Anatomy and Physiology of the Digestive Tract

The Human Digestive System: Definition and Function

The digestive system is a set of organs responsible for carrying out digestion. It transforms food into simple substances that pass into the blood and eliminates undigested remains. Its primary role is to provide nutrients to cells by digesting food.

Organs of the Digestive System

Mouth

• Lips: Fleshy muscles that surround the mouth.
• Palate: The roof of the mouth, separating the mouth and nostrils. The anterior part is the hard palate, and the posterior part is the soft palate.
• Uvula: A structure that hangs from the soft palate, preventing the passage of substances into the nostrils.
• Teeth: Used to crush food. Types include incisors, canines, premolars, and molars. Adults typically have 32 teeth (8 incisors, 4 canines, 8 premolars, 12 molars). Children's primary dentition lacks molars.
• Tongue: A muscular organ where the sense of taste resides. It mixes food with saliva, forming a *bolus*.

Pharynx

A duct located behind the mouth, containing lumps called tonsils (inflammation of which is *tonsillitis*). It communicates with the esophagus, nasal cavity, larynx, and middle ear (via Eustachian tubes). The *epiglottis* is a flap in the lower part of the pharynx that closes the entrance to the larynx, preventing food from entering the respiratory tract.

Esophagus

A tube about 25 cm long, located behind the trachea and in front of the spinal cord. It drives the food bolus from the pharynx to the stomach. Its muscles produce *peristaltic movements* so that the food reaches the stomach. It also produces mucus, which favors food transit.

Stomach

A muscular, J-shaped organ located in the abdominal cavity beneath the diaphragm. It dilates when filled with food. On its internal wall, there are gastric acids and mucus. The mucus stops the gastric acids from attacking the stomach's own walls. The *cardia* is a sphincter that communicates with the esophagus. The *pylorus* is another sphincter connecting to the duodenum of the small intestine.

Small Intestine

A long tube with curvatures called *intestinal loops*. It produces intestinal juice in the glands of its walls and absorbs nutrients, possessing a very high absorption surface. Folds in its internal walls retract into *intestinal villi*, which are finger-like projections that significantly increase the absorption surface area.

• Duodenum: The initial portion, connecting with the stomach. The liver secretes bile and the pancreas secretes pancreatic juice into this section.
• Jejunum: The middle portion (the longest stretch), where nutrients are primarily absorbed.
• Ileum: The final portion that ends into the large intestine through the *ileocecal valve*.

Large Intestine

An inverted U-shaped, thick tube located in the abdominal cavity, surrounding the small intestine.

• Cecum: The lower part of the abdomen, below the ileocecal valve. From its bag-like shape, the *vermiform appendix* emerges (inflammation of which causes *appendicitis*).
• Colon: Divided into ascending, transverse, descending, and sigmoid sections. Here, symbiotic bacteria develop, forming the *intestinal flora*.
• Rectum: The end of the large intestine, which terminates in the *anus*. The anus has a double sphincter that controls defecation.