small & large intestines

Comparison of the Small and Large Intestines

  • Location and Gross Anatomy
    • Small Intestine
    • Known as the primary digestive organ and part of the alimentary canal.
    • Length: Approximately 3.05 meters (10 feet) in a living person; about twice as long in a cadaver.
    • Diameter: Approximately 2.54 cm (1 in), comparatively smaller than the large intestine.
    • Surface Area: Enormous surface area of approximately 200 m², essential for absorption and digestion processes.
    • Structure: Subdivided into three regions:
      • Duodenum (25.4 cm or 10 in), starts at the pyloric sphincter, includes four segments (superior, descending, horizontal, ascending) and is retroperitoneal.
      • Jejunum (approximately 0.9 meters or 3 feet long).
      • Named 'jejunum' which means empty in Latin due to its appearance at death.
      • Ileum (about 1.8 meters or 6 feet).
      • Joins cecum, more developed mucosal folds, thicker and more vascular than jejunum.
    • Large Intestine
    • Terminal part of the alimentary canal.
    • Runs from the cecum to the anus, framing the small intestine on three sides.
    • Diameter: More than twice that of the small intestine (approximately 7.62 cm or 3 in).
    • Structure: Subdivided into four major regions:
      • Cecum (6 cm or 2.4 in long), continues absorption of water and salts.
      • Colon, further divided into ascending, transverse, descending, and sigmoid sections.
      • Rectum (20.3 cm or 8 in long), follows the contour of the sacrum.
      • Anus, the final part which opens to the exterior.

Small Intestine Adaptations for Absorption

  • Three main adaptations of the wall of the small intestine that increase its absorptive capacity:
    • Circular Folds (Plicae Circulares):
    • Deep ridges in mucosa and submucosa, facilitate absorption by causing chyme to spiral through the intestine.
    • Their structure allows for slowed movement, increasing nutrient absorption time.
    • Villi:
    • Small, vascularized projections (approximately 0.5 mm long) on the mucosal surface.
    • About 20-40 villi per square millimeter, substantially increase the surface area of the epithelium.
    • Each villus contains capillary beds (one arteriole and one venule) and a lymphatic capillary (lacteal) for nutrient transport.
    • Microvilli:
    • Cylinder-like extensions of the epithelial cell membrane (approximately 1 micrometer).
    • Form the brush border, housing enzymes for finishing carbohydrate and protein digestion.
    • Estimated up to 200,000,000 microvilli per square millimeter, greatly enhancing absorptive capacity.

Mechanical and Chemical Digestion in Small Intestine

  • Mechanical Digestion:

    • Movement includes segmentation and migrating motility complexes instead of traditional peristalsis.
    • Segmentation mixes chyme, combining it with digestive juices rather than forcing it through the tract, occurring up to 12 times per minute in the duodenum to 8 times in the ileum.
    • As absorption takes place, segments transition to transportation modes under the influence of hormones like modulin.
    • The ileocecal valve regulates the transition of chyme into the large intestine.

  • Chemical Digestion:

    • Completion of protein and carbohydrate digestion occurs here with pancreatic and intestinal juices.
    • Lipids often require bile and pancreatic lipase, highlighting the importance of slow, controlled delivery of chyme to prevent overwhelming the small intestine with high osmolarity.
    • About 90% of water intake is absorbed in the small intestine, using osmotic mechanisms.
    • Enzymatic activity occurs on microvilli membranes, enhancing absorption efficiency.

Unique Features and Functions of the Large Intestine

  • Unique Structural Features:

    • Tenia Coli:
    • Three bands of smooth muscle in the muscularis layer, contributing to colon structure and function.
    • Haustra:
    • Pouches formed by the contraction of tenii coli, aiding in mixing contents and absorption.
    • Epiploic Appendages:
    • Small, fat-filled sacs of peritoneum, their exact function remains unclear.

  • Functions:

    • Final absorption of nutrients and water, synthesis of vitamins, and conversion of liquid chyme into feces.
    • Mucosal structure is predominantly simple columnar epithelium with an abundance of goblet cells for mucus secretion aiding fecal movement.

Pathway of Food Waste

  • From entry into the cecum through its passage to the anus:
    • Cecum: Receives chyme from ileum and continues dehydration and absorption of salts.
    • Colon: Food residue passes through the ascending, transverse, descending, and sigmoid sections.
    • Rectum: Expands to store feces until prompted for defecation.
    • Anal Canal: Two sphincters control feces release; internal (involuntary) and external (voluntary).
    • Defecation Reflex: Triggered by stretching of the rectal wall, involves spinal cord-mediated contractions and voluntary control over the external anal sphincter.

Beneficial Roles of Bacterial Flora

  • Trillions of bacteria reside in the large intestine, known as bacterial flora, comprising over 700 species.

    • Roles:
    • Facilitate chemical digestion and absorption over non-pathogenic commensal means.
    • Synthesize essential vitamins (biotin, pantothenic acid, and vitamin K) and contribute to immune system function.
    • Prevent mucosal barrier breaching through immune activity prompted by bacterial components.

  • Digestive Functions:

    • Residue typically stays in large intestine for 12-24 hours allowing for water absorption and waste consolidation into feces.
    • The fecal composition includes undigested food, bacteria, and epithelial cells, accounting for up to 150 ml of waste from 500 ml of food entering daily.