Chapter 23: The Digestive System

23.1 Overview Of the Digestive System

  • The function of the digestive system:

    • Breaks down foods, releases nutrients, and absorbs them into the body.

    • The small intestine is the primary site for digestion and nutrient absorption into the blood or lymph.

    • Each digestive organ contributes to the nutrient absorption process.

  • Interrelation with other body systems:

    • The digestive system works cooperatively with other systems (e.g., cardiovascular and endocrine systems).

      • Ex. Arteries supply organs with oxygen and processed nutrients.

      • While intestinal veins (the hepatic portal system) drain the digestive tract and lead to the liver for nutrient processing.

        • the hepatic portal system, which directs blood from the digestive organs to the liver instead of back to the heart.

          • These nutrients are off-loaded for processing before blood completes its circuit back to the heart.

    • The digestive system provides nutrients and support to muscle and vascular tissues.

    • Hormones from endocrine glands ( and endocrine cells of the pancreas, the stomach, and the small intestine) contribute to control of digestion and nutrient metabolism.

    • The digestive system supports overall homeostasis by providing nutrients for endocrine function.


Digestive System Organs

  • The digestive system is divided into two main categories:

    • Alimentary canal organs

    • Accessory digestive organs

      • Critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body

  • Alimentary Canal Organs:

    • The alimentary canal is a one-way tube, approximately:

      • 7.62 meters (25 feet) in length during life

      • 10.67 meters (35 feet) in length after death (once smooth muscle tone is lost)

    • Also called the gastrointestinal (GI) tract or gut.

    • Main function: nourish the body.

    • Begins at the mouth and terminates at the anus.

    • Between these points, the canal is modified as:

      • Pharynx

      • Esophagus

      • Stomach

      • Small and large intestines

    • Both mouth and anus are open to the external environment.

      • so food and wastes within the canal are considered outside the body until absorbed.

    • Nutrients enter into and nourish the body’s "inner space."

  • Accessory Structures:

    • Each accessory digestive organ aids in the breakdown of food.

    • Mechanical digestion begins in the mouth with the teeth and tongue.

      • Salivary glands begin chemical digestion.

    • Once foods enter the small intestine, the gallbladder, liver, and pancreas release essential secretions (bile, enzymes) for digestion.

      • These structures are called accessory organs as they develop from the lining cells of the gut and augment its function.

        • Even after development they maintain connection to the gut by way of ducts.

      • Vital contributions from these organs are necessary for life.

    • Malfunctions can lead to significant diseases.

Histology of the Alimentary Canal

  • The 4 tissue layers of the alimentary tract have details that vary to fit their specific functions.

  • The alimentary tract is composed of four tissue layers throughout its length:

    • Mucosa

    • Submucosa

    • Muscularis

    • Serosa (continuous with the mesentery)

  • Starting from the lumen and moving outwards

  • Mucosa:

    • Referred to as a mucous membrane due to mucus production, characteristic of gut epithelium.

    • Composed of:

      • Epithelium (in direct contact with ingested food)

      • Lamina propria ( layer of connective tissue similar to the dermis)

      • Muscularis mucosa (thin, smooth muscle layer)

    • Epithelium:

      • In the mouth, pharynx, esophagus, and anal canal: primarily a non-keratinized, stratified squamous epithelium.

      • In the stomach and intestines: simple columnar epithelium.

      • The epithelium is in direct contact with the lumen.

      • Epithelial cells:

        • Goblet cells secrete mucus and fluid into the lumen;

        • enterendocrine cells secrete hormones into interstitial spaces between cells.

      • Lifespan of epithelial cells:

        • Mouth: a couple of days

        • Gut: about a week

      • Process of rapid cell renewal preserves health despite wear from food contact.

    • Lamina Propria:

      • Loose connective tissue with numerous blood and lymphatic vessels.

        • These vessels and blood transports nutrients absorbed through the alimentary canal to other parts of the body.

      • Serves an immune function by:

        • Housing clusters of lymphocytes (mucosa-associated lymphoid tissue - MALT).

          • Lymphocyte clusters substantial in distal ileum (Peyer's patches).

      • Plays a role in defending against pathogens encountered in the alimentary canal.

    • Muscularis mucosae:

      • Thin layer of smooth muscle in a constant state of tension, pulling the mucosa into undulating folds.

      • These folds increases surface area for digestion and absorption.

  • Submucosa:

    • Lies immediately beneath the mucosa.

    • A broad layer of dense connective tissue connects the mucosa to the underlying muscularis.

    • Contains blood and lymphatic vessels (transport absorbed nutrients).

    • Houses submucosal glands that release digestive secretions.

    • Serves as a conduit for the submucosal plexus (a dense nerve network).

  • Muscularis (also called the muscularis externa):

    • Composed of a double layer of smooth muscle (inner circular and outer longitudinal layers).

      • Contractions of layers promote mechanical digestion, exposes more food to digestive chemicals, and moves food along the canal.

    • Involves skeletal muscle in some regions for voluntary control in the most proximal and distal regions(e.g., esophagus, mouth).

      • This includes the mouth, pharynx, anterior part of the esophagus, and the external anal sphincter.

    • Basic two-layer structure found in the small intestine, modified in other organs.

    • Stomach provides churning function through the additional oblique muscle layer.

    • Colon has two layers but longitudinal layer appears as three narrow parallel bands (tenia coli).

  • Serosa:

    • Portion superficial to the muscularis in the abdominal cavity.

    • Consists of a layer of visceral peritoneum and loose connective tissue.

    • Differs from organs like the stomach, esophagus which have dense collagen sheaths (adventitia).

    • Adventitia helps stabilize the alimentary canal in place near the vertebral column.

Nerve Supply

  • As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves.

    • Without these nerves:

      • Food would lack taste.

      • Individuals would be unable to feel the structures of their mouth.

      • Individuals would be unable to avoid biting themselves while chewing, an action enabled by motor branches of cranial nerves.

  • Intrinsic innervation of much of the alimentary canal:

    • Provided by the enteric nervous system, which runs from the esophagus to the anus.

    • Contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system).

    • Enteric neurons are grouped into two plexuses:

      • Myenteric plexus (plexus of Auerbach):

        • Located in the muscularis layer of the alimentary canal.

        • Responsible for motility, especially the rhythm and force of contractions of the muscularis.

      • Submucosal plexus (plexus of Meissner):

        • Lies in the submucosal layer.

        • Responsible for regulating digestive secretions and reacting to the presence of food.

  • Extrinsic innervations of the alimentary canal:

    • Provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves.

    • Sympathetic activation (fight-or-flight response):

      • Restricts the activity of enteric neurons, thereby decreasing GI secretion and motility.

    • Parasympathetic activation (rest-and-digest response):

      • Increases GI secretion and motility by stimulating neurons of the enteric nervous system.

Blood Supply

  • The blood vessels serving the digestive system have two functions:

    • Transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen.

    • Supply the organs of the alimentary canal with the nutrients and oxygen needed for their cellular processes.

  • Specifically,:

    • The more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta.

    • Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta.

    • The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines.

  • The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system.

    • This venous network takes the blood into the liver where the nutrients are either processed or stored for later use.

    • Only then does the blood drained from the alimentary canal viscera circulate back to the heart.

    • To appreciate how demanding the digestive process is on the cardiovascular system, consider that while you are "resting and digesting," about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines.