Oral cavity:
A pair of lips projects the oral cavity (mouth). The lips are lightly vascularized, skeletal muscle tissue with an abundance of sensory nerve endings. Lips help retain food as it is being chewed and play a role in phonation (the modification of sound). The oral cavity contains tongue and teeth. Mammals can mechanically process a wide range of food because their teeth are covered with enamel (hardest material in the body) and because their jaws and teeth exert a strong force. The oral cavity is continuously bathed by saliva secreted by three pairs of salivary glands. Saliva moistens food, binds it with nucius (glycoproteins) and forms moist mass called bolus. Saliva also contains bicarbonate ions (HCO3-) which buffer chemicals in mouth and thiocynate ions (SCN-) and the enzyme lysozyme, which kill micro-organisms. It also contributes enzyme (amylase) necessary for initiation of carbohydrate digestion.
Pharynx and esophagus:
Air and swallowed foods and liquids pass from the mouth into the pharynx. The epiglottis temporarily seals off the opening (glottis) to the trachea so that swallowed food does not enter the trachea. Initiation of swallowing reflex can be voluntary but most of the time it is involuntary. When swallowing begins, sequential, involuntary contractions of smooth muscles in the walls of esophagus propel the bolus or liquid to the stomach.
Stomach:
Mammalian stomach is distensible sac with three main functions. It stores and mixes the food bolus received from the esophagus. It secretes substances enzymes, mucus and Hydrochloric acid (HCl) that start the digestion of proteins and helps control the rate at which food moves into small intestine via pyloric sphincter.
Structure:
Stomach is made up of inner mucous membrane containing thousands of gastric glands. Three types of cells are present:
(1) Parietal cells: Parietal cells secrete a solution containing HCl.
(2) Chief cells: Chief cells secrete pepsinogen the precursor of the enzyme pepsin. Both of the cells are in the pits of the gastric glands. The surface of the mucus membrane at the openings of the glands contain mucous cells that secrete mucus that coat the surface of stomach and protect it from HCl and digestive enzymes. The surfaces of esophagus and mouth have much thinner mucous cell layer than the stomach which is why vomiting can cause burning sensation in the esophagus or mouth.
(3) Endocrime cells: Endocrime cells in one part of the stomach mucosa release the hormone gastrin which travels to target cells in the gastric glands, further stimulating them.
Working:
When the bolus of food enters the stomach, it distends the walls of the stomach. This distention as well as the act of eating, causes the gastric pits to secrete HCl (as H+ and Cl-) and pepsinogen.
H+ions cause pepsinogen to be converted into active pepsin. As pepsin, mucus and HCl mix with and begin to break down proteins, smooth mucosal muscles contract and vigorously churn and mix the food bolus. About 3 – 4 hours after a meal, the stomach contents have been sufficiently mixed and are semi-liquid mass called chyme.
The pyloric sphincter regulates the release of chyme into small intestine.
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