Our bodies are made up of billions of cells, and each one needs a steady supply of essential nutrients—including carbohydrates, proteins, fats, vitamins, and minerals—to function properly. We get these nutrients from the foods we eat.However, before our cells can use these nutrients, the food must first be broken down into smaller, simpler molecules through the process of digestion. Only then can the nutrients be absorbed into the bloodstream and delivered to the cells that need them.

Digestion is a complex process that involves many organs, enzymes, and chemical reactions working together. On this page, we’ll take a closer look at how this system functions.

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

Overview of the Organs Involved in Digestion

The digestive system has three main jobs: to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although every organ plays a part, the small intestine is the system’s “workhorse,” where most digestion and absorption take place.

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

1. the gastrointestinal (GI) tract, and

2. the accessory organs.

The Gastrointestinal (GI) Tract

The GI tract is a long, continuous tube—about 25 feet in length—that runs from the mouth to the anus. Along the way, it includes the epiglottis, esophagus, stomach, small intestine, large intestine, and rectum.

Interestingly, the GI tract is considered to be outside the body. That’s because food and waste travel through it without ever entering the body’s internal tissues until absorption occurs. During absorption, nutrients move from the intestines into the bloodstream, where they can nourish the body’s cells.

The Accessory Organs

Despite their name, the accessory organs are essential to digestion. These include the salivary glands, liver, gallbladder, and pancreas. While food doesn’t pass through them, they connect to the GI tract through small ducts and release important digestive juices.

These secretions contain enzymes, bile, and acids that help break down food into smaller molecules, making it easier for the body to absorb nutrients. Together, the GI tract and the accessory organs form a highly coordinated system that turns the foods you eat into the fuel your body needs to live and thrive.

The digestive system breaks down the foods you eat, releases their nutrients, and absorbs those nutrients into the body. Although the small intestine is the workhorse of the system where the majority of digestion and absorption occurs, each of the digestive organs makes a vital contribution to this process.

Overview of the Digestive Process

The process of digestion involves five main steps: ingestion, mechanical digestion, chemical digestion, absorption, and excretion. Each step plays a crucial role in transforming food into the nutrients your body can utilize.

 Ingestion

Ingestion is the first step—it’s when food enters the gastrointestinal (GI) tract through the mouth. Here, food is chewed and mixed with saliva, which contains enzymes that start breaking down carbohydrates and lipids.

The act of chewing (called mastication) breaks food into smaller pieces, increasing its surface area so enzymes can work more efficiently. It also makes food small and soft enough to swallow safely.

Movement Through the Esophagus

Once food is chewed and moistened, it forms a soft mass called a bolus. The tongue and muscles in the throat push the bolus into the esophagus, a muscular tube that connects the throat to the stomach.

The bolus moves down the esophagus through a process called peristalsis—a series of rhythmic, wave-like muscle contractions that push food along the digestive tract (see Figure 3.8).

Peristalsis is so strong that it can move food to your stomach even if you’re standing on your head—though that’s definitely not recommended!

Digestion happens through two main types of actions: mechanical and chemical. These work together to turn the food you eat into nutrients your body can use.

Mechanical Digestion

Mechanical digestion is all about physically breaking food down into smaller pieces. It doesn’t change what the food is made of—it just makes it easier for your body to handle and for enzymes to reach more surface area.

It starts in your mouth, where you chew your food and mix it with saliva. The tongue helps move food around so it can be crushed and softened. In your stomach, the muscles churn and mix the food with digestive juices, breaking it down even more.

As the food moves through your intestines, a process called peristalsis pushes it along. Peristalsis is a wave-like squeezing motion that keeps food moving through your digestive system.

Segmentation

Another kind of muscle action, called segmentation, happens mostly in the small intestine. Instead of pushing food forward, the muscles squeeze sections of the intestine, moving food back and forth. This helps mix it with digestive juices and break it up into smaller parts, making it easier for your body to absorb nutrients.

Together, these movements keep your food moving, mixing, and breaking down until your body can absorb all the good stuff it needs.

