Life, as we know it, requires some degree of stability—both internally and externally. Consider a bacterium like E. coli that needs to maintain its internal copper concentrations within a narrow range: too much copper would kill the cell, while too little would impede important metabolic functions that rely on copper atoms as catalytic centers of enzymes. Keeping copper concentrations within the required range—copper homeostasis—is achieved through a negative feedback loop: the bacterium possesses internal sensors that react to sub-optimal copper levels by changing the production rate of proteins that pump copper out of the cell. This feedback system has its limits, though, and most bacteria succumb to too much copper in their environment—storing water in copper containers is an age-old strategy to keep it fresh.
Human cells not only need elaborate systems to achieve internal homeostasis of many types of molecules. In addition, they also require a precisely tuned environment. Our cells can count on a working temperature of close to 37°C, measured by thermometers in the brain and maintained through behavior (e.g., wardrobe adjustments) as well as through the regulation of blood flow to the limbs and through sweating. Our cells can also rely on a constant supply of nutrients through the blood stream, including glucose, measured in the pancreas and regulated through insulin secretion, and oxygen, measured in the major blood vessels and the kidneys and maintained through adjustments of the activity of breathing muscles and the production of red blood cells. Again, homeostasis is achieved through negative feedback loops: in response to measured deviations from a desired level, our body initiates responses that move us back toward the target value.
That homeostasis plays a major role in human health was already recognized by the ancient Greek philosophers. Hippocrates believed that health represented a harmonious balance of the “elements” that made up the human body, while disease was a state of systematic imbalance. For many important diseases, this view indeed provides an accurate description. In type 1 diabetes, for example, the pancreas cells responsible for the blood glucose level measurements are destroyed, and the homeostasis system breaks down. Chronic diseases, on the other hand, are often initially compensated by homeostatic systems; e.g., anemia caused by an accelerated breakdown of red blood cells can be compensated through an increased production of these cells, as long as the body possesses enough raw materials for this enterprise.
Complex systems can hardly be stable without at least some level of homeostasis. The earth’s biosphere is a prime example. Surface temperatures and atmospheric CO2 levels are both affected by biological activities. Higher atmospheric CO2 partial pressure leads to increased plant growth, causing an increased consumption of CO2 and thus maintaining homeostasis. Higher temperatures cause increased phytoplankton growth in the oceans that produces airborne gases and organic matter seeding cloud droplets; more and denser clouds, in turn, lead to an increased reflection of sunlight back into space and thus contribute to temperature homeostasis. These systems also have their limits, though; like an E. coli bacterium with too much copper, our planet’s homeostasis systems on their own may be unable to overcome the current onslaught of human activities on global temperatures and CO2 levels.