Large intestine

• Large intestine- general overview

  • Final part of digestive tract
  • 1.5m long tube extending from ileocecal junction to anus
  • Consists of cecum, appendix, colon (ascending,transverse,descending and sigmoid), rectum and anal canal
  • plays role in absorption of fluid and electrolytes from gut contents forming feces

  
• Course of large intestine

  • Begins in right inguinal region as cecum + associated appendix → continues upward as ascending colon through right lumbar region into right hypochrondrium→ bends to left just below liver forming right colic flexure→crosses abdomen as transverse colon to left hypochrondrium→below spleen bends downward forming left colic flexure→continues downward as descending colon and into left inguinal region→enter upper part of pelvic cavity as sigmoid colon→continues on post wall of pelvic cavity as rectum→terminates at anal canal

  
• distinct characteristics of large intestine

  • Precende of taniae coli
  • Omental appendices
  • Haustrations
• Taeniae coli

  • The taeniae coli are 3 separate longitudinal bands of smooth muscle that run along the length of the colon. Begins at base of appendix where thick longitudinal layer of appendix separates into 3 bands.
  • taeniae coli are shorter than the rest of the colon, which causes it to pucker and form pouches called haustra. This puckering effect creates the segmented appearance of the colon
  • They 3 are named based on their positions:
  • Taenia libera (free taenia)
  • Taenia omentalis (omental appendices attach)
  • Taenia mesocolica (mesocolic taenia, transverse and sigmoid mesocolon attaches)

  
• Haustra/haustrations

  • Pouches in btw taeniae found in colon and cecum
  • They are produced bc lenght of taeniae are shorter than lenght of colon
  • Give puckered sppearance of colon and cecum

  
• Appendices epiploicae

  • Small bags of viceral peritoneum filled with fat attached to taeniae of large intestine
  • Absent in appendix, rectum and anal canal
  • Numerous on side of sigmoid colon and posterior surface of transverse colon

  
• Cecum

  • pouch-like structure forming first part of large intestine
  • Located in right iliac fossa (right lower quadrant)
  • 7.5cm lenght and breadth
• Cecum relations

  • Intraperitoneal organ
  • cecum has no mesentery but its bound to abdominal wall by cecal folds
  • Superior: ascending colon
  • Posteromedial: appendix
  • medial: ileocecal junction

  
• Blood supply of cecum

  • arterial: coecal branches of ileocolicnartery
  • Venous drainage: drains into superior mesenteric vein which drains into portal vein
• Ileocecal junction

  • Junction btw terminal ileum( small intestine) and cecum
  • lies at right sacroilac joint
  • The horizontal plane of ileocecal junction represents beginning of ascending colon
  • The ileal orifice is btw the 2 ileocecal lips (sup and inf) that meet on both sides forming ridges called frenula of ileal orifice
  • When cecum is distended lips tighten and the orifice appears as ileal papilla
  • This papilla acts as a valve closing ileal orifice preventing reflux of contents from cecum into ileum

  
• Appendix

  • Narrow, worm-like intestinal structure arising from posteromedial wall of cecum 2cm bellow ileocecal junction, where the 3 taniae coli merge
  • 6-10 cm lenght

  
• Position of appendix,

  • Lies in right iliac fossa, although base if fixed tip point in any direction
  • Position is variable

  Retrocecal: lies posterior to cecum (usual position)

  Subcecal: lies below cecum

  Preileal: lies anterior to terminal ileum (dangerous)

  Postilea: lies posterior to terminal ileum

  Pelvic: descend into pelvis

  
• Relations of appendix

  • Intraperitoneal organs and very mobile
  • Has short triangular mesentery (mesoappendix) that attaches the proximal part of appendix to terminal ileum

  
• Surface anatomy of appendix

  • Surface projection of base of appendix is at the Mcburney point
  • Mcburney point: point at junction btw lat 1/3 and medial 2/3 of the line joining umbilicus to right anterior superior iliac spine
• Blood supply of appendix

  • Arterial: apendicular artery (branch of ileocolic artery branch of superior mesentery)
  • Venous: appendicular vein Which drains into ileocolic vein tributary of superior mesentery
  • Lymphatic drainage: pass into appendicular lymph nodes

  
• Colon

  • Longest part of large intestine
  • begins at ileocecal valve and ends into rectum
  • Subdivided into 4 segments: ascending, transverse, descending and sigmoid colon
  • Encircles small intestine

