How your body keeps the balance?

 Body keeps balance because of Homeostasis.

Now what is homeostasis? 
"Homeostasis is the relatively constant state maintained by the body".

 Mainly this means that over time, the internal body will maintain a stable environment. /
So, for example, if the external temperature is cold or hot, the internal body temperature will remain relatively constant at 98.6 degrees Fahrenheit or 37 degrees celsius , the internal body temp may vary a little above or below this number but that will still be considered normal. 

Homeostasis doesn’t just apply to body temperature, but many other conditions as well. The human body needs to maintain a certain range of nutrients in the blood stream, such as water, glucose, sodium, and other elements as well. We get our blood work done from time to time to check and make sure we don’t have high glucose or cholesterol levels, or too low levels of certain vitamins or minerals. This in essence is to check and make sure our body is maintaining homeostatic conditions. Set point is an important term when discussing homeostasis. 
"Set point is a normal range that the body tries to stay in".

For instance, the normal range for systolic blood pressure is between 80 and 120. So homeostasis is sought by the body to keep systolic blood pressure in this range or set point. However, the body can change the set point. In many people weight gain leads to higher systolic blood pressure. This could be due to many factors, but the point is that the body changed the blood pressure set point to keep homeostasis. So now, maybe after the weight gain, someone’s systolic blood pressure may be at 140, which is considered unhealthy and at a risk for some diseases, but the body felt to keep homeostasis it needed to change and pump blood harder. The ability of the body to maintain homeostasis is crucial to life, and each of the systems in the body plays important roles. It is often times easier to understand homeostasis by constructing a model to make the concept clearer. 

Many sources use a fish tank comparison. The water represents body fluid, or the internal environment. The glass that encloses the water or fluid would be the skin creating a barrier separating internal from external. The fish in the tank represent cells, which obviously want to stay alive. But the cells need nutrients from the external environment in order to survive. The cells need oxygen, so we can put an air pump in the tank, which would represent the lungs, pumping oxygen into the tank. A filter will get rid of wastes, which would represent the kidneys. A heater to keep a constant temperature, and the heater would represent the muscles. Food, or at least the nutrients contained in food, so, we hook up a feeder to our fish tank to feed those fish, or cells, and the feeder would represent the digestive system. So, fish tank is complete, and with everything working together, we have relatively stable and constant conditions, which equals homeostasis. Keep in mind that our fish tank model only covers a few of the aspects involved in maintaining homeostasis, as there are actually far more mechanisms involved in the body keeping at a state of homeostasis. And that, be the basics on homeostasis.