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The Digestive System: Anatomy

Fun Facts

• Length Comparison: The small intestine is longer than the large intestine.
    - Small Intestine: Approximately 20 feet long.
    - Large Intestine: Approximately 5 feet long.
• Travel Time: Food takes about 7 seconds to travel from mouth to stomach; gravity is not required for food movement through the digestive tract.
• Bacterial Diversity: There are approximately 500 species of bacteria that inhabit the gastrointestinal tract.
• Stomach Protection: The stomach must protect itself due to the production of hydrochloric acid (HCl) and a mucus lining. Breaks in this lining can lead to ulcers. The cause of ulcers was identified in 1982 to be the bacterium Helicobacter pylori, and treatment is now accomplished with antibacterial agents. This discovery earned its discoverers the 2005 Nobel Prize in Physiology or Medicine.

Road Map

The following topics will be covered:

• Functions of the Digestive System
• Overview of Digestive Processes
• Mouth and Associated Organs
• Pharynx and Esophagus
• Stomach
• Gallbladder, Pancreas, Liver
• Small Intestine
• Large Intestine

Functions of the Digestive System

The main functions of the digestive system are:

1. Passage for Food: Provides a route for nutrients to travel.
2. Breaking Down Food: Transform food into nutrient molecules.
3. Absorption: Molecules are absorbed into the bloodstream.
4. Expulsion: Removal of indigestible remains from the body.

Digestive Processes

Processing of food is accomplished through six essential activities:

1. Ingestion: The act of eating.
2. Propulsion: Movement of food through the alimentary canal.
3. Mechanical breakdown: This includes:
      - Chewing (mastication)
      - Mixing food with saliva
      - Churning food within the stomach
4. Digestion: A catabolic process that involves enzymes breaking down food molecules into chemical fragments.
5. Absorption: The passage of digested fragments from the gastrointestinal tract into blood or lymph.
6. Defecation: The elimination of indigestible substances via the anus in the form of feces.

Digestive Anatomy

Two Categories of the Digestive System

1. Alimentary Canal (also known as the gastrointestinal (GI) tract or gut):
      - A continuous muscular tube extending from the mouth to the anus.
      - Functions to digest food and absorb the digested fragments through its lining into the blood.
      - Organs included are: mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus.
2. Accessory Digestive Organs:
      - Teeth
      - Tongue
      - Gallbladder
      - Digestive Glands: These produce secretions to assist with food breakdown.
         - Salivary glands
         - Liver
         - Pancreas

Alimentary Canal and Related Accessory Digestive Organs

• Mouth (oral cavity)
• Tongue
• Salivary glands
    - Parotid gland
    - Sublingual gland
    - Submandibular gland
• Esophagus
• Stomach
• Small intestine
    - Duodenum
    - Jejunum
    - Ileum
• Large intestine
    - Cecum
    - Appendix
    - Colon (ascending, transverse, descending, sigmoid)
• Rectum
• Anus
• Pancreas
• Liver
• Gallbladder

Mouth and Associated Organs

• The mouth is where food is chewed and mixed with enzyme-rich saliva, which begins the process of digestion; the swallowing process also starts here.
• Associated organs include:
    - Tongue
    - Salivary Glands
    - Teeth

Mouth (Oral Cavity)

• Known as the oral (buccal) cavity.
• Oral orifice: The anterior opening of the mouth.
• Epithelium lining: The mouth's walls are lined with stratified squamous epithelium.
    - This lining consists of tough cells that resist abrasion.
    - Cells of the gums, hard palate, and parts of the tongue are keratinized to provide additional protection during eating.
• The mouth marks the beginning of mechanical digestion through chewing, moving, and forming a bolus of food.

Tongue

• The tongue occupies the floor of the mouth and is composed of interlacing bundles of skeletal muscle.
• Functions include:
    - Moving and mixing food with saliva during chewing.
    - Formation of a bolus (a small mass of chewed food).
    - Initiation of swallowing, speech, and taste.
• Muscle Composition:
    - Intrinsic muscles: Change the shape of the tongue.
    - Extrinsic muscles: Change the position of the tongue.

Salivary Glands

• Salivary glands produce saliva that serves several functions:
    - Cleanses the mouth
    - Dissolves food chemicals for taste
    - Moistens food
    - Initiates the breakdown of starchy foods using the enzyme amylase.
• Composition of Saliva:
    - Mostly water (97 – 99.5%).
    - Slightly acidic with a pH of 6.75 – 7.00.
    - Digestive enzymes include amylase and lingual lipase.
    - Proteins: mucin, lysozyme, immunoglobulin A (IgA), defensins which provide protection against microorganisms.

