A&P 2 - Small&Large Intestines

Unique Features of the Stomach

  • Muscularis Layer:

    • The stomach has three layers of smooth muscle:
    • Oblique Layer: Unique to the stomach; allows contraction at multiple angles.
    • Circular and Longitudinal Layers: Present in most other organs, but the stomach's oblique layer enhances its capacity to crush and squeeze food effectively.

  • Functions of the Stomach:

    1. Food Reservoir:
    • Holds food until it's ready to move further along the digestive tract.
    1. Secretion of Gastric Juice:
    • Aids in mechanical and chemical breakdown of food, mixing it for a more efficient digestion.
    1. Intrinsic Factor:
    • Vital for Vitamin B12 absorption, crucial for red blood cell production.
    1. Limited Absorption:
    • Capable of absorbing water, alcohol, and NSAIDs.
    1. Hormonal Production:
    • Secretes hormones such as gastrin (increases stomach activity) and ghrelin (appetite regulation).
    1. Protection from Pathogens:
    • Hydrochloric acid (HCl) secreted in gastric juice kills bacteria that may enter with food.

Anatomy of the Stomach

  • Regions of the Stomach:

    • Cardia: Connects to the esophagus.
    • Fundus: The upper left portion of the stomach.
    • Body: The central section comprising the majority of the stomach.
    • Pylorus: Connects to the small intestine.

  • Gastric Pits:

    • Depressions in the stomach lining that increase surface area and contain gastric glands responsible for producing gastric juices.

  • Cells in Gastric Pits:

    1. Chief Cells: Secrete pepsinogen, which is converted to the active form pepsin for protein digestion.
    2. Parietal Cells: Secrete hydrochloric acid and intrinsic factor.

Small Intestine Overview

  • Overview:

    • Long, coiled tube stretching approximately 18 feet (6 meters) from the pyloric portion of the stomach to the large intestine. Named "small" due to its narrow diameter compared to the large intestine (approximately 6 cm wide).

  • Components:

    1. Duodenum:
    • First C-shaped portion, about 25 cm long, connects to the stomach.
    1. Jejunum:
    • Middle portion, approximately 2.5 meters long, major site for absorption.
    1. Ileum:
    • Distal portion, about 3.5 meters long, continues to small intestine's function of absorption.

Absorption in the Small Intestine

  • Villi: Projections from the wall of the small intestine that increase surface area for absorption.

  • Microvilli: Tiny hair-like structures on villi that facilitate absorption by increasing surface area further.

  • Plicae: Folds in the intestinal lining that are lined with villi, enhancing the sodium and nutrient absorption.

  • Absorption Process: Nutrients are absorbed through epithelial cells in the villi and enter blood vessels or lymphatic systems for distribution.

Purpose and Location of the Small Intestine

  • Absorption and Enzyme Interaction: Most absorption occurs in the jejunum due to high surface area and vascular supply, while enzymes from the pancreas and bile from the bile duct are secreted into the duodenum to aid in food breakdown before absorption.

Large Intestine Overview

  • Structure:

    • Consists of cecum, colon (with ascending, transverse, descending, and sigmoid regions), rectum, and anal canal.
    • Diameter is 2-3 times larger than the small intestine (about 6 cm wide) but shorter in length (5-6 feet long).

  • Function:

    • The primary role is to absorb remaining water and nutrients, and form waste into feces through muscular contractions and secretions of mucus for lubrication.

Components of the Large Intestine

  1. Cecum:

    • Connects to the ileum of the small intestine; first component of the large intestine, located in the lower right quadrant.

  2. Colon:

    • Ascending Colon: Moves upward from the cecum.
    • Transverse Colon: Runs horizontally across the abdomen.
    • Descending Colon: Moves downward on the left side.
    • Sigmoid Colon: S-shaped portion leading into the rectum.

  3. Rectum and Anal Canal:

    • Final components of the digestive tract, storing waste until expelled.

  4. Appendix:

    • A small pouch attached to the cecum; functionally considered a reservoir for gut bacteria.

Peritoneum and Its Function

  • Peritoneum:

    • Membrane surrounding abdominal organs.

  • Mesentery:

    • Extension of the peritoneum attaching to the intestines, allowing for structure and some movement.

  • Extra/Retroperitoneal:

    • Organs like kidneys are located behind the peritoneum, while most gastrointestinal organs are within the peritoneal cavity.