Biology’s quest is to understand the rules that regulate life on every scale.

These rules are both surprising and profound: surprising because they explain connections among creatures that are not obvious; profound because these rules determine nature’s ability to produce the animals, plants, trees, and clean air and water on which we depend.

Fifty years ago, when the human population was about 3 billion, we were using about 70 percent of the Earth’s annual capacity each year. That broke 100 percent by 1980 and stands at about 150 percent now, meaning that we need one and one-half Earths to regenerate what we use in a year.

Wouldn’t it be terribly ironic if, while we race toward and discover more cures to all sorts of molecular and microscopic threats to human life, we continue to just sail on blissfully or willfully ignorant of the state of our common home and the greater threat from disregarding how life works on the larger scale?

The power of the small number of general rules that I will describe is their ability to reduce complex phenomena to a simpler logic of life.

Before my mind could even form the thought “Mad elephant! Run!” a primitive part of my brain, the amygdala, was signaling danger to my hypothalamus. This almond-sized command center just above the amygdala promptly sent out electrical and chemical signals to key organs. Through nerves, it signaled the adrenal glands that sit on top of my kidneys to release norepinephrine and epinephrine, also known as adrenaline. These hormones then circulated quickly through the bloodstream to many organs including: my heart, causing it to beat faster; my lungs, to open up airways and increase breathing rate; my skeletal muscles, to increase their contraction; my liver, to release stored sugar for a quick supply of energy; and smooth muscle cells throughout my body, causing blood vessels to constrict, skin hairs to stand on end, and blood to shunt away from the skin, intestine, and kidneys. The hypothalamus also sent a chemical signal, corticotropic releasing factor (CRF), to the nearby pituitary gland that triggered it to release a chemical called adrenocorticotropic hormone (ACTH) that traveled to another part of the adrenal gland and triggered the release of another chemical—cortisol, which increased blood pressure and blood flow to my muscles. All these physiological changes are part of what is known as the “fight-or-flight” response. Coined and described a century ago by Harvard physiologist Walter Cannon, these responses are aroused by both fear and rage, and quickly prepare the body for conflict or escape. We opted for escape.

“It has long been common knowledge that violent emotions interfere with the digestive process, but that the gastric motor activities should manifest such extreme sensitiveness to nervous conditions is surprising.”

He observed that emotional distress also ceased digestion in rabbits, dogs, and guinea pigs, and from the medical literature that also seemed to be true of humans. The connection between emotions and digestion suggested some direct role of the nervous system in controlling the digestive organs.

Cannon knew that all the outward signs of emotional stress—the pallor caused by the contraction of blood vessels, “cold” sweat, dry mouth, dilation of pupils, skin hair standing on end—occurred in structures that are supplied by smooth muscle and innervated by the so-called sympathetic nervous system. The sympathetic system comprises a series of neurons that originate from the thoracic-lumbar region of the spinal cord and travel out to clusters of nerve cells (called ganglia). From there, a second set of generally much longer neurons extend to and innervate target organs. Most of the body’s organs and glands receive sympathetic input, including the skin, arteries, and arterioles, the iris of the eyes, the heart, and the digestive organs. These same organs also receive input from nerves originating in the cranial or sacral parts of the spinal cord.

Cannon’s student Philip Bard subsequently demonstrated that the hypothalamus is the critical part of the brain for control of the so-called involuntary (autonomic) functions of the nervous system, including digestion, heart rate, respiration, and the fight-or-flight response. Both this part of the brain and these emergency responses are ancient.

One important clue to shock came from the then-novel approach of measuring soldiers’ blood pressures, not just their pulses. Healthy soldiers had pressures of about 120–140 (mmHg; the abbreviation stands for millimeters of mercury), while shock patients had pressures below 90. It was learned that if this fell to 50–60, the patient did not recover. A low blood pressure meant that vital organs would have difficulty obtaining sufficient fuel and disposing of waste. Early in his time in France, Cannon decided to measure the concentration of bicarbonate ions in the bloodstream of shock patients, a critical component of the blood’s buffering system. He discovered that the patients had lower levels of bicarbonate, which meant that the normally slightly alkaline blood had become more acidic. And he found that the more acidic the blood was, the lower the blood pressure and the more severe the shock were. Cannon proposed a simple possible therapy: administer sodium bicarbonate to shock victims.

One of the recent breakthroughs was in understanding the role of insulin in controlling blood sugar. Cannon noted how when sugar levels rise after a meal, the vagus nerves stimulate the pancreas to secrete insulin, which causes excess sugar to be stored. Conversely, if sugar levels fall, other nerves in the autonomic system trigger the adrenal glands to liberate sugar from the liver. In this way, Cannon said, “the organism automatically restricts the range over which the percentage sugar in the blood may shift.”

the importance of enzyme regulation is not in the gritty details but in the logic.