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COLON, RECTUM, AND ANUS - Coggle Diagram

- - - - Primitive Gut: Derived from endoderm, divided into foregut, midgut, and hindgut.
      - Hindgut: Develops into the distal transverse colon, descending colon, rectum, and proximal anus.
        
        Blood Supply: Inferior mesenteric artery.
      - Distal Anal Canal: Derived from ectoderm; receives blood supply from the internal pudendal artery.
      - Dentate Line: Divides the endodermal hindgut from the ectodermal distal anal canal.
    - - Large Intestine Divisions: Colon, rectum, and anal canal.
      - Colonic Wall Layers: Mucosa, submucosa, muscularis propria (inner circular, outer longitudinal), and serosa.
      - Colon Landmarks
        
        Length: Extends approximately 150 cm (3 to 5 feet).
        
        Cecum: Widest diameter (7.5–8.5 cm), thinnest muscular wall; most vulnerable to perforation, least vulnerable to obstruction.
        
        Sigmoid Colon: Narrowest part; extremely mobile; most vulnerable to obstruction. Volvulus is most common here.
      - Rectum: Approximately 12 to 15 cm in length.
        
        Landmarks: Valves of Houston (submucosal folds); Denonvilliers' fascia (separates rectum from prostate/seminal vesicles in men, or vagina in women).
      - Anal Canal:
        
        Anatomic Anal Canal: Extends from dentate line to anal verge.
        
        Surgical Anal Canal: Begins at anorectal junction, measures 2 to 4 cm.
        
        Anal Transition Zone: Mucosa proximal to the dentate line sharing characteristics of columnar, cuboidal, and squamous epithelium.
      - Sphincters:
        
        Internal Anal Sphincter: Inner smooth muscle layer that thickens in the distal rectum.
        
        External Anal Sphincter (Deep, Superficial, Subcutaneous): Surrounds the internal sphincter.
      - Pelvic Floor: Formed by the levator ani muscle (puborectalis, iliococcygeus, and pubococcygeus muscles).
      - C. Vascular Supply
        
        Colon Arterial Supply: Branches of the superior mesenteric artery (ileocolic, right colic, middle colic) and the inferior mesenteric artery (left colic, sigmoidal, superior rectal artery). Terminal branches communicate via the marginal artery of Drummond.
        
        Anorectal Arterial Supply: Superior rectal artery (IMA branch), middle rectal artery (internal iliac), inferior rectal artery (internal pudendal artery).
        
        Venous Drainage (Rectum): Superior rectal vein (drains to portal system via IMV). Middle rectal vein and inferior rectal vein (drain to systemic circulation via internal iliac vein/internal pudendal vein).
      - D. Nerve Supply
        
        Colon: Sympathetic (inhibitory, T6–T12 and L1–L3) and Parasympathetic (stimulatory, vagus nerve for right/transverse colon; S2–S4 nervi erigentes for left colon).
        
        Anorectum: Sympathetic (L1–L3) and Parasympathetic (nervi erigentes S2–S4) fibers form the pelvic plexus.
        
        Internal Anal Sphincter: Inhibited by both sympathetic and parasympathetic fibers.
        
        External Anal Sphincter/Puborectalis: Innervated by the inferior rectal branch of the internal pudendal nerve.
        
        Sensory Innervation: Anal canal below the dentate line receives somatic innervation via the inferior rectal branch of the pudendal nerve.
  - - - Water Absorption: The colon absorbs approximately 90% of water contained in ileal fluid (1000–2000 mL/d).
      - Sodium Absorption: Active via sodium-potassium (Na+/K+) ATPase.
      - Ammonia: Produced by bacterial degradation of protein and urea; absorption depends on intraluminal pH.
    - - Components: Acetate, butyrate, and propionate.
      - Function: Produced by bacterial fermentation of dietary carbohydrates; important energy source for colonic mucosa. Lack of dietary source or fecal diversion can cause mucosal atrophy ("diversion colitis").
      - Microflora: 30% of fecal dry weight is bacteria (10¹¹–10¹² bacteria/g). Anaerobes are predominant (Bacteroides species most common).
    - - Function of Bacteria: Breakdown of carbohydrates/proteins, metabolism of bile acids/estrogen/cholesterol, production of vitamin K, and colonization resistance (suppressing pathogens like Clostridium difficile).
      - Intestinal Gas: Major components are nitrogen, oxygen (swallowed air), carbon dioxide, hydrogen, and methane (the latter two produced by colonic bacteria).
    - - Motility: Characterized by intermittent contractions; High-Amplitude, Prolonged Duration, Propagated Contractions (HAPCs) create "mass movements" (4 to 10 times per day).
      - Defecation: Involves mass movement, increased intra-abdominal/rectal pressure, and relaxation of the pelvic floor.
        
        Rectoanal Inhibitory Reflex: Distention of the rectum causes reflex relaxation of the internal anal sphincter (the "sampling reflex").
      - Continence: Requires adequate rectal compliance, neurogenic control, functional sphincters, and formed stool.
        
        Internal Sphincter: Responsible for most resting, involuntary sphincter tone.
        
        External Sphincter: Responsible for most voluntary sphincter tone (squeeze pressure).