When the term digestion is mentioned, it is natural to think about the actions of the mouth, stomach, and small intestine in the processing of food for energy. While these actions are no doubt important in the breakdown of food, they actually are the result of complex processing mechanisms at the cellular level. This chapter will examine the physiology of the digestive system at the organ level. However, to effectively understand the structure and function of the digestive system, we must first understand the cellular and molecular basis of nutrient processing.
The purpose of digestion is to process food by breaking the chemical bonds that hold the nutrients together. This is necessary so that the body has an adequate source of energy for daily activity, as well as materials for the construction of new cells and tissues. Since these nutrients arrive in the digestive system as the tissues of previously living organisms, they are rarely in the precise molecular structure needed by a human body. For example, the blood of cows and chickens has evolved over time to meet the precise metabolic needs of the organism. When the tissues of these animals are consumed, our bodies must chemically alter the proteins and other nutrients found in the animal’s blood to form human blood proteins such as hemoglobin.
As is the case with almost all the nutrients (with the exception of water, minerals, and some vitamins), the body breaks down the nutrient into its fundamental building blocks, transports the digested nutrient into circulatory and lymphatic systems, and eventually uses these nutrients in the cells of the body for either energy or metabolic processes.