Keeping the balance just right is a process called homeostasis and much of your body's physiology is part of a feedback control circuit designed to do just that. Consider the needs of your body in ticking-over mode. Water is taken to your cells by the circulatory system through blood vessels, and its the kidney which is responsible for regulating your fluid loss. Energy is stored in muscles, liver and fat tissue, and is also kept at a constant level in your blood by the action of hormones . Nutrients such as amino acids are stored in the liver and muscle.
Amino acids can also provide energy but are needed as building blocks.
Other nutrients such as salts are carried in the blood - these are regulated by the kidney. Finally oxygen is carried by the red blood cells in the circulatory system - the heart and lungs are involved in moving the required amount of oxygen about. More energy is required by the muscles to do the work and if its cold, energy is also needed to keep your temperature constant. The way in which the body responds to these changes in conditions and maintains the status quo is essential for survival. Fuel for thought All functions of the body require energy as fuel and this is taken in the form of food.