Normal Movements of the Digestive Tract
The digestive tract includes the esophagus (or food tube), stomach, small intestine/bowel, and colon or large intestine/bowel. It begins at the mouth and ends at the anus.
Gut motility is the term given to the stretching and contractions of the muscles in the gastrointestinal (GI) tract. The synchronized contraction of these muscles is called peristalsis. These movements enable food to progress along the digestive tract while, at the same time, ensuring the absorption of the important nutrients.
Techniques of measuring these movements of the gut enable us to recognize the normal patterns of contraction in each of the regions. The types of contraction in the gut differ depending on the region and the type of food which has been eaten. Some contractions cause onward movement of the food, others cause mixing and grinding.
The esophagus, stomach, small intestine, and large intestine are the main regions of the GI tract. They are separated from each other by special muscles, called sphincters, which regulate the movement of ingested material from one part to another. Each part of the GI tract has a unique function to perform in digestion, and each has a distinct type of motility and sensation.
Esophagus and Stomach
Digestion begins in the mouth where food is chewed, mixed with saliva, and swallowed. The esophagus propels food from the mouth to the stomach. The stomach is large enough to temporarily store the food eaten at each meal. Solid food is gradually broken down by powerful muscle contractions in the lower end of the stomach. This muscular activity produces small food particles suitable to enter the small bowel, where processes of nutrient absorption begin.
Different types of food empty from the stomach at different rates; for example, fatty foods take longer to leave the stomach than other foods. Beverages are handled differently by the stomach, emptying more quickly into the small bowel and not requiring break-down into smaller particles. Normally, most of an average-sized meal has left the stomach after about 2 hours.
In the stomach the food stimulates the release of digestive juices (secretions) like hydrochloric acid and digestive enzymes that chemically further break down and mix with the food. The mixture is referred to as chyme.
The chyme then passes, in a regulated controlled manner, out of the stomach into the small bowel/intestine. In the small intestine, the muscular contractions occur irregularly, varying in strength and type. Here also, the different nutrients in food affect the type of contractions generated. After an average sized meal, the contractions continue for several hours, mixing the food and moving it along the intestine. These types of contractions last until most of the meal residues enter the large intestine. Different foods travel at different rates along the small intestine; for example, foods high in fat travel more slowly than fiber-rich foods.
After most of the food has left the small intestine, a different pattern of contractions appears. Bursts of powerful contraction, occurring about every 90 minutes during fasting and particularly at night, progress slowly down the intestine. These bursts clear residual food and secretions from the upper intestine, and thus act as a "housekeeper" in the intestine.
The average total length of the normal small bowel in adults is about 7 meters/22 feet. The small intestine has 3 segments:
- the duodenum,
- the jejunum, and
- the ileum.
Each part or section performs an important role in nutrient absorption.
Duodenum – The chyme first enters into the duodenum where it is exposed to secretions that aid digestion. The secretions include bile salts, enzymes, and bicarbonate. The bile salts from the liver help digest fats and fat soluble vitamins (Vitamin A, D, E, and K). Pancreatic enzymes help digest carbohydrates and fats. Bicarbonate from the pancreas neutralizes the acid from the stomach.
Jejunum – The chyme is then further transited down into the second or middle part of the small intestine, the jejunum. Mainly in the first half of the jejunum, the majority (about 90%) of nutrient absorption occurs involving proteins, carbohydrates, vitamins, and minerals.
Ileum – The ileum is the last section of the small intestine and leads to the large intestine or colon. The ileum mainly absorbs water, bile salts, and vitamin B12.
The ileocecal valve is a one way valve located between the ileum and the cecum, which is the first portion of the colon. This valve helps control the passage of contents into the colon and increases the contact time of nutrients and electrolytes (essential minerals) with the small intestine. It also prevents back-flow (reflux) from the colon up into the ileum, and minimizes the movement of bacteria from the large intestine up into the small bowel.
Large Intestine (Colon)
The first portion of the colon, the cecum, is shaped like a pouch, and is the area of storage for the contents arriving from the ileum. The primary function of the large intestine or colon is to absorb fluids and electrolytes, particularly sodium and potassium, and to convert remaining luminal contents into more solid stool. The colon absorbs on average 1-1.5 liters (about 1-1.5 quarts) of fluid every day and has a capacity to adapt its fluid absorption to as much as 5 liters/quarts per day if needed. Another function of the colon is to break down (ferment) dietary fiber to produce short chain fatty acids – substances that can be absorbed and provide added nutrition.
The patterns of contraction in the colon are not as well understood as those in the small intestine. It is known, however, that eating a meal stimulates contractions in the colon – the larger the meal the greater is the response.
Stretching of the rectum by stool produces relaxation of the muscles of the anus and surrounding structures. The rectal contents can then be discharged voluntarily.