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Structure and Function of the Digestive System

Movement of Food in the Alimentary Canal

Food moves through the alimentary canal via peristalsis and segmentation and moves from one organ to another through sphincters.
Peristalsis and segmentation are mechanisms for moving food in the digestive system. Peristalsis is waves of smooth muscle contraction in the walls of the alimentary canal. When the circular muscles contract, the diameter of the lumen decreases and when the longitudinal muscles contract the diameter of the lumen increases. The coordinated efforts of these muscles unidirectionally move digestive substances from one location to another. These movements are strong enough to work against gravity.

Peristaltic Movement of Food through the Esophagus

A food bolus moves down the esophagus via waves of muscular contraction called peristalsis. Muscles contract behind the food bolus while muscles in front of the bolus relax, keeping the movement unidirectional.
Segmentation is the coordinated contraction of circular muscles that moves the partially digested food in a back-and-forth motion. In segmentation, waves of muscle contractions shift directions to mix the contents in a particular location for a longer period of time while digestive processes are occurring. This serves to expose more of the food mixture of partially broken down food, stomach acids, and enzymes called chyme to the absorptive cells in the lumen. Segmentation is common in the small and large intestines. Peristalsis, churning, and segmentation are all generated by contractions of smooth muscles of the muscularis externa layer of the alimentary canal organs.

A sphincter is a thick band of circular muscle tissue in the muscularis externa used to regulate food moving from one organ to another. Sphincters are composed of smooth muscle tissue with the exception of the external anal sphincter, which contains skeletal muscle tissue. When the muscle contracts, the tubular opening closes and when it relaxes the sphincter opens the lumen to allow digestive contents to pass through. When waves of peristaltic contractions reach a sphincter, this triggers the sphincter to relax and allow contents through. There are seven sphincters found within the digestive system.

  • The upper esophageal sphincter regulates the movement of food from the pharynx to the esophagus.
  • The lower esophageal (gastroesophageal) sphincter regulates the movement of food from the esophagus to the stomach.
  • The pyloric sphincter separates the stomach from the start of the small intestine.
  • The hepatopancreatic sphincter is between the duodenum and the ducts from the gallbladder and pancreas.
  • The ileocecal sphincter is between the small and large intestines.
  • Internal and external anal sphincters are at the end of the alimentary canal and control defecation.
Peristalsis can be reversed (antiperistalsis) during vomiting when the stomach and sometimes the upper small intestine contents empty to the mouth, usually as a protective measure when exposed to irritants. Sensory neurons detect chemical stimuli or mechanical triggers (like in motion sickness) and the brain activates the vomiting center in the medulla oblongata. Somatomotor signals cause the stomach to contract and the lower esophageal sphincter between the esophagus and the stomach to relax, forcing food out of the stomach and up the esophagus. The diaphragm and abdominal muscles contract to increase the pressure in the abdominal cavity, putting pressure on the stomach, and decrease the pressure in the thoracic cavity, allowing the esophagus to open. The gag reflex is activated in the soft palate in the back of the oral cavity to support the vomiting action. After a deep inhale, respiration stops and the opening to the respiratory system, the glottis, closes to keep stomach contents from entering the respiratory system. During antiperistalsis, the esophageal region proceeding the vomit relaxes and the esophagus contracts following the vomit, causing a wavelike motion toward the mouth.

Anatomy of Vomiting

When irritation occurs in the stomach, the brain triggers the vomiting center. Muscle contractions in the stomach occur. This increased abdominal pressure forces the irritants up the esophagus and out of the mouth called antiperistalsis, or reverse peristalsis.