12.1: Overview of the Digestive System

The digestive system is a coordinated set of organs that break down food, absorb nutrients, and eliminate waste to support the body’s energy needs and overall health.

Master this section and you'll be able to:

• List the organs of the digestive tract from proximal to distal, and briefly state their functions.
• Identify the accessory digestive organs and briefly state their primary function.
• Describe the six core digestive processes and where they occur.

The digestive system breaks down the foods you eat by releasing enzymes that mix with food, free the nutrients, and allow those nutrients to be absorbed into the body. Although the small intestine does most of this work, every organ in the system contributes to the overall process. The figure below shows the digestive tract, a continuous tube that runs from the oral cavity through the esophagus, stomach, small intestine, and large intestine, ending at the anus. It also includes important accessory organs, such as the salivary glands, liver, gallbladder, and pancreas.

The digestive system works around the clock, even though we seldom notice the complex, coordinated tasks it performs. When you eat something as simple as an apple, you taste it only briefly, but long after you stop thinking about it, your stomach and intestines quietly digest it, absorb its nutrients, and move along anything your body cannot use. In the sections that follow, we will explore the structures that carry out digestion and the processes that turn food into usable fuel.

Figure \(\PageIndex{1}\):  Organs of the Digestive System. As food moves through the organs of the digestive tract it is broken down into component molecules which are absorbed into the body. The accessory organs (salivary glands, liver, pancreas and gallbladder) secrete digestive enzymes and other substances to assist and regulate the processes.  (Image credit: "Components of the Digestive System" by Jennifer Lange and Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0, modification of original by KnowledgeWorks .)

 

Digestive System Organs

The simplest way to understand the digestive system is to divide its organs into two main categories.

The first category is the digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, or simply "gut", which is the long tube food travels through.

The second category is the accessory digestive organs, which release exocrine secretions into the tract to help break food down and allow the body to absorb nutrients. Despite the term “accessory,” these organs are essential for normal digestive function.
 

Digestive Tract Organs

The digestive tract is a one-way tube about 7.62 meters (25 feet) long during life and closer to 10.67 meters (35 feet) after death, when smooth muscle tone is lost. Its primary function is to nourish the body. The tube begins at the oral cavity and ends at the anus, and along the way it is regionally modified as the pharynx, esophagus, stomach, and small and large intestines to meet the body’s needs.

In this system, structures closer to the oral cavity are considered “proximal,” and those closer to the anus are “distal.” Because both the oral cavity and anus open to the external environment, the materials within the tract — food and waste — are technically outside the body. Only through absorption, when nutrients move from the tract into the bloodstream, do they enter and support the body’s internal environment. 
 

Accessory Organs

Each accessory digestive organ contributes to the breakdown of food. In the oral cavity, the teeth and tongue begin mechanical digestion, and the salivary glands begin chemical digestion. Once food enters the small intestine, the gallbladder, liver, and pancreas release secretions such as bile and digestive enzymes that allow digestion to continue. These structures are called accessory organs because they originate as outgrowths of the developing gut (mucosa) and support its function. You could not survive without the essential roles of the liver and pancreas, and many serious diseases result when they malfunction. These organs are connected to the gut through a system of ducts. The accessory digestive organs are summarized in the Table below.

                  

Figure \(\PageIndex{2}\): The Alimentary Canal and its Accessory Organs. On the left is the tube called the alimentary canal with its main subdivisions. On the right the critical accessory organs. (Image credit: "Main Digestive Organs" by John Polos is licensed under CC BY-NC-SA 4.0, modification of original by KnowledgeWorks .)