  
• Ascending colon

  • 15-20cm long
  • Second part of large intestine
  • Begins from horizontal plane of ileocecal junction passes superiorly on right side of abdominal cavity to right lobe of liver, bends to form right colic flexure to continue as transverse colon
  • Position: lies in right lumbar region
  • Relations: retroperitoneal organ

  Anteriorly: coils of jejunum and ileum + greater omentum

  Posterior: quadratus lumborum, iliacus and transversus abdominis, lat cutaneous n of thigh, iliohypogastric and ilioinguinal nerves, lower part of right kidney

  
• Transverse colom

  • Third, longest and most mobile part of large intestine
  • Extends across abdomen from right colic flexure to left colic flexure
  • Attached to posterior abdominal wall by transverse mesocolon ( double layer of peritoneum that divided peritoneal cavity into supra/infracolic compartments)
  • Relations: intraperitoneal

  
• Colic flexures

  • Right colic flexure( hepatic) junction of ascending and transverse colon
  • Lies deep to 9-0 ribs, anterior to right kidney and inferior to right lobe of liver
  • Left colic flexure (splenic): at junction of transverse and descending colon
  • Usually lies deep to 8-9 ribs, more acute, less mobile
  • Attached to diaphragm through phrenicocolic ligament
  • Lies anterior to left kidney and snd inferior to pleen

  
• Descending colon

  • Fourth part of large intestine
  • Begins at left colic flexure
  • passes inferiorly to left side of abdominal cavity
  • Ends in left iliac fosse where it continuous with sigmoid colon
  • Narrowest part pf colon
  • Position: lies in left lumbar region
  • Retroperitoneal

  
• Sigmoid colon

  • Fifth part of large intestine
  • Begins in left iliac fossa as continuation of descending colon
  • Characterized by its S-Shaped loop of variable lenght
  • Ends at S3 vertebra where it joins to rectum (where the 3 taniae coli merge togethe)
  • Position: lies in left lower quadrant and extends into pelvis
  • Intraperitoneal, attached to posterior pelvic wall by double layer of peritoneum (sigmoid mesocolon)

  
• Blood supply of colon

  • Ileocolic artery( branch of SMA): lower part of ascending colon
  • Right colic artery (SMA): upper part of ascending and right colic flexure
  • Middle colic artery (SMA) supplies right 2/3 of transverse colon
  • Left colic artery (SMA) supplies left 1/3 of transverse colon, left colic flexure and descending colon
  • Sigmoid arteris (IMA): supplies sigmoid colon

  These arteries anastomose with each other to form a continuous arterial channel, the marginal artery of Drummond (juxtacolicartery), which parallels the colon and extends the length of it

  
• Venous drainage of colon

  • ileocolic vein (SMV) drains lower part of ascending colon .
  • right colic vein (SMV) drains upper part of ascending colon & right colic flexure .
  • middle colic vein (SMV) drains the right 2/3 of transverse colon .
  • left colic vein (IMV) drains the left 1/3 of transverse colon, left colic flexure, and the descending colon .
  • sigmoid veins (IMV) drain the sigmoid colon .
  • These veins are connected with each other to form a continuous venous channel, the marginal vein (juxtacolic vein), which parallels the colon and extends the length of it

  
• Lymphatic drainage of colon

  The lymph from the colon passes sequentially through:

  • epiploic lymph nodes : on the wall of colon .
  • paracolic lymph nodes : along the marginal artery .
  • intermediate colic lymph nodes : including ileocolic, right colic, middle colic, left colic, & sigmoid lymph nodes :
  • the ileocolic, right colic, & middle colic lymph nodes drains into the superior mesenteric lymph nodes .
  • • the left colic & sigmoid lymph nodes drains into the inferior mesenteric lymph nodes .
• Rectum

  • Lowest part of GiT is formed by rectum and anal canal
  • 13 cm long
  • Beings as continuation of sigmoid colon at S3 level and ends by piercing pelvic diaphragm to become continuous with anal canal at anorectal junction
  • Upper part is covered with peritoneum an Ant and lat aspects and middle 2/3 is covered with peritoneum on its anterior aspect only
  • Lower part of rectum dilates above pelvic diaphragm to form rectal ampulla.

