5. large intestine-f1
TRANSCRIPT
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Large Intestine Extent From ileocaecal junction to the anal orifice About 1.5 meters in length Placed in the abdominal & pelvic cavity
surrounding the coils of small intestine
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Subdivided into Cecum ▪ Sac, connects to
ileum Vermiform appendix Colon ▪ ascending ▪ transverse ▪ descending ▪ sigmoid Rectum Anal canal
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Cardinal features of large intestine Teniae coli ▪ Thickening of longitudinal muscularis Haustration ▪ Puckering created by teniae coli Epiploic appendages ▪ Fat-filled pouches of visceral peritoneum
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Intraperitoneal parts (Have a mesentery) Caecum Appendix Transverse colon Sigmoid colon
Retroperitoneal parts (No mesentery) Ascending colon Descending colon Rectum
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Caecum Beginning of large
intestine Length – 6cm, width-
7.5cm Blind sac Lies in right iliac fossa Covered by peritoneum on
all sides Ileum opens to medial side Appendix attached to
postero -medial aspect Interior shows ▪ iliocaecal orifice – with
ileocaecal valve ▪ Appendicular orifice
Ileocaecal valve prevents backflow of contents from colon to small intestine
1500 ml of chyme empty into caecum everyday
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Appendix Narrow worm-like Arises from postero -
medial wall of caecum 2 to 20 cm long. Average
is 9cm Suspended by peritoneal
fold meso –appendix Devoid of cardinal
features of large intestine Has a base, body and tip Base is constant and
teniae coli begins from here
Tip is least vascular and directed in various positions
Mesoappendix
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Appendix positions
A - Preileal B - Postileal C
–Subileal/promonteric D - Pelvic E – Subcaecal /inguinal F - Paracaecal G - Retrocaecal
A B
C
DE
F
G
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Mc Burney’s Point
Junction of lateral 1/3rd and medial 2/3rd of a line joining right anterior superior iliac spine and the umbilicus
Corresponds to the base of appendix Site of maximum tenderness in acute
appendicitis
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Ascending colon
15 cm long, 5cm in caliber
Continuation of caecum up to right colic flexure
Retroperitoneal
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Transverse colon
50 cm long Extends from right
colic flexure to left colic flexure
Suspended from posterior abdominal wall by peritoneal fold transverse mesocolon
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Descending colon
25 cm long, 3.5 cm in caliber
Extends from left colic flexure to left pelvic brim ( sigmoid colon)
Retroperitoneal Non mobile
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Sigmoid colon
Begins at left pelvic brim and ends at recto-sigmoid junction
“S” shaped About 40 cm long Suspended by
peritoneal fold sigmoid mesocolon
Sigmoid colon may get involved in volvulus because it is liable to twist on its mesentery
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Rectum
12 cm long, 4cm wide Recto-sigmoid
junction to ano -rectal junction
No cardinal features Lower part is dilated,
ampulla - storage of faeces
Lateral rectal curves – 2 to the right and 1 to the left
Rectal valves - mucosal folds
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Anal canal
Ano -rectal junction to anal orifice
3.8 – 4 cm long 2 sphincters ▪ Sphincter ani
internus – involuntary
▪ Sphincter ani externus – voluntary
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Anal columns – longitudinal mucosal folds Anal valves – crescentic folds connect lower end of
anal columns Anal papillae – project from free margin of anal valves Anal sinuses – recesses above the anal valves Anal glands – ducts open in to anal sinuses
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Anorectal junction – indicated by the superior end of anal columns
Inferior ends of anal valves forms a irregular line - pectinate line
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Digital examination per rectum is an important clinical procedure in assessment of the prostate
The following structures are palpable per rectum Anteriorly in the male - rectovesical pouch, full
bladder, seminal vesicles, displaced or enlarged ductus deferentes, membranous part of urethra when catheterized, and bulbo-urethral glands
Anteriorly in the female- vagina, cervix, ostium uteri, body of uterus when retroverted, recto-uterine fossa, and, pathologically, broad ligaments, uterine tubes, and ovaries
Laterally - ischial tuberosity and spine and sacrotuberous ligament
Posteriorly-pelvic surface of sacrum and coccyx.
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Female pevis
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Pectinate line – junction of superior and inferior part of anal canal
Part superior to pectinate line differs from the part inferior to pectinate line in arterial supply, innervation, venous and lymphatic drainage
Difference is due to different embryological origins of upper and lower parts
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Superior mesenteric artery
Inferior mesenteric artery
Middle colic A.
Right colic A.
Ileocolic A.
Left colic A.
Sigmoid arteries
Superior rectal A.
Arteries of Large IntestineMarginal arteries
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Microscopic Anatomy of LargeIntestine
Villi are absent Absorptive epithelia with
numerous goblet cells Intestinal crypts of
Leiberkuhn – simple tubular glands
Lined with simple columnar epithelium
▪ Epithelium changes at anal canal ( below pectinate line)
▪ Becomes stratified squamous epithelium
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Movements of Large Intestine Though sluggish movements it is important for
mixing, propulsive and absorptive functions. Mixing movements – segmentation
contractions ▪ Large circular contractions ▪ Appear at regular intervals ▪ Length of portion of colon involved in each
contraction is about 2.5 cm Propulsive movements – Mass peristalsis ▪ Propels the feces from colon towards anus ▪ Occurs only few times everyday
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Absorptive function ▪ Most of the absorption occurs in the proximal
half of the colon – absorbing colon. Distal colon functions for storage – storage colon
▪ Active absorption of water and electrolytes ( sodium, chloride)
▪ Absorption of nutrients from digested residue ▪ Maximum absorption capacity – 5 to 7 liters of
fluid and electrolytes each day Formation of feces and defecation ▪ After the absorption of nutrients, water and other
substances, the unwanted materials form feces. ▪ Composition of feces: ▪ About ¾ water ▪ About ¼ solid matter - 30% dead bacteria, 10 –
20% fat, 10-20% inorganic matter, 2- 3% protein, 30% undigested material
Function
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▪ Defecation ▪ Voiding of feces ▪ Most of the time rectum is empty of feces ▪ Defecation reflex When mass movement forces feces into the
rectum, the desire for defecation is normally initiated
Gastrocolic reflex – Defecation occurs by gastrocolic reflex mediated by intrinsic nerves of GI tract. In this , the distention of stomach by food causes contraction of rectum followed by desire for defecation. However it causes weak contraction of rectum. Strong contraction of rectum & relaxation of anal sphincters occur due to the reflex mediated by parasympathetic nerves
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Digestive function ▪ Numerous bacteria , especially
colon bacilli are present as normal flora
▪ They are capable of digesting small amount of cellulose
Excretory function ▪ Excretes heavy metals like mercury,
lead, arsenic through feces
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Secretory function ▪ Large intestinal juice – watery alkaline fluid ▪ 99.5% water and 0.5% solids ▪ Digestive enzymes are absent ▪ Concentration of bicarbonate ions is high ▪ Neutralizes the strong acids formed by bacterial flora ▪ Mucous ▪ Mucin present lubricates the mucosa and bowel contents so the movement of bowel is facilitated ▪ Mucin also protects the mucosa by mechanical
and chemical insult Synthetic function ▪ Bacterial flora synthesizes folic acid, vitamin B12 and vitamin K
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