5. large intestine-f1

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

Subdivided into Cecum ▪ Sac, connects to

ileum Vermiform appendix Colon ▪ ascending ▪ transverse ▪ descending ▪ sigmoid Rectum Anal canal

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

Intraperitoneal parts (Have a mesentery) Caecum Appendix Transverse colon Sigmoid colon

Retroperitoneal parts (No mesentery) Ascending colon Descending colon Rectum

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

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

Appendix positions

A - Preileal B - Postileal C

–Subileal/promonteric D - Pelvic E – Subcaecal /inguinal F - Paracaecal G - Retrocaecal

A B

C

DE

F

G

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

Ascending colon

15 cm long, 5cm in caliber

Continuation of caecum up to right colic flexure

Retroperitoneal

Transverse colon

50 cm long Extends from right

colic flexure to left colic flexure

Suspended from posterior abdominal wall by peritoneal fold transverse mesocolon

Descending colon

25 cm long, 3.5 cm in caliber

Extends from left colic flexure to left pelvic brim ( sigmoid colon)

Retroperitoneal Non mobile

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

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

Anal canal

Ano -rectal junction to anal orifice

3.8 – 4 cm long 2 sphincters ▪ Sphincter ani

internus – involuntary

▪ Sphincter ani externus – voluntary

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

Anorectal junction – indicated by the superior end of anal columns

Inferior ends of anal valves forms a irregular line - pectinate line

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.

Female pevis

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

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

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

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

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

▪ 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

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

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