Messengers: hormones
Glands drop chemical messages into your blood. Watch adrenaline, insulin and thyroxine ride out to the whole body.
Meet the glands
Nerves send fast messages down wires. Your body has a second, slower post service: glands. A gland is a small lump of tissue that makes a chemical message, called a hormone, and drops it into your blood. The blood carries it everywhere, and only the parts with the right lock respond to that key. Meet the main ones: a bean-sized boss deep in the brain (the hypothalamus), a pea hanging under it (the pituitary), a butterfly in your neck (the thyroid), two little hats that sit on top of your kidneys (the adrenal glands), and a long soft gland behind your stomach (the pancreas).
The endocrine system is the set of ductless glands that secrete hormones straight into the blood. A hormone acts only on cells that carry a matching receptor, which is how one chemical in the whole bloodstream can have a precise effect. Shown here: the hypothalamus (a brain region of about 4 g that links the nervous and endocrine systems), the pituitary (about 0.5 g, in a bony cup in the base of the skull; its front lobe, the adenohypophysis, and its back lobe, the neurohypophysis, are two different tissues), the thyroid (20 to 30 g, wrapped round the front of the windpipe), the adrenal glands (about 4 g each; their outer cortex makes cortisol and aldosterone, their inner medulla makes adrenaline) and the pancreas, which is mostly a digestive gland but holds about a million tiny islets that make insulin and glucagon. Not shown: the four rice-grain parathyroids behind the thyroid, the pineal gland, the ovaries and testes, and the kidneys the adrenals sit on, which are not in this model.
Scared!
A dog barks right behind you. In a split second your adrenal glands fire out adrenaline (orange). It pours into the big vein beside them and rides up into the heart. Your heart jumps from about 70 beats a minute to 120, pumping harder and faster. Then the blood goes to the lungs, and adrenaline opens the airways wide so you can gulp more air. Your pupils widen, your skin goes pale, and sugar is dumped into your blood for fuel. Your body is ready to run or to fight. It takes a few minutes to calm down again.
This is the fight-or-flight response. A fright makes the sympathetic nerves fire straight into the adrenal medulla, which releases adrenaline (epinephrine) and some noradrenaline into the blood within seconds. The right adrenal vein is about 1 cm long and empties straight into the inferior vena cava; the left one drains into the left renal vein first, as the dots do here. In the heart, adrenaline raises the rate and the force of each beat (a resting 60 to 100 beats a minute can jump past 120), and in the lungs it relaxes the muscle around the airways, which is why adrenaline injections are used for severe allergic reactions and adrenaline-like drugs for asthma. It also narrows the blood vessels of the skin and gut, widens those of the muscles, dilates the pupils and makes the liver release glucose. The effect fades in minutes as the hormone is broken down; the slower stress hormone cortisol, from the adrenal cortex, follows over the next hour.
After a meal
You eat a sandwich. The bread turns into sugar, and the sugar floods into your blood. The pancreas tastes the sugar and sends out insulin (green). Insulin first passes through the liver, then rides through the heart out to the whole body. It is the key that unlocks your cells so the sugar can go in and be burned for energy, or put away for later. As the sugar in the blood drops, the pancreas sends less insulin. If the pancreas cannot make insulin, or the cells stop listening to it, the sugar stays stuck in the blood: that is diabetes.
Beta cells in the islets of Langerhans sense the rise in blood glucose after a meal and release insulin. The pancreatic veins drain into the splenic and superior mesenteric veins and then the hepatic portal vein, so insulin reaches the liver first (which removes about half of it) before the rest passes through the hepatic veins, the vena cava, the heart and the lungs to the systemic arteries. Insulin makes muscle and fat cells move glucose transporters to their surface and take up glucose, and tells the liver to store glucose as glycogen and stop making new glucose; its partner glucagon, from the alpha cells of the same islets, does the opposite when blood sugar falls. Together they hold fasting blood glucose near 4 to 6 mmol/L. In type 1 diabetes the immune system destroys the beta cells and insulin must be injected; in type 2 the cells respond poorly to insulin (insulin resistance) and the beta cells cannot keep up.
The thermostat
The thyroid is a butterfly-shaped gland wrapped round the front of your windpipe. It makes thyroxine (blue), a hormone that sets how fast every cell in your body works, like the dial on a heater. Thyroxine leaves through the neck veins, drops into the heart, and goes out to every part of you. Dial up, and you feel hot, hungry and jittery, your heart races and you lose weight. Dial down, and you feel cold, tired and slow. The thyroid needs iodine to make it, which is why iodine is added to table salt.
The thyroid makes thyroxine (T4) and the more active triiodothyronine (T3), both built round iodine, and stores them in its follicles. They raise the basal metabolic rate of almost every tissue: heat production, heart rate, protein turnover, and growth and brain development in children. The gland is told how much to make by thyroid-stimulating hormone (TSH) from the front lobe of the pituitary, itself driven by TRH from the hypothalamus; high thyroid hormone levels switch TSH off again (negative feedback), so the loop behaves like a thermostat. Too much (hyperthyroidism, e.g. Graves’ disease) gives heat intolerance, weight loss and a fast heart; too little (hypothyroidism) gives tiredness, cold intolerance and weight gain, and untreated in infancy it stunts growth and brain development, which is why newborns are screened. Thyroid hormone acts slowly: the effect of a dose builds over days. The veins shown, superior thyroid to internal jugular, are one of three pairs that drain the gland.
The boss
Who tells the glands what to do? A bean-sized piece of the brain called the hypothalamus. It checks your temperature, your hunger, your thirst and your stress, and it sends short messages down a stalk to the pituitary, the pea hanging beneath it. The pituitary then sends its own messages (pink) into the blood: one tells the thyroid to make more thyroxine, another tells the adrenal glands to make cortisol, a slow stress hormone, others make you grow. When there is enough hormone about, the hypothalamus notices and stops asking. Two things skip the boss: adrenaline, which the nerves fire straight out of the adrenal glands, and insulin, because the pancreas tastes the sugar in your blood and decides for itself.
The hypothalamus is where the nervous system meets the endocrine system. Its neurons release hormones into a private set of tiny portal veins that run down the pituitary stalk to the front lobe (adenohypophysis), telling it to release TSH (to the thyroid), ACTH (to the adrenal cortex for cortisol), growth hormone, prolactin and the gonadotropins FSH and LH. Other hypothalamic neurons run right down into the back lobe (neurohypophysis) and release vasopressin, which makes the kidneys keep water, and oxytocin. The hypothalamus can also hold a pituitary hormone back (prolactin is kept in check by dopamine). Many of these chains, including the thyroid and cortisol ones, are negative feedback loops: the final hormone switches off the hypothalamus and pituitary, which keeps levels steady; oxytocin in labour is the famous exception, where more begets more. Pituitary hormones leave through the cavernous sinus into the jugular veins, as the dots do here, and reach their target glands with the arterial blood. Insulin release is mostly controlled directly by blood glucose, though nerves and gut hormones tune it; adrenaline release from the adrenal medulla is driven by nerves, not by the pituitary.