How Our Digestive System Works

Our gastrointestinal tract is vital for life: it is responsible for digesting food, absorbing nutrients, eliminating waste, and protecting us. From the mouth to the anus, each organ has a function.

From the Mouth to the Stomach: Starting the Digestive Process

The digestive process starts in the oral cavity, characterized by the grinding of food by the teeth and saliva, whose action through an enzyme called amylase breaks down carbohydrates. This food bolus, now chewed and moistened, passes through the throat (pharynx) and enters the esophagus. The esophagus is not just a tube; it is a muscle that, through rhythmic contractions (known as peristalsis), pushes the food towards the stomach.

Gastric Digestion: the Stomach as the Food Processor

Upon reaching the stomach, the food bolus mixes with gastric juice, an highly acidic environment (pH 1.5-3.5), due to the secretion of hydrochloric acid that not only kills bacteria but also converts pepsinogen into the enzyme pepsin, which begins to break proteins down into smaller fragments (peptides). The rhythmic contraction of the stomach muscles (gastric motility), aids in the mechanical grinding and mixing, transforming the bolus into chyme.

The Small Intestine: Where Main Nutrient Absorption Occurs

The chyme is gradually released into the duodenum, the first section of the small intestine. Here, the acidic pH is neutralized by the secretion of bicarbonate by the pancreas. In this area, the chyme meets three key assistants:

• Bile: is crucial for breaking down fats into small droplets, facilitating their digestion. The enzymes responsible for this action are the
• Pancreatic juice: this juice is a potent mix of enzymes that digest carbohydrates (amylase), proteins (trypsin and chymotrypsin) and fats (lipases).
• Intestinal enzymes: produced by the walls of the small intestine itself, these enzymes finish breaking down carbohydrates and proteins into their simplest units (sugars and amino acids).

The walls of the small intestine are covered by millions of tiny projections called villi and microvilli. These structures, which look like tiny fingers, greatly increase the surface area for absorption. Through them, the already digested nutrients (sugars, amino acids, fatty acids, vitamins, and minerals) cross the intestinal wall and enter the bloodstream or lymphatic system, ready to be distributed to every cell in your body.

Large Intestine: Water Absorption and Faeces Formation

The material not digested and not absorbed moves into the large intestine. The primary function of this section is the absorption of water and electrolytes, leading to the compaction of faeces. Additionally, in the large intestine live millions of "good" bacteria (the gut microbiota), which help break down some indigestible materials and produce certain essential vitamins (e.g., vitamin K, biotin). Faeces are stored in the rectum before being eliminated by defecation.

In summary, digestion is a finely regulated process that involves the coordinated action of multiple organs, enzymes, and transport mechanisms, ensuring the efficient conversion of food into components usable by each cell in the body.