Pharynx

• The pharynx allows the passage of food, fluids, and air.
    - The pathway of food is from the mouth to the oropharynx to the laryngopharynx.
• It is lined with stratified squamous epithelium and contains mucus-producing glands.
• The external muscle layers consist of two types of skeletal muscle:
    - Longitudinal Muscle Layer: Inner layer that runs longitudinally.
    - Pharyngeal Constrictors: The outer layer encircles the wall of the pharynx.

Esophagus

• The esophagus is a flat, muscular tube that extends from the laryngopharynx to the stomach.
• It is collapsed when not involved in food propulsion.
• The gastroesophageal (cardiac) sphincter encircles the cardial orifice (the junction of the esophagus and stomach).
    - The sphincter closes when food is not being swallowed.
    - Mucus cells adjacent to the sphincter help protect the esophagus from acid reflux.

Stomach

• The stomach acts as a storage tank and begins the chemical breakdown of proteins.
    - It converts the bolus into chyme, a paste-like mixture of food and gastric juices.
    - The empty stomach has a volume of approximately 50 ml but can expand to about 4 L.
• Major Regions of the Stomach:
    - Cardial Part (Cardia): Surrounds the cardial orifice.
    - Fundus: Dome-shaped region beneath the diaphragm.
    - Body: The central portion of the stomach.
    - Pyloric Part: The wide region that narrows into the pyloric canal- continuous to the duodenum through the pyloric valve which regulates stomach emptying.

Microscopic Anatomy of the Stomach

• Glands located in the fundus and body of the stomach produce most of the gastric juice.
• Types of Gland Cells in Stomach:
    1. Mucous Neck Cells: Secrete a thin, acidic mucus.
    2. Parietal Cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
      - Function of HCl:
        - Makes contents highly acidic (pH 1.5–3.5), denatures proteins, activates pepsin, breaks down plant cell walls, and destroys many bacteria.
      - Intrinsic Factor: Glycoprotein essential for vitamin B12 absorption in the small intestine.
    3. Enteroendocrine Cells: Secrete various chemical messengers (hormones), including serotonin, histamine, somatostatin, and gastrin.
    4. Chief Cells: Secrete pepsinogen and lipases.
      - Pepsinogen: Inactive enzyme activated to pepsin by HCl, initiating protein digestion.
      - Lipases: Responsible for breaking down approximately 15% of lipids.

Stomach: Regulation of Emptying

• Peristalsis propels chyme toward the pyloric valve.
• The pyloric valve opens briefly to allow small quantities of chyme to pass into the small intestine.
    - Controlled emptying prevents overwhelming the duodenum.
• The stomach typically empties within 4 hours; however, the presence of fatty chyme in the duodenum can extend this time to 6+ hours.

Liver, Gallbladder, and Pancreas

• The liver, gallbladder, and pancreas are accessory organs associated with the small intestine.
• Liver: Responsible for bile production.
• Gallbladder: Acts as a storage site for bile.
• Pancreas: Supplies most of the enzymes required to digest chyme and bicarbonate to neutralize stomach acid.

Liver

• The liver is the largest gland in the body, weighing about 3 lbs.
• It is divided into four lobes:
    - Right lobe
    - Left lobe
    - Caudate lobe
    - Quadrate lobe
• Primary Functions of the Liver:
    - Producing bile
    - Receiving nutrient-rich blood after absorption via the hepatic portal vein (hepatic refers to the liver)
    - Processing, storing, and distributing nutrients.

Liver: Bile

• Bile: A fat-emulsifying digestive fluid that assists in fat breakdown for absorption.
• It is a yellow-green, alkaline solution composed of:
    - Bile Salts: Derivatives of cholesterol that facilitate fat digestion and absorption.
    - Bilirubin: A waste product derived from heme during the breakdown of worn-out red blood cells (RBCs).
    - Other components include cholesterol, triglycerides, phospholipids, and electrolytes.
• Enterohepatic Circulation: A mechanism that recycles bile salts for subsequent use.
    - The liver secretes bile salts into the small intestine, where they assist in fat digestion and absorption.
    - The ileum (the last part of the small intestine) reabsorbs approximately 95% of bile salts, which return to the liver via the hepatic portal blood for resecretion.
    - Disruption of this circulation can lead to fat malabsorption, vitamin deficiencies, and an increased risk of gallstones.

Gallbladder

• The gallbladder is a thin-walled muscular sac located on the ventral surface of the liver.
• Primary Functions:
    - Stores bile
    - Concentrates bile by extracting water and ions, resulting in "stronger" bile for more effective fat emulsification.

Pancreas

• The pancreas is located deep to the greater curvature of the stomach and connects to the duodenum.
• Primary Function: To produce enzymes that break down food substances.
• The pancreas features both exocrine and endocrine components with differing functions:
    - Exocrine Part: Produces pancreatic juice.
    - Endocrine Part: Secretes insulin and glucagon.