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In chemical digestion, special fluids called secretions release enzymes that break down large nutrients into their smaller, usable parts—for example, turning starch into glucose (a simple sugar).

Enzymes are helper molecules that speed up chemical reactions in the body. They make it easier for certain compounds to react, but the enzymes themselves don’t get used up or changed in the process—they can keep working again and again.

One of the most important chemical reactions in digestion is called hydrolysis. In hydrolysis, water is used to split one large molecule into two smaller ones.

For example, table sugar (sucrose) is made of two smaller sugars, glucose and fructose. Before your body can absorb it, sucrose must be broken down into these two parts. The enzyme sucrase helps this happen by bringing a sucrose molecule and a water molecule together so the bond between them can be broken.

Watch the animation below to see how this hydrolysis reaction works in the body.

Functions of the Digestive Organs

Now that you’ve learned the basics of the digestive organs and how the digestive process works, let’s take a closer look at what happens in each part of the gastrointestinal (GI) tract.

To make it easier to understand, imagine eating a peanut butter and jelly sandwich. This simple meal contains all the major nutrients your body needs—carbohydrates, proteins, fats, vitamins, and minerals.

As the sandwich moves through your digestive system, each organ plays a special role in breaking it down into smaller parts that your body can use. By the end of the journey, those nutrients will be absorbed into your bloodstream and delivered to the cells that need them for energy, growth, and repair.

Let’s follow the sandwich step by step to see how your body turns it into fuel.

Following a Peanut Butter and Jelly Sandwich Through the Digestive System

Now that you know the basic organs and steps of digestion, let’s walk through what happens to a peanut butter and jelly sandwich after you take your first bite. This sandwich contains carbohydrates, proteins, fats, vitamins, and minerals—all of which must be broken down before your body can use them. Each organ in the gastrointestinal (GI) tract plays a special role in turning that sandwich into nutrients your cells can absorb.

Mouth: Where Digestion Begins

Digestion starts the moment the sandwich enters your mouth. This is where both mechanical and chemical digestion begin.

Your teeth grind and crush the food into smaller pieces, while your salivary glands release saliva—a mixture of water, enzymes, and other helpful chemicals. One enzyme, salivary amylase, starts breaking down carbohydrates in the bread. Another enzyme, lingual lipase, begins breaking down fats (triglycerides) in the peanut butter.

By the time you finish chewing, the food has become a soft, moist ball called a bolus that’s ready to be swallowed.

Esophagus: The Food Highway

The esophagus is a muscular tube that carries the bolus from your mouth to your stomach. No chemical digestion happens here—just movement. The bolus travels through the esophagus by peristalsis, a wave-like motion of muscles that pushes food downward.

Stomach: The Mixer and Grinder

Your stomach connects the esophagus to the first part of the small intestine, called the duodenum. Although it’s about the size of your fist when empty, it can stretch to hold up to 4 liters of food and liquid!

The stomach serves as a temporary holding chamber. You can eat a meal in minutes, but digestion takes much longer, so the stomach slowly releases food into the small intestine over several hours. Meals high in fat or protein (like peanut butter) take longer to digest than carbohydrate-rich foods.

When the sandwich reaches the stomach, strong muscular contractions churn and mix it with gastric juices, turning it into a thick, soupy mixture called chyme. These gastric juices contain hydrochloric acid (HCl), which:

• Creates a very acidic environment that kills many bacteria and germs, and

• Helps unfold proteins in the peanut butter and bread.

The stomach also produces enzymes that begin breaking down proteins and fats into smaller components.

Small Intestine: The Workhorse of Digestion

Next, the chyme enters the small intestine, where most digestion and absorption occur. The small intestine is divided into three parts:

1. Duodenum (closest to the stomach)

2. Jejunum (middle section)

3. Ileum (final section)

In the duodenum, the pancreas and gallbladder release digestive juices that help break food down further.

• The pancreas, located behind the stomach, releases pancreatic juice—a fluid that contains bicarbonate to neutralize stomach acid and enzymes that digest carbohydrates, proteins, and fats.

