Digestive System Lecture Notes

Parts of the Digestive System

• Composed of two main parts:
  • Gastrointestinal tract (GI tract, or alimentary canal).
  • Accessory digestive organs.
• Forms a continuous tube from the mouth to the anus through which ingested material is propelled.
• Includes:
  • Oral cavity.
  • Pharynx.
  • Esophagus.
  • Stomach.
  • Small intestine (duodenum, jejunum, and ileum).
  • Large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal).
• Accessory digestive organs are found along the GI tract path and connected to it, assisting in the digestion of ingested material.
• Include:
  • Teeth.
  • Tongue.
  • Salivary glands.
  • Liver.
  • Gallbladder.
  • Pancreas.

Embryological Development of the Digestive System

• Primary germ layer: endoderm.
• Mesoderm also contributes to development.
• Development begins at the end of week 3:
  • Folding of embryo transforms the endoderm and lateral parts of the mesoderm into a primitive tube.
  • Connects to the mouth and anus.
• Dorsal mesentery:
  • Double-layered membrane that helps anchor the gut tube to the posterior wall.
• Ventral mesentery:
  • Membrane that connects a portion of the gut tube.
  • Forms two mesenteries associated with the fully developed GI tract.
  • Membranous folds supporting some digestive organs.
• Gut tube divisions:
  • Foregut (proximal).
  • Midgut (medial).
  • Hindgut (distal).
• Portions of the tube expand to form abdominal organs.
• Some accessory digestive organs develop as outgrowths from the gut tube, often rotating/shifting before final form.

Foregut, Midgut, and Hindgut

Foregut:

• Gastrointestinal organs:
  • Stomach.
  • First part of the duodenum (small intestine).
• Accessory digestive organs:
  • Liver.
  • Gallbladder.
  • Pancreas.

Midgut:

• Gastrointestinal organs:
  • Most of the duodenum.
  • Jejunum.
  • Ileum.
  • Cecum.
  • Ascending colon.
  • Proximal 2/3 of the transverse colon.

Hindgut:

• Gastrointestinal Organs:
  • Distal 1/3 of transverse colon.
  • Descending colon.
  • Sigmoid colon.
  • Rectum.
  • Superior part of the anal canal.

Peritoneum

• Serous membrane lining abdominopelvic cavity.
• Serous membranes include:
  • Outer parietal layer lining the internal surface of the body cavity.
  • Inner visceral layer lining the external surface of some organs in the cavity.
  • Potential space between layers containing serous fluid.
• Peritoneum consists of two layers with a potential space in between called the peritoneal cavity.

Parietal Peritoneum

• Outer layer of the peritoneum, lining the walls of the abdominopelvic cavity.

Visceral Peritoneum

• Inner layer of the peritoneum, lining the surface of organs within the abdominopelvic cavity.

Peritoneal Cavity

• Potential space between the two layers of the peritoneum.
• Contains peritoneal fluid, which reduces friction during the movement of abdominopelvic organs.
• Divided into two spaces:
  • Greater sac.
  • Lesser sac.
• Formed as a result of changes in the dorsal and ventral mesenteries and the development of certain digestive organs.
• Peritoneum forms folds (mesenteries) as it reflects from the walls of the abdominopelvic cavity (parietal peritoneum) onto the surface of the organs (visceral peritoneum).
• Mesenteries include:
  • Mesentery proper.
  • Greater omentum.
  • Lesser omentum.
  • Mesocolon.
• Greater and lesser omenta develop from the dorsal and ventral mesenteries.

Lesser Omentum

• Peritoneal fold extending from the lesser curvature of the stomach, connecting the stomach and duodenum to the liver.
• Develops from the ventral mesentery.

Mesocolon

• Mesentery of the large intestine.
• Peritoneal fold suspending parts of the large intestine from the posterior abdominal wall.
• Parts named according to the part of the large intestine they suspend:
  • Transverse mesocolon (suspends the transverse colon).
  • Sigmoid mesocolon (suspends the sigmoid colon).

Mesentery Proper

• Commonly referred to as “the mesentery.”
• Peritoneal fold that suspends most of the small intestine from the posterior abdominal wall.

Intraperitoneal vs. Retroperitoneal Digestive Organs

• Classification based on relationship to the peritoneum.
• Intraperitoneal:
  • Organs completely surrounded by visceral peritoneum.
• Retroperitoneal:
  • Organs only covered by the parietal peritoneum on their anterior surfaces.
  • 'Retro' = behind.
• Intraperitoneal organs are associated with the peritoneal folds.

Intraperitoneal Digestive Organs:

• Stomach.
• First part of the duodenum.
• Jejunum.
• Ileum.
• Cecum.
• Transverse colon.
• Sigmoid colon.
• Liver (except for a small area).
• Gallbladder.
• Tail of the pancreas.

Retroperitoneal Digestive Organs:

• Esophagus.

• Most of the duodenum.

• Most of the pancreas.

• Ascending colon.

• Descending colon.

• Rectum.

• Mnemonic: SAD PUCKER

Functions of the Digestive System

• Six main functions:
  • Ingestion.
  • Motility.
  • Secretion.
  • Digestion.
  • Absorption.
  • Elimination of wastes.

Ingestion

• Introduction of solids and liquids into the oral cavity.
• First step in digesting and absorbing nutrients.
• Bolus: soft mass of ingested material chewed and mixed with saliva.

Motility

• Movement of ingested material through the GI tract.
• Involves peristalsis and mixing.
  • Peristalsis: wave-like muscular contractions propelling material along the GI tract.
  • Occurs in the esophagus, small intestine, and large intestine.
  • Mixing: churning movements that dispense ingested material and combine it with digestive secretions.
  • Occurs in the stomach and small intestine.

Secretion

• Production and release of substances into the lumen of the GI tract, such as mucous, acid, bile, and digestive enzymes.
  • Mucous: serves as a protective function, coating the lumen to prevent damage.
  • Acid, bile, and digestive enzymes aid in chemical digestion.
• Digestive secretions produced in the stomach, small intestine, liver, and pancreas.
• Chyme: paste-like substance formed when ingested material is broken down and mixed with secretions.

Digestion

• Breaking down material into smaller, usable components through chemical and mechanical digestion.
  • Mechanical: physical breakdown through chewing in the oral cavity and mixing in the stomach and small intestine.
  • Chemical: breakdown by digestive enzymes into smaller molecules.
  • Begins in the oral cavity, mainly occurring in the small intestine.

Absorption

• Passage of nutrients, vitamins, electrolytes, and water into blood and lymphatic vessels within the wall of the GIT.
  • Occurs in the small intestine.
  • Large intestine involved in water absorption.

Elimination of Wastes

• Removal of indigestible material and waste products secreted by digestive organs through defecation.
• These materials and waste products are compacted into feces in the large intestine.