Before the nutrients in food can nourish your body, digestion must split food into smaller molecules that can be absorbed into the bloodstream and delivered to cells throughout the body.

Digestion is a coordinated process involving specialized organs and enzymes that work together to break food into molecules that can be absorbed. Let’s begin by looking at the organs that carry out digestion.

Before reading further, watch the video below to see how the digestive system works in action.

Overview of the Organs Involved in Digestion

Each digestive organ performs a specialized job, but they all work together to digest food, absorb nutrients, and eliminate waste. Most digestion and nutrient absorption occur in the small intestine.

To better understand how the digestive system works, it helps to divide its organs into two groups:

• The gastrointestinal (GI) tract
• The accessory organs

THE GASTROINTESTINAL (GI) TRACT

The gastrointestinal (GI) tract is a continuous muscular tube—about 25 feet long—that extends from the mouth to the anus. As food travels through the esophagus, stomach, small intestine, large intestine, rectum, and anus, it is mechanically and chemically converted into forms the body can absorb.

Although food passes through the GI tract, the inside of this tube is technically considered outside the body until nutrients cross the intestinal wall and enter the bloodstream.

THE ACCESSORY ORGANS

Despite their name, the accessory organs are essential to digestion. They include the salivary glands,  and the liver, gallbladder, and pancreas. Although food never enters these organs, they produce and secrete saliva, bile, and digestive enzymes into the gastrointestinal (GI) tract to help digest food.

These secretions—including enzymes, bile, and other digestive fluids—work together with the GI tract to convert food into nutrients your body can absorb and use. Unlike enzymes, bile does not chemically digest food. Instead, it breaks large fat droplets into smaller droplets, increasing their surface area and making them easier for the enzyme lipase to digest.

Note: The gallbladder does not produce bile. Instead, it stores and concentrates bile made by the liver before releasing it into the small intestine when fatty foods are eaten.

Overview of the Digestive Process

Digestion occurs in five major steps: ingestion, mechanical digestion, chemical digestion, absorption, and excretion. Together, these steps convert food into nutrients the body can absorb and use.

INGESTION

Ingestion begins when food enters the gastrointestinal (GI) tract through the mouth. Chewing (mastication)—the first stage of mechanical digestion—splits food into smaller pieces and mixes it with saliva.

MOVEMENT THROUGH THE ESOPHAGUS

After chewing, food forms a soft mass called a bolus. The tongue and throat muscles push the bolus into the esophagus, a muscular tube connecting the throat to the stomach.

The bolus travels to the stomach by peristalsis—rhythmic, wave-like muscle contractions that move food through the digestive tract. Peristalsis is so effective that it can move food even if a person is upside down.

As food moves through the digestive tract, it is broken down by two complementary processes: mechanical digestion and chemical digestion.

MECHANICAL DIGESTION

Mechanical digestion physically breaks food into smaller pieces without changing its chemical composition. It begins with chewing and continues as muscles in the stomach churn food into a semiliquid mixture called chyme. In the small intestine, muscular contractions continue mixing chyme with digestive secretions.

SEGMENTATION

Unlike peristalsis, which moves food forward, segmentation occurs mainly in the small intestine and repeatedly mixes chyme with digestive juices. This mixing increases contact between nutrients and the intestinal lining, improving digestion and absorption.

CHEMICAL DIGESTION

Chemical digestion uses enzymes and other digestive secretions to break large nutrient molecules into smaller molecules that can be absorbed. For example, enzymes break starch into glucose, a simple sugar.

Enzymes are specialized proteins that speed up chemical reactions without being used up. Different enzymes digest carbohydrates, proteins, and fats into molecules small enough to cross the intestinal wall.

One important chemical reaction is hydrolysis, in which water helps split large molecules into smaller ones. For example, the enzyme sucrase breaks sucrose into glucose and fructose, which can then be absorbed.

ABSORPTION

After digestion, nutrients move from the small intestine into the bloodstream through a process called absorption. Most carbohydrates, proteins, vitamins, minerals, and water enter the bloodstream directly. Most fats are absorbed into the lymphatic system before entering the bloodstream.

EXCRETION

The final step of digestion is excretion. Materials that cannot be digested or absorbed move into the large intestine, where water is removed before the remaining waste forms feces. The feces are stored in the rectum until they are eliminated through the anus.

Functions of the Digestive Organs

Now let’s follow a meal through the digestive system and see how each organ helps turn food into nutrients your body can absorb and use.

Every organ in the digestive system has a specialized role in breaking down food, absorbing nutrients, and eliminating waste. The table below follows a peanut butter and jelly sandwich as it travels through your digestive tract to see what happens at each step.

Although little digestion by human enzymes occurs in the large intestine, it is home to trillions of microorganisms collectively called the gut microbiome. These microbes ferment some of the fiber that escapes digestion in the small intestine, producing compounds that nourish colon cells, support immune function, and synthesize vitamin K and some B vitamins.

 Diet strongly influences the gut microbiome. Fermented foods provide beneficial microorganisms (probiotics), while certain fibers (prebiotics) nourish the helpful bacteria already living in your intestine.

Figure 3.11 provides a visual summary of where digestion and absorption occur throughout the digestive tract.

Attributions

This section is an adaptation of “The Digestive System” in Nutrition: Science and Everyday Applications, V.1.0 by Alice Callahan, Heather Leonard, and Tamberly Powell under a Creative Commons Attribution-NonCommercial 4.0 International License.