Pancreatic Juice

• The exocrine pancreas produces approximately 1200 to 1500 mL per day of this secretion.
• It is a watery, alkaline solution (pH 8) that neutralizes the acidic chyme that comes from the stomach.
• Composition of Pancreatic Juice:
    - Contains digestive enzymes that break down macromolecules:
      - Proteases: Act on proteins.
      - Amylase: Acts on carbohydrates.
      - Lipases: Act on lipids.
      - Nucleases: Act on nucleic acids.

Small Intestine

• The small intestine is the principal organ of digestion and absorption, extending from the pyloric sphincter to the ileocecal valve, where it joins the large intestine.
• Dimensions:
    - Length: 2–4 meters (7–13 feet).
    - Diameter: 2.5–4 cm (1.0–1.6 inches).
• Subdivisions:
    - Duodenum: Approximately 25 cm (10 inches) long; connects to the pancreas.
    - Jejunum: Approximately 2.5 meters (8 feet) long.
    - Ileum: Approximately 3.6 meters (12 feet) long; connects to the large intestine at the ileocecal valve.

Small Intestine Absorption

• The considerable length of the small intestine and its structural adaptations contribute to a large surface area for nutrient absorption, approximately 200 m² (comparable to the size of a tennis court).
• Increased Surface Area Is Achieved By:
    - Circular Folds: These enforce a spiraling motion for chyme, ensuring extended time for nutrient absorption.
    - Villi: Fingerlike projections of the mucosa containing a dense capillary bed and a lymphatic capillary known as a lacteal for absorption.
    - Microvilli: Extensions of the mucosal cells referred to as brush border that house enzymes (brush border enzymes) to assist in carbohydrate and protein digestion.

Intestinal Digestive Juice

• Primary Functions:
    - Provides a liquid medium for digestion and absorption.
    - Protects the intestinal lining.
    - Lubricates the intestinal contents.
• Composition:
    - Mostly composed of water, with some mucus secreted by duodenal glands and goblet cells of the mucosa.
• Approximately 1-2 liters of digestive juice is secreted daily in response to:
    - Distension of the intestinal wall
    - Irritation of the mucosa
    - The presence of acidic chyme.

Large Intestine

• The primary function of the large intestine is to absorb water and eliminate feces.
• Three Unique Features of the Large Intestine:
    1. Teniae Coli: Three bands of longitudinal smooth muscle.
    2. Haustra: Pocket-like sacs created by the tone of the teniae coli.
    3. Epiploic Appendages: Fat-filled pouches that protrude from the serosa.

Subdivisions of the Large Intestine

• The large intestine serves as a part of the mucosa-associated lymphoid tissue (MALT) of the immune system and is a bacterial storehouse that can recolonize the gut when necessary.
• Cecum: The first part of the large intestine.
• Appendix: Consists of lymphoid tissue and has a twisted shape that makes it susceptible to blockages.

Divisions of the Colon

• The colon consists of several regions:
    - Ascending Colon: Ascends the right side of the abdominal cavity.
    - Transverse Colon: Travels across the upper abdominal cavity.
    - Descending Colon: Descends the left side of the abdominal cavity.
    - Sigmoid Colon: The S-shaped portion that traverses the pelvis.

Subdivisions of the Large Intestine

• Rectum: Houses three rectal valves that prevent feces from passing alongside gas (flatus).
• Anal Canal: The terminal segment of the large intestine which opens to the exterior at the anus.
    - Contains two sphincters:
      - Internal Anal Sphincter: Composed of smooth muscle.
      - External Anal Sphincter: Composed of skeletal muscle.

Review Questions

• Includes lab activities and the Anatomage table.

Updates and Reminders

• New Exam 3 Dates: 4/15 and 4/16
    - In-Class Review: 4/13 or 4/14
    - Tutor Led Review: To Be Determined (TBD)
• Holiday Week Schedule Changes:
    - No class on Thursday, 4/2;
    - M/W sections will meet twice (Urinary System, 1st Reproductive Lecture)
    - T/Th sections will meet only on Tuesday, 3/31 (Urinary System)
    - Week of 4/6: No class Monday, 4/6; Wednesday follows Monday's schedule
    - M/W sections will meet only on Wednesday (2nd Reproductive Lecture)
    - T/Th sections will meet twice (Reproductive Lectures 1 and 2)
• Combining Sections: Dr. James on maternity leave (estimated to start week of 4/20).
    - M/W 12 pm (sections 4&5) will meet in Mondor 201
    - T/Th 10 am (sections 6&7) will meet in Mondor 201
    - T/Th 12 pm (sections 8&9) will meet in Mondor 201