• The gallbladder, a small sac under the liver, stores and releases bile, a fluid made by the liver. Bile acts like a detergent—it emulsifies fats, breaking large fat droplets into smaller ones so they can mix with watery digestive juices and be more easily digested.

As chyme moves through the small intestine, peristalsis continues to push and mix it along the tract.

Absorption: Nutrients Enter the Body

Most nutrients are absorbed in the ileum, the last section of the small intestine. The inside of the small intestine is perfectly designed for absorption—it has an enormous surface area of over 200 square meters, about the size of a tennis court!

This huge surface area comes from the many folds, villi, and microvilli that line the intestinal wall.

• Villi are tiny, finger-like projections covered with absorptive cells (called enterocytes).

• The surface of these cells is covered with even smaller projections called microvilli, which together form the brush border—so named because it looks like the bristles of a brush under a microscope.

These structures increase the area for nutrient absorption, allowing sugars, amino acids, fatty acids, vitamins, and minerals from your sandwich to move into the bloodstream (or lymph system, in the case of fats).

Now that you have an overview of the digestive organs and the digestive process, let’s discuss in more detail what types of mechanical and chemical digestion take place in each of the organs of the GI tract. Let’s imagine eating a peanut butter and jelly sandwich that contains carbohydrates, proteins, fats, vitamins, and minerals. How does each organ participate in breaking this sandwich down into units that can be absorbed and utilized by cells throughout the body?

By the time the peanut butter and jelly sandwich reaches the large intestine, most of its nutrients have already been digested and absorbed. What’s left—usually less than 10% of what you ate—includes indigestible fiber and other materials that your body can’t break down.

Large Intestine: Absorbing Water and Forming Feces

The main job of the large intestine (or colon) is to absorb water from the remaining material. This water comes not only from the food and drinks you consume but also from the digestive juices produced by the stomach, pancreas, and intestines.

As water is reabsorbed, the once-liquid mixture (called chyme) becomes thicker and turns into a semisolid known as feces. Feces contain undigested food particles, unabsorbed nutrients, millions of bacteria, old intestinal cells, minerals (salts), and just enough water to move smoothly through the body.

Bacteria: The Helpful Residents of the Colon

Even though no new chemical digestion happens here (except for a little help from bacteria), the large intestine is full of life. It’s home to trillions of bacteria—known as the gut flora or microbiota—which actually outnumber all the human cells in your body!

Most of these bacteria are harmless and many are beneficial. They:

• Help break down certain materials that your body can’t digest on its own.

• Support a healthy immune system.

• Produce some important vitamins, such as vitamin K, biotin, and pantothenic acid.

Elimination: The Final Step

The feces are stored in the rectum, which acts as a temporary holding area, until they leave the body through the anus in a process called defecation.

At this point, your body has completed its journey of digestion—from taking a bite of your sandwich to absorbing nutrients and finally eliminating waste.

(See the figure below for a summary of the functions of the digestive organs.)

Most of the nutrients from the peanut butter and jelly sandwich have now been digested and absorbed. Any components that still remain (usually less than ten percent of food consumed) and the indigestible fiber move from the small intestine to the large intestine (colon). A main task of the large intestine is to absorb much of the remaining water. Water is present not only from the solid foods and beverages consumed, but also from the digestive juices released by the stomach and pancreas. As water is reabsorbed, liquid chyme becomes a semisolid, referred to as feces. Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old cells from the lining of the GI tract, inorganic salts, and enough water to let it pass smoothly out of the body.

Feces is stored in the rectum (a temporary holding area) until it is expelled through the anus via defecation. No further chemical breakdown of food takes place in the large intestine except that accomplished by the bacteria that inhabit this portion of the GI tract. There are trillions of bacteria residing in the large intestine (referred to as the bacterial flora), exceeding the total number of cells in the human body. This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and many are even beneficial—facilitating chemical digestion and absorption, improving immune function, and synthesizing vitamins such as biotin, pantothenic acid, and vitamin K.

The figure below summarizes the functions of the digestive organs.

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.