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Illustration A shows the parts of the human oral cavity. The tongue rests in the lower part of the mouth. The flap that hangs from the back of the mouth is the uvula. The airway behind the uvula, called the pharynx, extends up to the nostrils and down to the esophagus, which begins in the neck. Illustration B shows the two salivary glands, which are located beneath the tongue, the sublingual and the submandibular. A third salivary gland, the parotid, is located behind the pharynx.
Digestion of food begins in the (a) oral cavity. Food is masticated by teeth and moistened by saliva secreted from the (b) salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by the National Cancer Institute)

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave-like movements called peristalsis that push the food toward the stomach, as illustrated in [link] . The peristalsic wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing.

Photo shows food moving down the esophagus, which is a muscular tube. Muscles constrict behind the food. The constriction moves down, pushing the food ahead of it, from the mouth to the stomach.
The esophagus transfers food from the mouth to the stomach through peristaltic movements.

A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; however, in humans, there is no true sphincter, but the esophagus remains closed when there is no swallowing action. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

Stomach

A large part of digestion occurs in the stomach, shown in [link] . The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available.

Illustration shows the human lower digestive system, which begins with the stomach, a sac that lies above the large intestine. The stomach empties into the small intestine, which is a long, highly folded tube. The beginning of the small intestine is called the duodenum, the long middle part is called the jejunum, and the end is called the ileum. The ileum empties into the large intestine on the right side of the body. Beneath the junction of the small and large intestine is a small pouch called the cecum. The appendix is at the bottom end of the cecum. The large intestine travels up the left side of the body, across the top of the small intestine, then down the right side of the body. These parts of the large intestine are called the ascending colon, the transverse colon and the descending colon, respectively. The large intestine empties into the rectum, which is connected to the anus. The pancreas is sandwiched between the stomach and large intestine. The liver is a triangular organ that sits above and slightly to the right of the stomach. The gallbladder is a small bulb between the liver and stomach.
The human stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

The stomach is also the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin (secreted by cells in the stomach lining) breaks peptide bonds and cleaves proteins into smaller polypeptides. Another cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. The highly acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles completely mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme . Chyme passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter.

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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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