What are the 8 different components of the large intestine / colon?
Caecum Appendix Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anus
What are the 4 functions of the large intestine
- digestion
- minimal
- enteric bacteria ferment some indigestible carbohydrates and synthesize vitamin B and K - absorption
- most remaining water
- electrolytes
- vitamins produced by bacteria - Propulusion
- haustral contractions and mass movements propel faeces to rectum - Defecation
What are the 2 types of contractions in the LI?
Haustral contractions:
- every 30 minutes
- short lived slow contractions
- mostly in ascending and transverse colon
- initiated by ENS when individual haustra fill with food
Mass movements:
- 3 to 4 times daily
- powerful prolonged contractile waves
- gastrocolic reflex: presence of food in stomach produces reflex mass movement
What is the gastrocolic reflex?
- presence of food in stomach produces reflex mass movement to propel food in large intestine towards rectum
What is the defecation reflex?
- mass movement of faeces into rectum stimulates visceral afferent neurons (distension)
- visceral afferents initiates PNS spinal reflex at S2-4
- PNS initiates contraction of rectum, and relaxation of internal anal sphincter
- message also reaches brain to allow conscious decision on relaxation of external anal sphincter
Describe the caecum including the location, ileocecal junction, nerve supply, and vascular supply
Anatomy:
- first part of LI
- between ileum of SI and ascending colon
- 7.5 cm length and breadth
Location:
- right inguinal region
- corresponds to right iliac fossa
Ileocecal junction:
- ileocecal vavle controls passage of chyme through ileocecal orifice
- formed by superior and inferior ileocecal folds
Vascular:
a. ileocolic artery (terminal branch of SMA)
b. ileocolic vein (drains to SMV)
Neuro:
- PNS: CNX Vagus
- SNS: abdominopelvic planchnic (lower Tx segments)
Describe the appendix including anatomy, function, vascular and innervation
Anatomy:
- arises from posteromedial aspect of caecum
- inferior to ileocaecal junction
- 6-10cm length
- intraperitoneal
- small twisted lumen (susceptible to blockage)
Function:
a. immunity: contains masses of lymphoid tissue, produces and stores lymphocytes
b. reservoir for beneficial gut bacteria
Neuro:
- visceral afferents (enter at T10)
Vascular:
a. appendicular a (branch of ileocolic from SMA)
b. ileocolic v (to SMV)
Describe the anatomy of the ascending, transverse and descending colon
Ascending:
- between caecum and right colic / hepatic flexure
- secondarily retroperitoneal (covered by peritoneum anteriorly and laterally)
Transverse:
- 45cm long
- crosses abdo wall at level of umbilicus
- largest and most mobile section of LI
- between right colic / hepatic flexure and left colic / splenic flexure
- two ligaments (phreniocolic to diaphragm and transverse mesocolon)
Descending:
- between left colic / splenic flexure and sigmoid colon
- retroperitoneal (covered anteriorly and laterally)
Describe the sigmoid colon
- S shaped loop
- inferior continuation of descending colon
- 40cm long
- extends from iliac fossa to rectum at S3
- intraperitoneal
Rectosigmoid junction: signified by termination of teniae coli
What is the rectosigmoid junction?
The junction between the sigmoid colon and the rectum at approx S3
- signified by the termination of teniae coli
What are the 4 histological features of the large intestine?
Teniae coli:
- 3 separate thickened bands of muscle in the longitudinal mm layer of muscularis
- extend from caecum to beginning of rectum
- muscular tone puckers colonic wall into haustra
Haustra:
- pocket like sacs in walls of LI
Crypts:
- in mucosal layer of LI
- contain goblet cells that produce mucous to protect LI wall and ease faecal passage
Omental / epiploic appendices:
- small fatty projections of omentum hanging from surface of colon
What is the arterial supply of the LI?
Ascending colon and hepatic flexure:
- branches from SMA
- ileocolic and Rt colic a (from SMA)
Transverse colon:
- middle colic a (SMA)
- Rt colic a (SMA)
- Lt colic a (IMA)
Descending colon:
- Lt colic
- superior sigmoid
(both from IMA)
Which components of teh LI are supplied by branches of the SMA, and which from the IMA?
SMA:
- ascending colon
- hepatic flexure
IMA:
- descending colon
- sigmoid colon
Both:
- transverse colon
What are the 4 branches of the SMA that supply the LI?
- Multiple intestinal branches of SMA (ascending colon)
- Ileocolic (ileocaecal junction, caecum)
- Right colic (ascending, hepatic flexure, part of transverse)
- Middle colic (transverse)
What are the 2 branches of the IMA that supply the LI?
- Left colic (transverse and descending)
2. Superior sigmoid aa (sigmoid colon)
What is the venous drainage of the LI?
Draining into SMV:
- ascending colon and hepatic flexure (branches of SMV, ileocolic and Rt colic)
- transverse (SMV)
Draining into IMV:
- descending and sigmoid
What is the innervation of the LI?
Ascending colon, hepatic flexure and transverse colon:
- PNS: CNX Vagus
- SNS: T10-12
Descending and sigmoid:
- PNS: pelvic splanchnic (S2-4)
- SNS: Lx part of sympathetic trunk and superior hypogastric plexus
What is the anorectal flexure?
the sharp postero-inferior angle at the terminal rectum, between the rectum and the anus (at tip of coccyx)
What are the peritoneal reflections of the rectum in males and females?
Females:
- rectouterine pouch (pouch of Douglas)
- peritoneum reflecting from rectum to posterior vagina and uterus
Males:
- retrovesicle pouch
- peritoneum reflecting from rectum to posterior bladder
Describe the anatomy of the rectum
- between sigmoid colon (S3) and anus (tip of coccyx)
- rectal ampulla = dilated distal part of rectum directly above pelvic floor, relaxes to accomodate faecal mass
- puborectalis m forms a sling around inferior rectum and has a role in maintaining continence
Describe the vascular and neuro supply of the rectum
Arterial:
- branches of IMA (superior rectal, middle rectal, internal rectal)
Venous:
- superior rectal v (into portal system)
- middle and inferior rectal v (into systemic)
Neuro:
- intrinsic: ENS
- PNS: pelvic splanchnic (S2-4)
- SNS: Lx part of sympathetic trunk and superior hypogastric plexus
Describe the anatomy of the anal canal, including location, histology and features
Location:
- terminal aspect of LI
- between pelvic diaphragm and anus
- begins at tip of coccyx and descends 2-3cm
HIstology:
- rectal: simple columnar
- anal: stratified squamous
Features:
- anal columns: longitudinal folds of mucosa
- anal sinuses: recesses bw columns that exude mucous when compressed by faeces
- anal sphincters: internal (involuntary control) and external (voluntary control)
- pectinate line: junction between superior and inferior anal canal, different neurovascular supply
Describe the innervation and vascular supply of the anal canal
Arterial:
- superior to pectinate line: superior rectal (from IMA)
- inferior to pectinate line: inferior rectal (from internal iliac)
Venous:
- superior to pectinate ine: internal rectal plexus (to superior rectal, IMV and portal system)
- inferior to pectinate line: internal rectal plexus (to inferior rectal and caval system)
Neuro:
- superior to pectinate line: inferior hypogastric plexus, PNS S2-4, visceral afferents
- inferior to pectinate line: somatic motor and sensory from pudendal (S2-4), sensitive to pain, touch and temperature
