Metabolism-The Technical Definition

Metabolism means a state of change. Metabolism in its broadest sense defines all chemical actions that occur within the cells of the body. It includes the break down of nutrients into their simplest building blocks, a process known as catabolism, the synthesis of more complex cellular structures (anabolism), and using nutrients and oxygen to produce adenosine triphosphate (ATP), the energy rich molecules that power cellular activity. Metabolism depends on the digestive and respiratory systems to make nutrients and oxygen available to the blood and on the cardiovascular system to distribute these needed substances throughout the body. Metabolism is regulated largely by hormones secreted by endocrine system glands. 

To carry out normal metabolism, body cells need a continuous supply of glucose, their major fuel for producing cellular energy, or ATP. Blood sugar levels are normally maintained around 90 milligrams of glucose per 100 milliliters of blood. 

When the blood experiences a sugar spike, something that happens when you eat a snack high in sugar content like a couple of doughnuts, the body reacts by secreting insulin from the pancreas into the blood stream. Insulin accelerates the uptake of glucose by most body cells. It also encourages the storage of excess glucose as glycogen in the liver and muscles that is kept as an energy reserve. Consequently, blood sugar levels return to more normal levels and insulin release diminishes. 

When blood sugar levels drop below normal, say because you skipped a meal, glucagon is released from the pancreas. Glucagon targets the liver, causing it to release its glucose reserves from glycogen into the blood. Consequently blood sugar levels return to the normal range. 

Metabolism, as you can see, is a complex process by which the body powers itself.