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13.2 - Upper GI Tract

The gastrointestinal (GI) tract is the site for the ingestion, digestion and absorption of nutrients. It is divided in the the upper and lower GI tracts.

The upper GI tract is the site of ingestion, mechanical digestion and the first stages of chemical digestion. It consists of the mouth, pharynx, esophagus and stomach.

Processes that occur in the upper GI tract include ingestion, mastication (chewing), swallowing, mechanical and chemical digestion.

Mouth (Oral cavity)

The mouth (oral cavity) is the site of ingestion; the taking of nutrients into the body. It also initiates the oropharyngeal stage of the swallowing reflex.

Movement

• The lips and tongue move the food during mastication (chewing) to form a bolus of chewed food.
• Once the bolus is formed the swallowing centre in the medulla initiates the swallowing reflex

Digestion

• Mastication is the initial mechanical digestion of the food. The tongue, cheeks, lips and teeth participate in the breaking of food into smaller pieces.
• During mastication the saliva is mixed with the food. Saliva contains salivary amylase, a hydrolase (enzyme) that breaks the polysaccharide starch into disaccharide units.

Pharynx

The pharynx is the common tube connecting the oral and nasal cavities to the trachea and esophagus.

Movement

• The pharynx contracts in a peristaltic motion to push the bolus into the esophagus during the swallowing reflex. While the bolus is in the pharynx, the swallowing centre in the medulla inhibits respiration.

Esophagus

The esophagus is a muscular tube connecting the pharynx to the stomach. It participates in the esophageal stage of swallowing.

Movement

• The pharynx contracts in a peristaltic motion to push the bolus of food towards the cardiac sphincter; the opening to the stomach.
• Stimulated to contract in a primary peristaltic wave by the vagus nerve. If the primary peristaltic wave does not succeed in pushing the bolus into the stomach a second, more powerful peristaltic wave is initiated.

Stomach

The stomach is a muscular organ that serves as a storage site for undigested food. It also continues the processes of mechanical and chemical digestion.

The cardiac sphincter connects the esophagus to the stomach and the pyloric sphincter connects the stomach to the small intestine. The stomach has four main regions: the fundus (stores undigested food), body (middle section of the stomach), antrum (site of most vigorous contractions) and pylorus (where undigested food exits the stomach). The stomach expands to several times its size just after a meal to accommodate undigested food in a process called receptive relaxation.

Gastric juices are produced in the stomach and contain several chemicals including mucus (lines and protects the stomach), hydrochloric acid, protein-digesting enzymes and intrinsic factor (needed for vitamin B-12 absorption).

Secretions of the stomach are stimulated by the hormone gastrin and by the presence of food molecules in the stomach, especially peptides.

Movement

• The stomach contains three layers of smooth muscle: circular, longitudinal and transverse. The muscle layers contract to churn the undigested food.

Digestion

• Mechanical: The stomach churns the undigested food and hydrochloric acid secreted by the parietal cells physically breaks up the food into smaller pieces
• Chemical: Pepsinogens secreted by the chief cells activates to become the enzyme pepsin, a protein-digesting enzyme that breaks proteins into peptides; smaller amino acid chains
• Once the stomach has mixed the undigested food with the gastric juices the mixture is called chyme

Factors affecting stomach emptying

Increases rate of stomach emptying:

• Fluid chyme
• Large volume of chyme in stomach
• Low volume of chyme in small intestine

Decreases rate of stomach emptying:

• Chyme is not yet fluid
• Low volume of chyme in stomach
• Distension of the small intestine
• High concentration of fats in the small intestine
• Enterogastric reflex initiated by the vagus nerve
• Enterogastrones (hormones)
• Nervous system suppression during times of intense pain or emotion

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