Although we are rarely aware of it, our digestive tract is constantly moving. Even at rest the smooth muscle of the circular and longitudinal muscle layers maintain a reasonable level of activity that establishes a resting tension and is responsible for the normal diameter and length of the digestive tract.
Superimposed upon this resting tension we have two major forms of movement:
Peristalsis
Food is transported through the digestive tract from the mouth towards the anus. This is enabled by strong contractions of the muscularis externa that propel the food the in the correct direction.
The exact mechanism of these propulsive movements is not yet fully understood, but one proposal is that the circular muscle layer contracts and reduces the diameter of the digestive tract behind the food. The longitudinal muscle adjacent to the food then contracts and this squeezes the food into the adjacent (relaxed) portion of the tract (see diagram on the right).
This cooperative action of the two muscle layers is coordinated by the enteric nervous system and produces the propulsive movements that keep food moving in the preferred direction.
This means that you don’t require gravity to encourage the movement of food along your digestive tract. This is very good news for people who are considering a career as an astronaut or who like to drink beer while standing on their heads at parties.
Segmentation
This form of movement is designed specifically to ensure that food is mixed effectively in the digestive tract. This assists in the mechanical breakdown of food (mechanical digestion), ensures effective mixing of food and the secretions of the accessory glands and optimises absorption by constantly changing the food in contact with the digestive tract epithelium.
Segmentation appears to be produced by contraction of short segments of circular muscle with the relaxation of the circular muscle in between (see diagram on the right).
By alternating the portions of circular muscle that contract and relax lumps of food are progressively divided and then mixed with lumps of food from adjacent segments. This produces very effective mixing but no propulsion of the food along the digestive tract.
The most significant amount of segmentation occurs in the small intestine.
Now that we have reviewed some of the functional anatomy of the digestive system and some of the basic concepts functional mechanisms we are ready to consider the core physiology of the digestive system.
The easiest way to be this is to follow a meal as it travels through the digestive tract. As this meal moves along the digestive tract we will see how it is transformed and subsequently absorbed or eliminated as waste product.
As we move through the digestive tract we will come across the secretions of the three accessory organs and deal with their physiology at that time. So sit back, relax and enjoy the journey.