In the face of grueling stress, it’s easy to romanticize the body’s commitment to balance. A dark and lovely liver serenely regulates its life-sustaining chores like some kind of untrammeled deity. Nearby, plump kidneys churn out requests for water and salt—twin cherubs of the torso. Even Claude Bernard, the 19th-century French physiologist credited with devising the concept of such internal give-and-take, spoke of his theory with beauty and grace, saying “a free and independent existence is possible only because of the stability of the internal milieu.” (Bernard also vivisected the family dog, so the romance only gets you so far.)
We call this balancing act homeostasis. Walter Bradford Cannon coined the term in 1926, but study of it took off in the 1960s, when scientists applied control theory—a branch of engineering concerned with dynamic, changing systems—to human anatomy. They found it to be filled with receptors and sensors that are constantly assessing for changes, like drops in blood oxygenation, fluctuations in sugar, and external threats. These sentinels stay in contact with systems equipped to issue streams of calibrated instructions to the parts of the body that can act to maintain stability. It’s a lot like cruise control: Your car will make the necessary tweaks to keep you at the speed you set it to, regardless of whether you are climbing a hill or careening down one. In this metaphor, the slope is a stand-in for the concept of stress—something that challenges homeostasis and necessitates adjustments.
Excerpted from Popular Science