Digestive System Lecture Notes

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.

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.

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

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.

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

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

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).

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

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.

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.

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.

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.

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.

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.