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Medicine LibreTexts

22.1C: Organs of the Digestive System

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  • The organs of the digestive system can be divided into upper and lower digestive tracts. The upper digestive tract consists of the esophagus, stomach, and the small intestine; the lower tract includes all of the large intestine, the rectum, and anus.


    Outline the relationship, structure, and function of the digestive organs


    Key Points

    • The gastrointestinal tract is made up of upper and lower tracts.
    • Food moves from the mouth to the stomach via the esophagus.
    • The small intestine has three parts: the duodenum, jejunum, and ileum.
    • The large intestine has four parts: the cecum, colon, rectum, and anus.

    Key Terms

    • small intestine: A winding, digestive tube and the site of large scale nutrient absorption comprised of the duodenum, jejunum, and ileum.
    • esophagus: An organ in vertebrates that is a muscular tube through which food passes from the pharynx to the stomach.
    • stomach: An organ in animals that stores and breaks down food in the process of digestion.
    • large intestine: The second to last part of the digestive system comprised of the cecum and colon.

    The human body uses a variety of mental and physiological cues to initiate the process of digestion. Throughout our gastrointestinal (GI) tract, each organ serves a specific purpose to bring our food from the plate to a digestible substance from which nutrients can be extracted.

    The Digestive Tube

    This diagram shows the relationship between the various organs of the digestive system. It shows how the oral cavity connects to the esophagus and descends into the stomach and then the small intestine. It then connects to the large intestine, then the rectum, and, finally, the anus.

    The organs of the gastrointestinal tract: This diagram shows the relationship between the various organs of the digestive system. It shows how the oral cavity connects to the esophagus and descends into the stomach and then the small intestine. It then connects to the large intestine, then the rectum, and, finally, the anus.

    Our digestive system is like a long tube, with different segments doing different jobs. The major organs within our digestive system can be split into two major segments of this tube: the upper gastrointestinal tract, and the lower gastrointestinal tract.

    The Upper Gastrointestinal Tract

    The upper gastrointestinal, or GI, tract is made up of three main parts:

    1. The esophagus.
    2. The stomach.
    3. The small intestine.

    The Lower Gastrointestinal Tract

    The lower GI tract contains the remainder of the system:

    1. The large intestine.
    2. The rectum.
    3. The anus.

    The exact dividing line between upper and lower tracts can vary, depending on which medical specialist is examining the GI tract.

    Food Breakdown and Absorption: The Upper GI Tract

    When we take a bite of food, the food material gets chewed up and processed in the mouth, where saliva begins the process of chemical and mechanical breakdown. The chewing process is also known as mastication.

    When we mix up food with saliva, the resulting mushy wad is called a bolus. The bolus gets swallowed, and begins its journey through the upper gastrointestinal tract.

    The Esophagus

    The upper GI tract begins with the esophagus, the long muscular tube that carries food to the stomach. The throat cavity in which our esophagus originates is known as the pharynx. As we swallow, the bolus moves down our esophagus, from the pharynx to the stomach, through waves of muscle movement known as peristalsis. Next the bolus reaches the stomach itself.

    The Stomach

    The stomach is a muscular, hollow bag that is an important part of the upper GI tract. Many organisms have a variety of stomach types, with many segments or even multiple stomachs. As humans, we have only one stomach.

    Here our bolus gets mixed with digestive acids, furthering breakdown of the bolus, and turning the bolus material into a slimy mess called chyme. The chyme moves on into the small intestine, where nutrients are absorbed.

    The Small Intestine

    The small intestine is an impressive digestive tube, spanning an average of 20 feet in length. The twists and turns of the small intestine, along with tiny interior projections known as villi, help to increase the surface area for nutrient absorption.

    This snaking tube is made up of three parts, in order from the stomach:

    1. The duodenum.
    2. The jejunum.
    3. The ileum.

    As the chyme makes its way through each segment of the small intestine, pancreatic juices from the pancreas start to break down proteins. Soapy bile from the liver, stored in the gallbladder, gets squirted into the small intestine to help emulsify—or break apart—fats.

    Now thoroughly digested, with its nutrients absorbed along the path of the small intestine, what remains of our food gets passed into the lower GI tract.

    Waste Compaction and Removal: The Lower Gastrointestinal Tract

    The Large Intestine (Colon)

    Following nutrient absorption, the food waste reaches the large intestine, or colon. The large intestine is responsible for compacting waste material, removing water, and producing feces —our solid-waste product.

    Accessory organs like the cecum and appendix, which are remnants of our evolutionary past, serve as special pockets at the beginning of the large intestine. The compacted and dried-out waste passes to the rectum, and out of the body through the anus. Healthy gut bacteria in the large intestine also help to metabolize our waste as it finishes its journey.