Balance is Everything
Homeostasis is a continuously occurring dynamic process that seeks to maintain balance between body processes and a stable internal environment. The Chinese describe it as a balance between Yin and Yang.
A healthy horse at rest may seem as tranquil as a quiet bay on a sunny summer day, but the truth is that even a healthy horse is more like the ocean side, with waves coming in and out, high and low tide. When your horse has a serious injury or illness, it's tsunami or hurricane time!
A (relatively) simple example would be temperature regulation in different weather situations. Under neutral conditions, the blood vessels to the skin are neither overly constricted nor dilated. When very hot, blood flow to the surface blood vessels increases, which also triggers increased sweating and if more cooling is needed breathing increases to allow for heat loss in expired air. When very cold, balance of blood flow to the skin and sweating swing to the other side of baseline by decreasing. When the temperature stress resolves, the blood vessels return to their normal neutral state, sweating abates, breathing is normal.
Maintaining temperature is actually much more complicated, involving the brain, autonomic nervous system and mediators such as dopamine and epinephrine. Even the process of creating sweat is wondrously complicated. When you get into the realms of internal fluid balance, pH regulation, urine production, the immune system, detoxification reactions, etc., the complexity mounts.
Cytokines are small proteins released by cells which they use to communicate with each other. The surface of a cell is covered with receptors that will recognize specific types of cytokines (or hormones, or amino acids, etc.). Over 45,000 cytokines have been identified since they were first described in the 1960s and researchers agree they are just beginning to uncover them all. They are particularly active in the immune system. For every cellular activity influenced by cytokines there will be some that trigger and others which suppress. Balance.
Underneath the healthy horse's tranquil exterior billions of cells are busily doing their jobs. Functions are being turned on and off as needed to keep the basics of body temperature, electrolyte levels, blood volume, and a host of other things stable within a fairly narrow range. When this balance is threatened by something like an infection, activity ramps up significantly until the challenge is successfully eliminated, at which point homeostasis is achieved again.
Many things we do to care for our horses, such as providing shelter, vaccinations, dewormings, help them to maintain homeostasis, but the most important of all is nutrition. They can survive all sorts of nutritional shortfalls, but surviving is not thriving. They need adequate quality protein to build cells, make cytokines, hormones and antibodies. Structurally important minerals of calcium, phosphorus and magnesium also play critical roles in muscle activity, nerve transmission and energy generation. Antioxidant vitamins and minerals protect the cells from damaging free radicals being generated all the time from immune system activity and the burning of fuels. Proper nutrition makes homeostasis possible, and that balance is the definition of health.