big breakthrough came in 1921 when Canadians Frederick Banting and Charles Best discovered insulin. Patients with type 1 diabetes had been dying within months of diagnosis for as long as records existed; there was nothing doctors could do. Banting and Best found that an extract from the pancreas could keep diabetic dogs alive, and in 1922 they tried it on a 14-year-old diabetic boy named Leonard Thompson, who was near death. He recovered within days. Banting and his collaborator John Macleod won the Nobel Prize in 1923, and millions of diabetics have been kept alive by insulin ever since.

The 20th century saw a steady stream of new hormones discovered: thyroid hormones, adrenaline, growth hormone, cortisol, sex hormones, and many more. Today the field of endocrinology is one of the major branches of medicine, and many drugs work by changing hormone levels or how the body responds to them.

For the brain that controls many endocrine processes, see the nervous system. For the system that delivers hormones, see the circulatory system.