Homeostasis and Hormones

Homeostasis is keeping the inside of the body within a narrow range, whatever happens outside. Cells, tissues and organs die if the fluid around them drifts too far (see Libretti and Puckett).

The idea is old. In 1865 Claude Bernard wrote of a milieu intérieur, an internal environment, and in 1926 Walter Cannon named the process homeostasis, from Greek words for “similar” and “standing still” (see Libretti and Puckett). The value the body aims for is its set point.

The Human Body lesson gives a first look at homeostasis. This lesson goes further into the loops and the hormones behind them.

Every homeostatic loop has three working parts:

• a receptor, which detects a change, called the stimulus;
• a control center, which compares the reading with the set point and decides what to do;
• an effector, a muscle or gland that makes the response.

Negative feedback reverses the change, so the variable swings back toward the set point. Many hormones are controlled this way: a gland senses the level of the substance it regulates (see SEER).

Positive feedback pushes the change further, so it builds until some event ends it. In labour, contractions of the womb trigger the release of oxytocin, which causes more contractions, which release more oxytocin. The loop stops once the baby is born (see You and Your Hormones). Blood clotting works in a similar way: thrombin, a key clotting enzyme, speeds up its own production through positive feedback loops (see Zavyalova and others), so a clot forms fast.

A healthy core temperature is 37 ± 0.5 °C. The body's thermostat is in the preoptic area of the hypothalamus. Thermoreceptors in the skin sense surface temperature, and others in the spinal cord, the organs and the hypothalamus itself sense core temperature (see Osilla and others).

When the body is too warm, blood vessels in the skin widen, called vasodilation, and sweat glands release sweat. When it is too cold, the vessels narrow, called vasoconstriction, skeletal muscles shiver, and body hair stands up (see Tansey and Johnson). Behaviour helps too, like taking off a coat.

The pancreas has clusters of cells called the islets of Langerhans. Beta cells there release insulin when blood glucose is high, and alpha cells release glucagon when it is low (see SEER).

Insulin lets muscle, liver and fat cells take glucose up from the blood. As glucose falls back to normal, insulin release slows (see You and Your Hormones). Glucagon works the opposite way. It acts on the liver, which breaks down its glycogen store into glucose and also makes new glucose (see You and Your Hormones).

Diabetes is what happens when this loop fails. In type 1 diabetes, thought to be an autoimmune reaction, the body stops making insulin, and people need insulin every day. In type 2 diabetes, the body does not use insulin well; it develops over many years, and it makes up about 90 to 95% of diagnosed cases (see CDC). In type 2, there is too little insulin for the body's needs and the cells respond to it less, called insulin resistance (see You and Your Hormones). People who take insulin or some diabetes medicines can go dangerously low if they skip a meal and exercise, so they follow their care plan (see CDC and NIDDK).

Doctors use blood tests to diagnose diabetes. The CDC gives these ranges (see CDC):

Keeping the right amount of water in the blood is called osmoregulation. Antidiuretic hormone, ADH, also called vasopressin, is made in the hypothalamus and released from the posterior pituitary gland (see You and Your Hormones).

When the blood gets too salty, osmoreceptors in the hypothalamus detect it and ADH is released. Low blood volume or pressure, sensed by baroreceptors, also triggers it. In the kidney, ADH makes the tubules take water back into the blood, so urine is more concentrated and less water is lost. Alcohol blocks ADH release, which is why it makes people pass more urine (see You and Your Hormones).

Endocrine glands have no ducts. They release hormones straight into the blood, which carries them around the body. Exocrine glands, like sweat glands, use ducts to carry their product to a surface (see SEER). The pancreas is both: one part sends digestive enzymes down a duct, and another part releases hormones (see SEER).

The pituitary is often called the master gland, because many of its hormones control other glands (see You and Your Hormones). A few of the main glands (see You and Your Hormones):

• Pituitary: growth hormone, thyroid-stimulating hormone and others from the front lobe. ADH and oxytocin are released from the back lobe (see SEER).
• Pineal: melatonin, which affects daily cycles (see SEER).
• Thyroid: thyroxine and triiodothyronine, which contain iodine. Parathyroids: parathyroid hormone, the main regulator of blood calcium (see SEER).
• Adrenals: the outer cortex makes cortisol, which raises blood glucose, and aldosterone. The inner medulla makes adrenaline, also called epinephrine, when sympathetic nerves fire in stress (see SEER).
• Ovaries and testes: oestrogens and progesterone, and testosterone (see SEER).

Hormones reach almost every cell, but only target cells with the matching receptor respond, like a key fitting a lock (see SEER). One hormone can cause different responses in different tissues. Adrenaline speeds up the heart, but makes the liver release sugar (see You and Your Hormones).

Protein and peptide hormones, like insulin, glucagon and oxytocin, cannot pass through the cell membrane. They bind receptors on the surface, which start a chain reaction inside the cell (see You and Your Hormones). The response is relatively rapid (see SEER).

Steroid hormones, like cortisol, oestrogen, progesterone and testosterone, are made from cholesterol and pass through the membrane. Their receptors are inside the cell, and some go into the nucleus and change how the cell uses its DNA (see You and Your Hormones). Because this works by making new proteins, it is relatively slow (see SEER).

The two systems share the job of control, and they are linked: the brain controls the pituitary, and many hormones affect nerve cells (see You and Your Hormones).

The idea of a steady inside, and the chemicals that keep it steady, came together over about 60 years.