  
• Anterior relations of rectum

  Males :

  • upper 1/3 (retrovesical pouch) sigmoid colon, coils of ileum
  • Lower 1/3: posterior surface of urinary bladder, seminal vesicles, vans deferens, prostate

  Females:

  • upper 1/3: retrouterine pouch, sigmoid colon, coils of ileum
  • Lower 1/3: posterior surface of vagina

  
• blood supply of rectum

  • Superior rectal: branch of inf mesenteric
  • Middle rectal: internal iliac
  • Inferior rectal: internal pudendal

  Venous: Superior rectal (tributary of IM drains into splenic vein

  Middle rectal( tributary of internal iliac vein)

  Inferior rectal (tributary of internal pudendal vein

  
• Lymphatic drainage of rectum

  • Lymph vessels end in pararectal nodes embedded in perirectal connective tissue
  • Lymph vessels thrn accompany superior redtal artery to inferior mesenteric lymph nodes
  • Lymph vessels of lower rectum follow middle rectal artery to internal iliac nodes
• NERVE SUPPLY of rectum

  •Sympathetic and parasympathetic nerves from inferior hypogastric plexuses
• Anal canal

  • Terminal part of GIT
  • Situated bellow level of pelvic diaphragm
  • Lies in anal triangle of perineum btw right and left ischional fossa
  • Extends from anorectal junction to anus

  
• Relations of anal canal

  • Anteriorly: both sexes: perineal body. Males (membranous urethra and bulb of penis); females ( lower end of vagina).
  • Posteriorly: anococcygeal ligament and tip of coccyx
  • Laterally: ischioanal fossa
  • all around: surronded by sphincter muscles ( keeps canal closed)

  
• Interior of anal canal

  3 parts:

  • upper mucous part (15mn long)
  • middle part (15 mn long)
  • Lower part 8mn long
• Upper mucous part of internal anal canal

  • 15mn long, endoderma, origin
  • lined with mucous membrane
  • Mucous membrane shows 6-10 vertical folds known as anal columns of morgani
  • Lower end of anal colums are united by short transverse folds of mucous membrane known as anal valves
  • Above each valve there is a depression in mucosa known as anal sinus
  • anal valves together form a transverse line that runs along allaround anal canal known as pectinate line
  • Pectinate line: situated just opposite to middle of internal anal sphincter. Junction btw endidermal and ectodermal part of anal canal

  
• Internal of anal canal

  • Anal valves: shows epithelial projections called anal papilla
  • anal sinus: contains anal glands that secrete secretions with peculiar smell
  • simple columnar epithelium

  
• Middle/transitional zone of internal anal canal

  • lines with mucous membrane
  • anal columns are absent
  • Dense venous plexus: lies btw mucosa and muscle coat
  • Lower limit of pectan has withish appearance so its known as white line of hilton
  • Non-stratified columnar epithelium
  • bluish-red color
• Lower cutaneous part of internal anal canal

  • 8mn long
  • lined by true skin containing sebacious gland
  • keratinised by stratified columnar epithelium
  • Resembles pigmented skin: sebacious glands, sweat and hair arr present

  
• Arterial supply of anal canal

  • Blood supply: above pectinate line: superior rectal artery: branch if inf mesenteric. Bellow pectinate line : inferior rectal artery
  • Venous drainage: inferior rectal venous plexus: lies in submucosa of anal canal. Drains into supeior rectal vein. Communicates freely with external plexus drains into middle and inferior rectal veins which drains into inferior vena cava.
  • External venous plexus lies outside muscular coat of rectum and anal canal. Communicates freely with internal anal plexus
• Anal canal os divided into

  • anal mucosa (upper part): columnar epithelium
  • Middle part: stratified squamous non-keratinized
  • lower cutaneous part: stratified squamous keratinized.

  
• • anal canal is terminal part of the gastrointestinal tract, playing a crucial role in excretion of feces. It is about 2.5 to 4 cm long in adults and extends from rectum to anus
• Musculature of anal canal

  • Internal Anal Sphincter: thickened continuation of inner circular muscle layer of rectum. It is involuntary and composed of smooth muscle.
  • External Anal Sphincter: Composed of skeletal muscle, it is voluntary and surrounds internal anal sphincter. It has three parts:
  • Subcutaneous part
  • Superficial part
  • Deep part
  • Puborectalis Muscle: Part of levator ani, it forms a sling around junction of rectum and anal canal, contributing to anorectal angle and playing a key role in maintaining fecal continence.