Making (and breaking) the status quo

Thermostat

Humans love to maintain the status quo. In fact, one of the basic requirements for life is the ability maintain internal conditions (this is called homeostasis). It is therefore not surprising that our bodies are packed full of systems that run like thermostats.

Thermostats regulate themselves by a process called negative feedback. To understand this process, let’s look back at one of the earliest documented thermostats: a poultry egg incubator. If you don’t keep the eggs warm, no chickens. But if you keep them too warm, sunny side up. You need a heat-sensing mechanism to give feedback and tell the system to chill out when the temperature rises above the optimal range. Inventor Cornelius Drebbel had this figured out in the 1600s when he built a state of the art, self-regulating poultry incubator that caught the attention of the village chronicler. When the incubator temperature was below or within range, the apparatus was firmly shut to the outside world, but when the temperature was too high, it caused expansion of a tube of alcohol, creating a gap, letting cooler air come in. The chronicler wrote “he was able, by means of a strange and amusing device, to hatch duck and chicken eggs all year round, yes even in the middle of winter without using ducks or chickens for this!” Clearly, it was a pretty big deal.

“Strange and amusing” negative feedback systems are a pretty big deal inside of living organisms as well. A classic example of a biological thermostat lies in thyroid hormone regulation.

Your pituitary gland secretes thyroid-stimulating hormone (TSH), which, as its name suggests, stimulates your thyroid. Your thyroid secretes hormones that kick your metabolism into gear and sensitize you to the effects of catecholamines (the fight or flight hormones). These are good things, up to a point. If you’ve got too much thyroid hormone (hyperthyroidism), you may experience a host of unpleasant symptoms related to your metabolism being constantly cranked up and your fight or flight mechanism being kept on a hair trigger: nervousness, irritability, difficulty sleeping, weight loss, and constantly feeling hot, to name a few.

Luckily, thyroid hormones feed back to your pituitary and tell it to stop releasing TSH. This is classic negative feedback: pituitary secretes TSH, TSH tells thyroid to secrete hormones, thyroid hormones ultimately tell pituitary to stop secreting TSH. When thyroid hormones are low again, this feedback inhibition is lifted, and the pituitary resumes secretion of TSH.

It’s a self-regulating cycle!


Biological systems use thermostat-like mechanisms time and again. They let us maintain homeostasis and keep us alive. But sometimes, we need to break the status quo. If you have, say, an eight-pound baby ready to be pushed out of your uterus, negative feedback is not your friend. Luckily, we have something called (surprise, surprise!) positive feedback.

We’ll get to childbirth in a moment, but first, let’s examine a familiar (though non-biological) example of positive feedback: the Internet meme. A meme gets posted, and people see it. Having seen it, more people post it, letting even more people see it, and thus it grows. It grows until what was started by one person has amused or annoyed an alarmingly enormous slice of the Internet population. The meme self-perpetuates.

The very same principle makes birth possible. The baby presses down against the cervix, causing it to stretch. Nerves sense the stretching and signal for the brain (specifically, the hypothalamus) to cue the release of the hormone oxytocin from the pituitary gland. Oxytocin causes the uterus to contract, pushing the baby down more and causing the cervix to stretch further. More stretching means more oxytocin, means more uterine contractions, means more stretching, and so on all the way until the baby is born. Once the baby has been delivered, the cervix is no longer being stretched, so it stops the signal for the release of oxytocin.

Birth: more useful than memes, but governed by the same principle.


Do note that last sentence: once the baby has been delivered, the cervix is no longer being stretched, so it stops the signal for the release of oxytocin. Unlike negative feedback loops, whose actions are constant and indefinite, positive feedback loops must have some way of breaking the cycle. Control of blood sugar is governed by negative feedback. We always want to keep it under control. Activation of the digestive enzyme, pepsin, however, is under positive feedback control. When we digest food, we need a lot of pepsin, and when we’re not digesting, well, best not to digest our own stomachs. Maintenance of blood pressure? Negative feedback. Erection? Positive feedback.

Yes, now you’re listening.

If your body’s thermostats get out of whack, a good doctor can serve as a handyman. He works with your internal control apparatuses, replacing broken pieces by prescribing medications to set things back in balance. But considering the sheer number of feedback loops constantly operating in our bodies, it’s quite amazing how well they do on their own.




Rachel Corbin is a medical student at Washington University in St. Louis. She studied biology at Yale and, upon graduation, relocated to rural China for two years to teach in a high school while studying Chinese. She enjoys dancing, learning languages, and talking to strangers.


Cover image: Pointing at Progress, by starmanseries

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