Physiology Topics
Saliva
lubricates, dissolves and begins chemical breakdown of food as it is chewed
in the mouth. Acetylcholine (Ach) from the vagus nerve stimulates saliva secretion.
Most saliva is secreted by the major salivary glands: parotid, submandibular and sublingual. Serous cells in the salivary glands secrete amylase, a carbohydrate digesting enzyme. Mucus cells secrete mucin protein, which combines with water to form mucus. Salivary duct cells secrete ions ( Na+, Cl- and K+) and bicarbonate. Saliva also contains lingual lipase.
Salivary amylase hydrolyzes bonds between glucose units in starches, breaking polysaccharides into smaller oligosaccharides, trisaccharides and disaccharides. Lingual lipase starts the digestion of triacylglycerols into fatty acids and monoglycerides.
Swallowing starts when the bolus is forced to the back of the oral cavity and into the pharynx by movement of the tongue in the voluntary stage of swallowing. The involuntary stage starts with the passing of the bolus into the pharynx. The respiratory passageways close, temporarily interrupting breathing. As the bolus reaches the esophagus, a progression of coordinated contractions and relaxations of the muscularis (peristalsis) pushes the bolus towards the stomach.
If the lower esophageal sphincter fails to close, the stomach contents can reflux into the inferior portion of the esophagus, a condition known as gastroesophageal reflux disease (GERD). Stomach acid can irritate the esophagus wall, resulting in a burning sensation called heartburn. Chronic GERD can cause Barrett's esophagus: metaplasia from squamous to columnar epithelium. 5% to 10% of Barrett's patients will develop esophageal cancer.
Continue to "Stomach " or take a quiz: [Q1] [Q2].
Need more practice? Answer the review questions below.
1- What are the functions of saliva?
2- How is saliva secretion stimulated?
3- List the mayor salivary glands.
4- List the types of cells in salivary glands and their secretions.
5- What is amylase? What does it do?
6- How is mucus formed?
7- Which enzyme not produced by salivary glands is present in saliva? What does it do?
8- Describe the voluntary stage of swallowing.
9- Describe the involuntary stage of swallowing.
10- What is GERD?
11- What is Barrett's esophagus and how it relates to cancer?