Infracolic Compartment: Large Bowel

general

~ 5 ft long

forms arch from cecum to anus

Subdivisions

Cecum

Ascending Colon

Transverse Colon

Descending Colon

Sigmoid Colon

Rectum

Anal Canal

differs from SI

more distensible

Taeniae coli

3 narrow bands (libre, omentalis, mesocolica)

equidistant around tube from appendix to rectum

end: expand & fuse w/ one another --> more robust, distinct layer

1/6 shorter than colon

haustra coli

epiploic (omental) appendages

fat storage bodies

enclosed & suspended w/in peritoneum

plica semilunares (semilunar folds) replace plica circularis of SI

mucosa evenly developed & w/o villi

functions:

remove water from food residue

store & compact stool

Mesenteries

fusion fascia

previously peritonealized colon + posterior body wall peritoneum

area dorsal to ascending & descending colon (secondarily retroperitoneal)

Clinical correlation: surgical approach for ascending & descending colon b/c no vessels/nerves traverse

mesoappendix: extension of dorsal mesentery

transverse mesocolon

suspends transverse colon from post. body wall

along transverse line (~LV1-LV2)

sigmoid mesocolon

suspends sigmoid colon

across L. pelvic brim to 3rd sacral segment

peritonealized, blind pouch

RLQ

ileocecal valve

terminal ileum invaginates into cecum --> superior & inferior lips

not competent; contraction of terminal ileum prevents reflux of cecal contents into ileum

appendix

blind-ending diverticulum

~ 8 cm long

retrocecal position (64%)

taenia coli form complete long. layer

mucosa & submucosa filled w/ lymphoid nodules

Appendicitis

inflammation d/t blockage of lumen

overgrowth of epithelial lining

impaction via coprolith (fecal stone)

referred pain: GVA fibers to T10 dermatome @ umbilicus

McBurney's point: contact w/ parietal peritoneum of post. body wall (2/3 distance from umbilicus to ASIS)

from cecum to R.colic flexure (hepatic)

retroperitoneal

narrower than cecum

relations:

anterior: SI & greater omentum

lateral: (R. paracolic gutter &) transversus abdominis

medial: post. body wall & SI

posterior: post. body wall & R. kidney

from R. colic flexure to L. colic flexure (splenic)

attachments:

gastrocolic lig. to stomach

posterior: mesocolon to post. body wall

anterior: omental apron of greater omentum

phrenicocolic lig.: L. colic flexure to L. diaphragm

relations:

superior: liver, gall bladder, stomach & spleen

anterior: ant. body wall

inferior: SI

posterior: mesocolon

from L. colic flexure to pelvic brim

retroperitoneal

relations:

anterior: SI

lateral: (L. paracolic gutter &) transversus abdominus

medial: SI & L. kidney

posterior: post. body wall

from descending colon to rectum

S-shaped; L.side --> R.side --> ending midline

diverticulosis

evaginations of colonic mucosa at weakest pts (where blood vessels penetrate)

most often in descending & sigmoid colon

d/t increased pressure generated by colon to move feces of low fiber content

diverticulitis: when material becomes lodged in pockets causing inflammation

partially peritonealized

proximal 1/3: peritoneum laterally & anteriorly

mid 1/3: peritoneum anteriorly only

distal 1/3

rectal ampulla: dilated portion resting on pelvic diaphragm; exists below peritoneum

peritoneum reflected to pelvic visceral or lateral & post. pelvic wall

musculature

outer long.: teniae coli fan out --> bands ant. & post.

inner circ.: continuous w/ colon

transverse rectal folds

3 shelves (mucosa, submucosa & some circular m.)

protrude from walls of rectum

cause external curvatures & flexures (L=2; R=1)

assist in supporting fecal mass

ampulla

dilated portion superior to pelvic diaphragm

in anal triangle b/w opposing ischioanal fossae

from pelvic diaphragm to anus

angles 90' posteriorly d/t puborectal sling (at jxn of rectum & anal canal)

ischioanal fat surrounds & allows for expansion during elimination

feature:

anal sphincters:

internal (IAS)

involuntary; autonomically innervated

external (EAS)

circularly arranged skeletal m.

inferior rectal bbr. of pudendal n. (S2-4)

anal columns:

anal valves:

anal sinuses:

vertical folds d/t veins deep to mucous membrane

venous anastomses connect base of contiguous anal columns & bridge across anal sinuses; course creates pectinate line

depression b/w anal columns & valves

Hilton's White Line (intersphincteric line)

pt where EAS meets IAS

anal verge: transitional epithelium changes to perianal skin

Transitional area (anal pecten)

pectinate line above & Hilton's White line below

demarcates epithelial change of mucosa

continuous w/ inner circ. layer

Hemorrhoids

internal

prolapse of rectal mucosa

internal rectal venous plexus affected; trapped by contracted sphincters --> engorged & ulcerated veins

bright red blood d/t elaborate anastomoses

external

thrombosed vv. of external rectal venous plexus

directly under skin; painful

d/t increased intra-abdominal pressure (i.e. chronic constipation, straining at stool; pregnancy & portal HTN)

thinner muscular layers; incomplete outer (long.) layer