Gastrointestinal Anatomy and Physiology

Introduction

  • Overview of the digestive system, focusing on the small intestine and large intestine.

Small Intestine (SI)

  • Definition: The small intestine (abbreviated as SI) is a crucial part of the digestive system responsible for further digestion and nutrient absorption. It connects from the pylorus of the stomach to the proximal part of the large intestine.

  • Anatomy:

    • Pylorus: Narrow ending/hallway from the stomach into the small intestine.

    • Location: Attaches to the dorsal wall of the abdomen via the mesentery.

    • Mesentery: A film that supports and contains the intestinal structure, keeping it organized.

  • Functions:

    • Main Function: The primary function of the small intestine is to absorb nutrients (also referred to as ingesta). It further digests food already initiated in the stomach.

    • Water Absorption: It absorbs some water, though more is absorbed in the large intestine.

  • Medical Terminology:

    • Combining Form for Intestine: "Entero"

    • Terms:

    • Enteritis: Inflammation of the small intestine.

    • Gastroenterology: Study of the stomach and intestines.

    • Enterotomy: Surgical incision into the intestine to remove blockages.

  • Segments of the Small Intestine:

    • Duodenum: The most proximal segment, where acidic food from the stomach is neutralized.

    • Combining form: "Duodeni" or "Duodeno."

    • Jejunum: The middle portion that connects the duodenum to the ileum.

    • Ileum: The distal portion, connecting the jejunum to the large intestine.

    • Distinguishing fact: "Ileum" with an 'e' pertains to the intestine, while "ilium" with an 'i' refers to a pelvic bone.

Large Intestine (LI)

  • Definition: The large intestine includes several segments and varies in shape and size across species, primarily involved in the absorption of water and waste processing.

  • Anatomy: Consists of four main parts:

    • Cecum: Structure at the beginning of the large intestine. Functions as a fermentation site in nonruminant herbivores (hindgut fermenters).

    • Combining form: "Ceca."

    • Colon: Where most water absorption occurs (90% of water absorption).

    • Combining form: "Colo."

    • Rectum: Terminal section of the large intestine, stores waste before excretion.

    • Combining form: "Rectal."

    • Anus: The opening through which waste is expelled from the body.

    • Combining form: "Anal."

  • Differences Across Species:

    • In nonruminate herbivores like rabbits and horses, the cecum is large and vital for digestion.

    • In monogastrics (e.g., dogs, cats), the cecum is smaller, functioning minimally.

  • Physiological Roles:

    • Water Absorption: The rectum is where 90% of water absorption occurs, allowing for fluid administration rectally if needed.

    • Digestion and Waste: The colon further metabolizes materials and prepares waste for defecation.

Accessory Organs of Digestion

  • Functions:

    • Excrete enzymes for chemical digestion.

    • Process food for nutrient extraction.

    • Regulate blood sugar levels.

    • Moisten food for easier passage through the digestive tract.

  • Four Main Accessory Organs:

    1. Salivary Glands: Produce saliva containing enzymes.

    2. Liver: Key functions include:

    • Glycogen Storage: Stores excess glucose as glycogen.

    • Vitamins and Iron Storage: Stores vitamins and iron necessary for bodily functions.

    • Toxins Filtering: Removes harmful substances from the blood.

    • Protein Production: Produces various proteins including clotting factors (e.g., albumin).

    • Bile Production: Bile aids in the digestion of fats by emulsifying them.

      • Gallbladder: Stores bile produced by the liver.

      • Not present in species like horses or rats.

      • Combining form: "Cholecysto."

      • Cholecystitis: Inflammation of the gallbladder.

    1. Pancreas:

    • Located near the duodenum; secretes pancreatic enzymes essential for digestion such as:

      • Lipase: Breaks down lipids.

      • Amylase: Breaks down carbohydrates.

      • Trypsin: Breaks down proteins.

    • Insulin: Hormone regulating blood sugar; an essential factor in diabetes management.

Digestion and Nutrient Metabolism

  • Overview of Digestion: The process involves breaking down food into usable nutrients; both mechanical (chewing) and chemical (enzyme action) components are involved.

  • Key Terms:

    • Prehension: Grasping or taking hold of food.

    • Mastication: The act of chewing food.

    • Peristalsis: Wave-like muscle contractions that move food through the digestive tract.

  • Metabolism: Converting digested food into energy; includes:

    • Anabolism: Building up of cells and substances.

    • Catabolism: Breaking down of cells and substances.

  • Absorption: Process of transferring digested nutrients into the bloodstream, often occurring in the small intestine.

    • Assimilation: Refers to the transformation of nutrients into practical forms utilized by the body.

Conclusion

  • Summary of the roles played by the small intestine, large intestine, and accessory organs in digestion.

  • Importance of understanding the structure and function of the GI tract for practical veterinary and medical applications.