Gastrointestinal system: Structure and function

The primary functions of the gastrointestinal system are digestion, which is breaking down food that we eat into nutrients; absorption, which is getting those nutrients into the bloodstream; and excretion, which is getting rid of digestive waste as feces.

The gastrointestinal system is made of hollow, connected organs that make up a canal, called the gastrointestinal tract, as well as a couple of accessory organs that are not part of the canal but play an important role in the food digestion process.

Alright, from top to bottom, the gastrointestinal tract starts with the mouth, then the esophagus, the stomach, the small intestine, the large intestine, and ends with the anus.

Digestion starts after food ingestion, which is putting food into the mouth. The first step is mastication, which is chewing on food to physically break it into small chunks.

This process is also referred to as mechanical digestion. During mastication, the tongue mixes food with saliva secreted from salivary glands.

Saliva contains some chemical substances, called enzymes, which start breaking down chemical bonds between food molecules.

This process is called chemical digestion, and it continues at different levels of the gastrointestinal tract as food moves along.

As mastication goes on, we get a food-saliva mixture, called a bolus, which is ready to be swallowed. Swallowing takes place through a muscular tube, called the esophagus.

To move this food bolus along, muscles of the esophagus contract and produce a wave-like movement that pushes food downward into the stomach. This movement is called peristalsis, and it also continues throughout the GI tract.

When the food arrives in the stomach, it mixes with gastric juice consisting of acid and gastric enzymes, which turns the food bolus into a pulpy soup, called chyme, made of much smaller food particles.

Next, chyme moves to the first part of the small intestines, called the duodenum, where bile secreted by the liver and pancreatic enzymes digest it even further. The end result is chyle, which is a kind of milky fluid ready to be absorbed.

Chyle then moves to the second portion of the small intestine, called the jejunum, which has a specialized mucosa, or lining, that allows nutrients to be absorbed into the bloodstream by way of a network of local capillaries.

After nutrients are absorbed, the food residue keeps moving along the canal by means of peristalsis. It moves from the last part of the small intestine, called the ilium, into the large intestine, or colon.

The remaining nutrients and water get absorbed, leaving semi-solid materials, called excrements or feces.

Excrements collect in the last segment of the large intestine, called the rectum, which is connected to the anus.

When the rectum is full, it sends a message to the brain that triggers the need to defecate, which is called excretion.