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Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.
In order to maximize digestion and absorption, your digestive tract regulates how much food can enter the small intestine at one time. In this lesson, you will learn how the digestive system regulates the amount of food entering the small intestine, and how food is propelled through this section of the digestive tract.
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We previously learned that the pyloric sphincter controls the flow of chyme as it passes out of the stomach and into the small intestine. Chyme is the name given to the partially digested food mass. The pyloric sphincter is a tight valve, and, therefore, very little chyme is allowed to exit the stomach at one time. Because so much digestion happens in this first section of the small intestine, this tight control gives the small intestine adequate time to complete digestion. However, this is not the only mechanism by which the digestive system regulates the flow of chyme through this section of the digestive tract. There is also an important reflex that kicks into gear.
When the first part of the small intestine is filled with chyme, its wall is stretched. We also see that the presence of chyme in the small intestine makes the environment acidic due to the acid secretions from the stomach. These factors trigger the enterogastric reflex. The enterogastric reflex inhibits gastric motility and the secretion of gastric acid. The prefix 'entero' refers to intestine and the suffix 'gastric' refers to the stomach. Therefore, you can think of the enterogastric reflex as a reflex that starts in the intestine and affects the stomach. An even better way to think about the enterogastric reflex is to think of it as a way that your digestive system puts on the brakes. When the brakes are applied, the stomach motions and secretions slow down, which causes the stomach to empty slower. This gives digestion within the intestine time to catch up.
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These kinds of controls help you get the most nutrients out of your food. Yet, the food remnants must keep moving through the small intestine to avoid a traffic jam. Food is propelled through your small intestine by peristalsis, which is a wavelike series of muscular contractions. You might recall that peristalsis is also how food moves through your esophagus as it travels from the throat to the stomach. During peristalsis, the longitudinal muscles within the small intestine wall contract, and then the circular muscles contract, pushing the food down the tract. This coordinated contraction of smooth muscle keeps food moving on its one-way path through your digestive system.
Peristalsis is what moves food along the small intestine, but we also see two other movements within this organ. While these two movements do not push food along the tract like peristalsis, they do mix the chyme with the digestive juices and bring particles of food into contact with the wall, where they can be absorbed. Segmentation is one of these movements. It is a localized contraction of circular smooth muscles that constrict the intestine into segments. This is a rhythmic movement that involves the contraction and relaxation of adjacent segments of muscles as if the small intestine is being momentarily pinched closed along its path. Segmentation acts to slosh the chyme back and forth, almost like it is being tossed around in a washing machine. This fully mixes the chyme and allows it to come in contact with the wall.
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Pendular movements are another type of movement that takes place in the small intestine. Pendular movements are described as alternating contraction and relaxation of the longitudinal muscles, causing a portion of the small intestine to shorten and lengthen. This is almost like watching a slinky as it stretches and contracts. This creates a mixing of the chyme as it spills the chyme back and forth. So, we see that segmentation involves the contraction of the circular muscles of the small intestine, and pendular movements involve the longitudinal muscles. These movements together help mix the chyme with digestive juices and expose the chyme to the wall of the small intestine where special adaptations in the wall surface, allowing for maximum nutrient absorption.
Let's review:
The enterogastric reflex is triggered by the presence of acidic chyme in the small intestine. It is a reflex that inhibits gastric motility and the secretion of gastric acid, essentially putting the brakes on the emptying of the stomach so digestion in the small intestine has time to catch up.
Food is propelled through the small intestine by peristalsis, which is a wavelike series of muscular contractions. Two additional movements do not push food along the tract like peristalsis, but they do mix the chyme with the digestive juices and bring particles of food in contact with the wall where they can be absorbed. Segmentation is one of these movements. It is defined as a localized contraction of circular smooth muscles that constrict the intestine into segments. Pendular movements are a different movement. They are defined as alternating contraction and relaxation of the longitudinal muscles, causing a portion of the small intestine to shorten and lengthen.
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