Homeostasis is a condition that the body strives to maintain all the time. The word Homeostasis comes from the Greek words; homoios which means "similar" and the word stasis with the meaning "stopping" or "standing" and it refers to a stable and steady state within its surroundings in a biological system.
Several mechanisms and processes regulate the internal environment in order to maintain homeostasis. It is mainly animals (when mentioning ‘animals’ that also refers to humans) that have the ability to actively influence their situation. This is done in various ways, often evolutionarily adapted to the environment the animal lives in. Many animals can instinctively flee from an unfavorable environment, such as high or low heat, salinity or oxygen content. Inside the animal there is an active system with three main components: a receptor, which detects the change, a control-organ which ensures that homeostasis is restored and an effector performing the control-organs orders, to describe it simplified.
Many vertebrates has a thermostat-function in the form of negative feedback, which besides the differences in heat also detects and regulates the blood pressure, oxygen content, carbon dioxide concentration, glucose concentration and pH in the body.
In humans and other endothermic-creatures is the energy consumption determined in order to maintain the internal processes of the basal metabolic rate.
The physical transfer of heat from a living organism is exchanged to the surroundings by conduction, convection, radiation and evaporation. When external cooling lowers the body temperature it will activate several mechanisms to restore the balance in the body; homeostasis.
Thermo regulation is a phenomenon that animals use to maintain a favorable temperature for survival.
Water and ion balance
Living organisms must continuously maintain a steady level of water and ions. This is regulated at the cellular level. Most animals have some form of transport epithelia that act as transport paths for water, ions and other substances that's going in and out of the body.
All animal and its cells need energy in the form of adenosine triphosphate (ATP) to survive and grow, something that is received from the food. Through various processes, the manufacturing and regulation of ATP is done mainly through glycolysis. Too high and too low intake of energy disturbs homeostasis. This is regulated mainly by a "memory" of the body by a temporary storage of glycogen in the muscles and the liver. A layer of fat is built when the glycogen reserves are full, and are taken from when the reserves of glycogen and glucose in the blood are completely drained (gluconeogenesis). Although proteins may in emergencies be used to produce ATP.
The energy regulation is the basic to understand when trying to figure out how weight losses, as well as weight gains are achieved. As you can see, that body will try to prevent you from both putting on weight, and also lose weight because of the homeostasis.
Our body is a very complex creation, and by understanding how it reacts during certain circumstances, and most important; why it act in that way is a great knowledge in order to gain a better self-understanding and why our body reacts as it does.