Travels of a Bolus

Digestion starts in the mouth. But, before food ever enters the mouth, clusters of cells called acini which make up the three salivary glands kick into gear. Saliva is made of mostly water. It does however contain other elements. The main enzyme called amylase starts to help digest starches into maltose. Further, saliva provides alkaline buffering and fluid. The mucus in saliva helps bind the bolus together and helps it slide easily down the esophagus. Salivation can't do all the work alone so, this is where teeth come into play. They are effective at breaking up and grinding food particles as well as mixing/injecting saliva into them. The tongue and to a lesser extent the cheeks and lips help keep the food aligned with the grinding surfaces. Now that the bolus is sufficiently broken up and coated, we swallow. The tongue plays the leading role at first. This is the only voluntary part of swallowing or deglutition. The tongue pushes the bolus back into the mouth which causes the soft palette to rise and close the nasopharynx. Next the larynx rises and glottis closes. The epiglottis falls to cover the closed glottis. (breathing is momentarily interrupted here). Pharyngeal muscles relax and the UES (upper esophageal sphincter) relaxes. Now inside the esophagus, primary peristalsis occurs. At this point the LES (lower esophageal sphincter) and stomach relax in order to receive the bolus. Upon entering the stomach, the LES or gastroesphageal sphincter closes to prevent regurgitation.

Once in the stomach, peristaltic contractions further mix the food with gastric secretions. The contractions in the pyloric region then move the chyme trough the pyloric sphincter and into the duodenum of the small intestine where absorption will begin. Roughly 90% of absorption takes place here. Next stop is the jejunum where further mixing will take place. The last part of the SI we move into is the ileum. Pressure in the terminal ileum opens the ileocecal sphincter as the cecum of the large intestine fills, the sphincter closes to stop the backflow of chyme. Segmentation occurs as we move up the acending colon and into the transverse colon. A slightly different segmentation takes place here as the colon creates little compartments called haustra. This occurs in the descending colon as well. This helps further absorb water and salts from what is now fecal matter. As we move into the sigmoid colon, we stop just short of the rectum. After some time, the rectum