The Digestive System and Nutrients

Organization of the Digestive Tract and Upper Digestion

The gastrointestinal tract is a continuous muscular tube that begins at the mouth and terminates at the anus. The entire digestive journey starts with ingestion, which is the process of taking food into the digestive system through the mouth. Once food enters, processing occurs through two distinct yet complementary methods: mechanical digestion and chemical digestion. Mechanical digestion physically breaks food into smaller pieces to increase surface area without altering chemical bonds. Chemical digestion, on the other hand, uses digestive enzymes to break chemical bonds within macromolecules, reducing them into forms that the body can readily utilize.

Both forms of digestion begin immediately in the mouth. As chewing takes place, mechanical actions break the food apart, while chemical processes are initiated by glandular secretions. Salivary amylase is produced by the salivary glands and begins carbohydrate digestion in the mouth. Simultaneously, lingual lipase is secreted by glands on the tongue; however, this enzyme begins lipid digestion mainly after reaching the acidic stomach. As these secretions moisten the oral contents, the tongue mixes chewed food with saliva to form a swallowed mass called a bolus.

Swallowing directs the bolus toward the rest of the gastrointestinal tract while safeguarding nearby structures. During swallowing, the epiglottis covers the opening of the larynx to help prevent food from entering the respiratory tract, ensuring that the bolus moves safely past the airway. Once past this junction, peristalsis takes over. Peristalsis consists of rhythmic smooth muscle contractions that propel food along the gastrointestinal tract. At the junction where food enters the stomach, the lower esophageal sphincter acts as a muscular valve. The lower esophageal sphincter prevents stomach acid and partially digested food from regurgitating into the esophagus.