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20: The Digestive System

  • Page ID
    12540
  • Skills to Develop

    • Identify the organs of the alimentary canal from proximal to distal
    • Identify the accessory digestive organs

    The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process (Figure 20.1).

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    Figure 20.1 Components of the Digestive System All digestive organs play integral roles in the life-sustaining process of digestion.

    Digestive System Organs

    The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body.

    Accessory digestive organs, despite their name, are critical to the function of the digestive system.

    Alimentary Canal Organs

    Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”

    Accessory Structures

    Each accessory digestive organ aids in the breakdown of food (Figure 20.2). Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes— essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

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    Figure 20.2 Layers of the Alimentary Canal The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.

    The Mouth

    The cheeks, tongue, and palate frame the mouth, which is also called the oral cavity (or buccal cavity). The structures of the mouth are illustrated in Figure 20.3.

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    Figure 20.3 Mouth The mouth includes the lips, tongue, palate, gums, and teeth.

    The Pharynx

    The pharynx (throat) is involved in both digestion and respiration. It receives food and air from the mouth, and air from the nasal cavities. When food enters the pharynx, involuntary muscle contractions close off the air passageways.

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    Figure 20.4 Pharynx The pharynx runs from the nostrils to the esophagus and the larynx.

    The Esophagus

    The esophagus is a muscular tube that connects the pharynx to the stomach. It is approximately 25.4 cm (10 in) in length, located posterior to the trachea, and remains in a collapsed form when not engaged in swallowing. As you can see in Figure 20.5, the esophagus runs a mainly straight route through the mediastinum of the thorax. To enter the abdomen, the esophagus penetrates the diaphragm through an opening called the esophageal hiatus.

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    Figure 20.5 Esophagus The upper esophageal sphincter controls the movement of food from the pharynx to the esophagus. The lower esophageal sphincter controls the movement of food from the esophagus to the stomach.

    Stomach

    There are four main regions in the stomach: the cardia, fundus, body, and pylorus (Figure 20.6). The cardia (or cardiac region) is the point where the esophagus connects to the stomach and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus.

    Below the fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.

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    Figure 20.6 Stomach The stomach has four major regions: the cardia, fundus, body, and pylorus. The addition of an inner oblique smooth muscle layer gives the muscularis the ability to vigorously churn and mix food.

    Histology

    The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal, but with adaptations to the mucosa and muscularis for the unique functions of this organ. In addition to the typical circular and longitudinal smooth muscle layers, the muscularis has an inner oblique smooth muscle layer (Figure 20.7). As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles.

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    Figure 20.7 Histology of the Stomach The stomach wall is adapted for the functions of the stomach. In the epithelium, gastric pits lead to gastric glands that secrete gastric juice. The gastric glands (one gland is shown enlarged on the right) contain different types of cells that secrete a variety of enzymes, including hydrochloride acid, which activates the protein- digesting enzyme pepsin.

    Small Intestine

    The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum (Figure 20.8).

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    Figure 20.8 Small Intestine The three regions of the small intestine are the duodenum, jejunum, and ileum.

    Histology

    The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli (Figure 20.9). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs.

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    Figure 20.9 Histology of the Small Intestine (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli. From left to right, LM x 56, LM x 508, EM x 196,000. (credit b-d: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Large Intestine

    The large intestine runs from the appendix to the anus. It frames the small intestine on three sides. Despite its being about one-half as long as the small intestine, it is called large because it is more than twice the diameter of the small intestine, about 3 inches.

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    Figure 20.10 Large Intestine The large intestine includes the cecum, colon, and rectum.

    Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages (Figure 20.11).

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    Figure 20.11 Teniae Coli, Haustra, and Epiploic Appendages

    Histology

    There are several notable differences between the walls of the large and small intestines (Figure 20.12). For example, few enzyme-secreting cells are found in the wall of the large intestine, and there are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is simple columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the movement of feces and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts as well as vitamins produced by your intestinal bacteria.

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    Figure 20.12 Histology of the large Intestine (a) The histologies of the large intestine and small intestine (not shown) are adapted for the digestive functions of each organ. (b) This micrograph shows the colon’s simple columnar epithelium and goblet cells. LM x 464. (credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    * Enteric neuron.

    LAB 20 EXERCISES 20-1

    A

    Label the parts of a hollow organ: Mucosa

    Image_455.pngImage_454.png

    6

    5

    2

    4

    1

    * Submucosa * Muscularis externa (circular and longitudinal layers) * Serosa * Muscularis mucosa * Peyer’s patch.

    1

    2

    3

    4

    image

    LAB 20 EXERCISES 20-2

    ORAL CAVITY

    B Label the parts of a hollow organ: Villus * Lacteal * Goblet cell * Intestinal crypt * Intestinal gland

    5

    3

    A

    B

    Label the components of the oral cavity:

    Uvula * Gingiva * Oral vestibule * Hard palate

    5

    * Soft palate.

    Label the parts of a tooth: Enamel * Dentin

    1

    4

    3

    4

    Cementum * Pulp cavity * Apical foramen * Root canal * Crown * Root.

    5

    1

    2

    2

    3

    6

    7

    LAB 20 EXERCISES 20-3

    image

    ORAL CAVITY & STOMACH

    A

    B

    Label the 3 major salivary glands: Label the parts of the stomach: Fundus *

    1

    5

    8

    Body * Cardiac region * Pyloric canal * Pyloric antrum * Pyloric sphincter * Cardiac sphincter * Lesser curvature * Greater curvature.