 

 

     

Organs of the Digestive System

Tract Organ	Accessory Organ
Oral Cavity: Ingestion. Site of mechanical digestion; start of chemical digestion.	Salivary Glands: Produce saliva; empty into oral cavity through ducts.  The three pairs of salivary glands are: parotid glands submandibular glands sublingual glands Teeth: Grinding, tearing, crushing of food. Tongue: Moves food to contact teeth; moves food into pharynx.
Esophagus: Moves food to stomach.
Stomach: Chemical digestion of proteins and mechanical breakdown of food.  Moves food to small intestine.
Small Intestine: Finish chemical digestion and absorbs food. Moves food to large intestine. Three regions are: Duodenum Jejunum Ileum	Liver: Produces bile that emulsifies fat. Empties into duodenum through hepatic ducts. Gallbladder: Stores bile.  Empties into duodenum through cystic duct. Pancreas: Produces pancreatic juice which contains enzymes to digest lipids, proteins, and starch. Empties into duodenum through pancreatic duct(s).
Large Intestine: Reabsorbs water and absorbs some vitamins and minerals; bacteria ferment soluble fiber.  Five regions are: Cecum Ascending Colon Transverse Colon Descending Colon Sigmoid Colon
Rectum: Collects and stores fecal matter.

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The Six Core Processes of Digestion: An Overview

The proper functioning of the gastrointestinal (GI) tract is imperative for our well being and life-long health. A non-functioning or poorly-functioning GI tract can be the source of many chronic health problems that can interfere with your quality of life.

Here is a look at the importance of the digestive system functions grouped into six core processes:
 

Getting Food Ready for the Trip: Ingestion, Moistening, Breakdown, and Swallowing

Digestion begins in the mouth. A brain reflex triggers the flow of saliva when we see or even think about food. Saliva moistens the food and begins its chemical breakdown, and the salivary glands together produce about three liters of saliva each day. The teeth contribute mechanical breakdown by chewing the food into smaller pieces so digestive enzymes can reach it more easily. Once food is moistened, shaped, and ready to swallow it is called a bolus.

Swallowing begins when the muscles of the tongue and mouth move the bolus into the pharynx. This step requires the coordinated action of about twenty-five muscles working at the same time.

The pharynx is a short passageway, only about five inches long, that carries both food and air. A small flap of tissue called the epiglottis closes over the opening to the trachea to keep food from entering the airway and causing choking. The bolus is then directed into the esophagus, a muscular tube that uses coordinated waves of contraction called peristalsis to move food toward the stomach.

While food travels through the digestive tract, it is technically passing through the body rather than being inside the body. Only after nutrients are absorbed do they truly enter the internal environment.

Figure \(\PageIndex{3}\): Stages of Swallowing (Deglutition). Tongue, soft palate, and epiglottis work together to safely move a food bolus from the mouth into the esophagus. In the first panel, the tongue pushes the bolus toward the pharynx. In the middle panel, the soft palate elevates to block the nasal cavity while the larynx rises and begins to close. In the final panel, the epiglottis folds down to cover the trachea so the bolus can enter the esophagus, preventing food from entering the airway. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).

Secretion

Cells along the digestive tract, and in nearby glands, release fluids that help you break down food and protect your organs. With every meal, several liters of fluid enter the digestive tract. These secretions include enzymes, mucus, acids, and bicarbonate-rich fluids, as well as hormones that help coordinate the whole process.

In the mouth, salivary glands release saliva that contains the enzyme amylase, which begins breaking down starches into smaller sugars. In the stomach, glands in the lining secrete hydrochloric acid (HCl), which helps unfold proteins and kills many microbes, and pepsin, an enzyme that begins protein digestion. The stomach also secretes thick mucus that protects its own tissue from being damaged by the acid.

As food moves into the small intestine, the pancreas, liver, and gallbladder add their own secretions. The pancreas releases digestive enzymes that break down carbohydrates, proteins, and fats, along with bicarbonate that neutralizes stomach acid so the intestinal lining is not harmed. The liver produces bile, which is stored in the gallbladder and then squirted into the small intestine to help emulsify fats so enzymes can reach them more easily. Hormones such as gastrin, secretin, and cholecystokinin (CCK) act as chemical messengers that tell different organs when to increase or decrease their secretions, depending on what kind of food has arrived. It is important understanding these secretions matters because problems such as ulcers, pancreatitis, or gallbladder attacks often involve abnormal secretion or blockage of these fluids, which can cause serious pain, bleeding, or changes in vital signs.

Mixing and Propulsion

Mixing and propulsion describe how the digestive tract keeps food moving and blends it with digestive juices. Once you swallow, waves of smooth muscle contraction called peristalsis push the food bolus from the esophagus down toward the stomach. In the stomach, strong muscular contractions churn the food and mix it with acid and enzymes to form a semi-liquid called chyme. This churning action increases contact between the food and digestive secretions so that chemical digestion can proceed efficiently.

As the food — now called chyme — enters the small intestine, the muscle in the intestinal wall continues to move it along, but now with a combination of peristalsis and a back-and-forth action called segmentation. Segmentation does not push food very far forward. Instead, it repeatedly squeezes and relaxes different segments of the intestine so that chyme is thoroughly mixed with enzymes and brought into close contact with the absorptive surface of the intestinal wall. Farther along, in the large intestine, slower peristaltic waves and occasional stronger “mass movements” propel the remaining material toward the rectum.  
 

Digestion

Food is the body’s source of fuel. The nutrients in food give the body’s cells the energy they need to operate. Digestion is the process of breaking food down into small enough pieces that your body can absorb and use. It has two main parts: mechanical digestion and chemical digestion. Mechanical digestion is the physical breakdown of food into smaller pieces. It starts in the mouth, where teeth tear, grind, and mash food. In the stomach, strong muscular contractions continue this process by churning the food into a semi-liquid mixture called chyme. This mechanical breakdown increases the surface area of the food, making it easier for enzymes to do their job.

Chemical digestion uses enzymes and other chemicals to break large molecules into their building blocks. In the mouth, salivary amylase begins breaking down starch. In the stomach, acid helps unfold proteins, and the enzyme pepsin begins to digest them. Most chemical digestion happens in the small intestine, where enzymes from the pancreas and the intestinal lining break carbohydrates into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol. Bile from the liver helps with fat digestion by emulsifying large fat droplets into smaller ones.  
 

Absorption

During absorption, the nutrients that come from food (such as proteins, fats, carbohydrates, vitamins, and minerals) actually enter the body’s internal environment by passing through the wall of the intestinal tract and into the bloodstream. Most absorption happens in the small intestine, which is lined with folds, villi, and microvilli that greatly increase the surface area, making it look like a plush carpet under the microscope. This huge surface allows nutrients to move efficiently from the lumen of the intestine into blood vessels and lymphatic vessels. Simple sugars from carbohydrates and amino acids from proteins are absorbed into tiny blood capillaries, while most fats are packaged into small droplets and absorbed into lymphatic vessels called lacteals. Water, vitamins, and electrolytes are also absorbed, mainly in the small intestine, with additional water and electrolyte absorption in the large intestine.

Once nutrients are absorbed, they do not just float around randomly. Blood from the intestines first travels through the hepatic portal vein to the liver, where many substances are processed, stored, modified, or detoxified before they enter the general circulation. Glucose can be stored as glycogen or released back into the blood to maintain a stable blood sugar level. Amino acids may be used to build new proteins, and fats can be stored in adipose tissue or used for energy. Cells throughout the body use these absorbed nutrients to make ATP, repair tissues, support growth, and maintain normal function.

Waste Compaction and Elimination

Waste compaction and elimination are the final processes of digestion, where the body deals with what it cannot digest or absorb. After most nutrients and a large amount of water have been removed in the small intestine, the remaining material enters the large intestine. Here, the colon slowly moves the contents along while absorbing additional water and electrolytes, which helps compact the material into feces. Helpful gut bacteria living in the large intestine also break down some remaining substances, producing gases and certain vitamins, such as vitamin K, that can be absorbed. Mucus is secreted to help the contents move more smoothly and protect the lining of the colon.

As feces become more solid, they are stored in the rectum until stretch receptors in the rectal wall are stimulated, triggering the defecation reflex. This reflex involves coordinated contraction of the rectum and relaxation of the internal anal sphincter, along with voluntary control over the external anal sphincter, which allows a person to decide when it is socially appropriate to go to the bathroom. If stool moves too slowly, too much water is absorbed and constipation can occur; if it moves too quickly, there is not enough time to reabsorb water, leading to diarrhea and possible dehydration.