Human Digestive System: Functions and Food Processing

Key Digestive Functions

The digestive system is responsible for performing essential digestive functions. These include the following processes:

• Ingestion: The entry of food into the digestive tract through the mouth. Here, food is crushed by the teeth and mixed with saliva.
• Digestion: The transformation of food into simpler substances, called nutrients, so they can be used by cells. Digestion occurs through two types of actions:
  • Mechanical: Involves cutting, grinding, and mixing food. These actions are conducted primarily in the mouth and stomach. They cause physical changes in foods, such as reducing their size and mixing components to facilitate chemical action.
  • Chemical: Consists of the transformation of food into simpler compounds through the action of substances that cause chemical changes in them.
• Absorption: The process by which nutrients from digestion cross the gut wall to be transported through the blood vessels throughout the body.
• Egestion: The elimination of undigested or unusable substances from the body.

These digestive functions allow foods, as they pass through the gut, to be transformed into nutrients, which are then transported into cells where they will be utilized.

Food Ingestion and Digestion Process

The digestive process begins with the ingestion of food through the mouth. Inside, digestion starts through two primary processes:

• Mastication: This is a mechanical action where food is broken down thanks to the action of the teeth.
• Insalivation: Foods are mixed with saliva, which initiates a chemical action on them. Saliva is produced by three pairs of glands: parotid, submandibular, and sublingual.

These two processes are facilitated by the tongue, a muscular organ that also houses the sense of taste.

Food, once chewed and soaked with saliva, forms the bolus, which is pushed into the pharynx and then the esophagus through swallowing.

The esophagus propels the bolus into the stomach through contractile movements of its walls, called peristalsis. It enters the stomach through the cardia, where it accumulates and is subjected to gastric digestion, comprising:

• A chemical action caused by gastric juices secreted by the lining of the stomach. These juices contain, among other substances, hydrochloric acid, which chemically acts on food.
• A mechanical action, consisting of peristalsis of the stomach walls, which move food and mix it with digestive juices.

As a result of gastric digestion, a thick, very acidic mixture called chyme forms. This chyme exits slowly from the stomach through the pylorus and enters the small intestine.

In the duodenum, the chyme is mixed with bile produced by the liver, with intestinal juice produced by the small intestine, and pancreatic juice produced by the pancreas. Bile helps digest fats, while intestinal and pancreatic juices complete chemical digestion. The end result is a whitish porridge called chyle, which contains water, nutrients resulting from digestion, and undigested products.