12.2 Digestive System Processes and Regulation
Learning Objectives
By the end of this section, you will be able to:
- Discuss six fundamental activities of the digestive system, giving an example of each
- Compare and contrast the neural and hormonal controls involved in digestion
The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Table 12.2.1 provides an overview of the basic functions of the digestive organs.
Table 12.2.1. Functions of the Digestive Organs
| Organ | Major functions | Other functions |
| Mouth |
|
|
| Pharynx |
|
|
| Oesophagus |
|
|
| Stomach |
|
|
| Small intestine |
|
|
| Accessory organs |
|
|
| Large intestine |
|
|
Digestive Processes
The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption and defaecation.
The first of these processes, ingestion, refers to the entry of food into the gastrointestinal tract through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.
Food leaves the mouth when the tongue and pharyngeal muscles propel it into the oesophagus. This act of swallowing, the last voluntary act until defaecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (Figure 12.2.1). These waves also play a role in mixing food with digestive secretions.
Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive secretions, creating an acidic “soup” called chyme. Segmentation, which occurs mainly in the small intestine, consists of localised contractions of circular muscle of the muscularis layer of the gastrointestinal tract. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and