Human Digestive System: Structure and Initial Processes

The Human Digestive System Structure

The digestive system is made by the gastrointestinal tract and accessory glands.

The Digestive Tract

The digestive tract begins in the mouth and ends at the anus. It includes the oral cavity, pharynx, esophagus, small intestine, and the large intestine.

Accessory Glands

Accessory glands are organs that pour their secretions into the digestive tract. These include the salivary glands (producing saliva in the mouth), gastric glands (in the stomach), and intestinal glands (in the intestine). The liver and pancreas are also glands that discharge their secretions into the digestive tube.

Initial Stages of Digestion

Mastication (Chewing)

Mastication is a mechanical process that breaks food into pieces. This is done by the teeth and tongue. Teeth are specialized for different functions:

• Incisors: for cutting
• Canines: for tearing
• Molars: for grinding

Insalivation and Saliva Production

When we enter food into the mouth, sometimes just smelling or seeing it begins the production of saliva. Saliva is a watery fluid produced by the salivary glands (located under the tongue and sides). When saliva mixes with the crushed foodstuff, it produces the food bolus.

Functions of Saliva

Saliva performs several duties:

• Facilitates the passage of the bolus into the pharynx and esophagus.
• Initiates the digestion of starch molecules.
• Destroys some of the bacteria that food contains.

Swallowing (Deglutition)

Once the bolus is formed, swallowing is produced by the action of the tongue. The progress of the bolus is realized thanks to peristaltic movement.

Digestion in the Stomach

The bolus remains in the stomach for 3 or 4 hours, where it is mixed with gastric juice. The resulting mixture is chyme, which is a fluid mass that is more than just the fluid bolus.

Gastric Juice Composition and Action

Gastric juice contains:

• Pepsin: An enzyme that begins the digestion of proteins.
• Hydrochloric Acid (HCl): Activates pepsin and kills bacteria.

The inner wall of the stomach is lined with mucus that protects it and prevents it from being attacked by the acidic environment and hydrochloric acid.

The Small Intestine

The small intestine measures about 6–7 meters, extending from the pylorus (where it joins the stomach) to the ileocecal valve (which separates it from the large intestine). It is divided into three sections:

1. Duodenum: The short portion where the liver and pancreas secretions flow into.
2. Jejunum: The longest portion.
3. Ileum: The final segment that results in the large intestine.

The Role of the Liver and Bile

The liver is a large organ located on the right side of the abdomen and partially covers the stomach. It performs the most functions in the human body. It produces bile, which is poured into the duodenum through the ampulla of Vater.

The secretion of bile is not done directly; rather, it is stored in the gallbladder and is released only when food enters the intestine. Bile contains a substance called bile salts that facilitate the digestion of fats.