    4

    7

    1

    2

    2

    3

    9

    3

    image

    6

    LAB 20 EXERCISES 20-4

    ABDOMINAL CAVITY

    A

    B

    Label the 4 types of teeth: in one quadrant Label the anatomy of the liver: Right lobe *

    Left lobe * Gall bladder * Hepatic portal vein * Hepatic artery * Inferior vena cava * Falciform ligament.

    4

    7

    2

    2

    1

    5

    4

    3

    6

    3

    1

    LAB 20 EXERCISES 20-5

    image

    BILE * PANCREAS AND INTESTINES

    A

    B

    Label the path of bile and pancreatic secretions: L. Hepatic duct * R. Hepatic duct

    5

    1

    * Cystic duct * Common Bile duct * Common Hepatic duct * Pancreatic duct * Accessory pancreatic duct * Hepato-pancreatic ampulla * Pancreas * Duodenum * Gall bladder

    Label the following: Stomach * Duodenum *

    Jejunum * Ileum * Colon.

    image

    4

    1

    10

    2

    6

    3

    4

    7

    2

    5

    8

    3

    9

    11

    image

    COLON & RECTUM

    A

    Label the parts of the colon: Cecum * Ascending colon * Transverse colon * Descending colon * Sigmoid colon * Appendix * Mesocolon * Hepatic flexure * Splenic flexure * Haustra * Teniae Coli.

    B

    Label the following: Rectum * Anus * Rectal valve * External Anal sphincter * Anal columns.

    7

    1

    8

    5

    1

    9

    2

    3 2

    10

    4 3

    6

    11

    4

    LAB 20 EXERCISES 20-7

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    HISTOLOGY

    A

    B

    Histology of the liver: Hepatic triad * Central vein * Hepatic artery branch * Hepatic Portal vein branch * Bile ductule * Hepatocyte

    * Sinusoid.

    Histology of the liver: Hepatic triad *

    2

    Central vein * Hepatocyte * Sinusoid.

    4

    5

    1

    6

    7

    3

    2

    3

    4

    image

    1

    LAB 20 EXERCISES 20-8

    image

    HISTOLOGY

    A

    B

    (see lab list)

    Identify this tissue and its relevant

    (see lab list)

    1

    3

    components:

    Label the parts of this gland:

    1

    4

    2

    2

    3

    Organ:

    Organ:

    MODELS: Hollow Organs, Teeth, Live/Gall Bladder, Upright Torsos, Mid-Sagittal Head

     Oral vestibule

     Philtrum

     Labia

     Oral cavity

    • Hard & Soft palate

    • Uvula

    • Pharynx (oro- and laryngo-)

    • Esophagus

       Stomach

    • Cardiac sphincter

    • Cardiac region

    • Fundus

    • Body

    • Pyloric antrum

    • Pyloric canal

    • Pyloric sphincter

    • Rugae

    • Lesser & greater curvature

       Small intestine

    • Duodenum

    • Jejunum

    • Ileum

    • Ileocecal valve

       Large intestine

    • Haustra

    • Taeniae coli

    • Epiploic appendages

    • Cecum

    • Vermiform appendix

    • Ascending colon

    • Hepatic flexure (R )

       Large intestine (cont.)

    • Splenic flexure (L)

    • Descending colon

    • Sigmoid colon

       Rectum

    • Rectal valves

       Anal canal

    • Anal columns

       Pancreas

    • Pancreatic duct

    • Hepato-pancreatic ampulla (or papilla)

    • Accessory pancreatic duct

       Teeth

    • Crown

    • Root

    • Enamel

    • Cementum

    • Dentin

    • Pulp

    • Apical foramen

       Dental Formula: 2 *1 *2 *3

    • Incisors * canines * pre-molar (bicuspids) * molars

      (tricuspids)

       Tongue

    • Filiform * Fungiform * Circumvallate * Foliate papillae

    • Sulcus terminalis

    • Median lingual sulcus

    • Foramen cecum

    • Taste buds

    • Minor salivary glands

    • Transverse colon

    • Body

    • Base

      Lab List continues on

       Salivary glands (torso)

    • Parotid (gland & duct)

    • Submandibular

    • Sublingual

       Liver/ Gall bladder

    • Lobes: left * right * caudate * quadrate

    • Left/ * Right and Common hepatic ducts

    • Hepatic portal vein

    • Hepatic vein

    • Hepatic arteries

    • Cystic duct

    • Common bile duct

    • Inferior vena cava

       Hollow Organ models:

    • Mucosa

    • Muscularis mucosa

    • Lacteal

    • goblet cell

    • crypt

       Intestinal gland

    • Submucosa

       Lymphatic nodule (Peyer’s patch)

       Lymphatic vessels

       Enteric neurons

    • Muscularis Externa:

       Circular *

       Longitudinal

    • Adventitia or serosa

    • Peritoneal cavity

      • Visceral & parietal peritoneum

      • Mesentaries

      • Greater omentum

      • Mesocolon

    • Histology:

    • Liver (also see liver histology model)

    • Lobules

      • Triad

      • Central vein

      • hepatocytes

      • Sinusoids

      • Kupffer cells (macrophages)

    • Esophagus

    • Stratified squamous epithelium

    • Tongue

      • Skeletal muscle

      • Taste buds

        • taste pore

        • receptor & support cells

    • Intestines

    • Brush border (microvilli)

      • Simple columnar epithelium

      • Goblet cells

      • Villi

      • Intestinal crypt

      • Intestinal glands

      • MALT

    • Salivary glands

      • Acini

      • Ducts

    • Pancreas

      • Acini

      • Ducts

      • Pancreatic islets (islet of Langerhans)