Overview
The large intestine is the
final part of the digestive system. It measures approximately 1.5 metres in
length and 6.5 cm in diameter. It is comprised of the Caecum, Appendix,
Ascending Colon, Transverse Colon, Descending Colon, Sigmoid Colon, Rectum and
Anal Canal. The overall functions of the
large intestine are to complete absorption, maintain fluid levels along with
electrolyte balance, allow bacterial degradation of complex carbohydrates, formation
of faeces and facilitation of its expulsion.
The gut develops from a
primitive endodermal tube and comprises of the fore, mid and hind-gut areas.
The caecum, ascending and right 2/3 of the transverse colon are part of the
mid-gut whereas the distal 1/3, descending, sigmoid, rectum and anal canal are
hind-gut structures.
Gross Anatomy
THE CAECUM
Development
The developing mid-gut
elongates to form a loop that projects into the proximal umbilical cord due to
lack of space. The loop has a cranial and caudal end, with both meeting at an
apex attaching to the vitelline duct. The caudal limb develops an outpouching
that becomes the caecum.
Surface Anatomy
The mid-point of a line drawn
from the right anterior superior iliac spine to the superior point of the
symphysis pubis approximately marks the middle of the lower border of the
caecum.
Macro Anatomy
The caecum is a blind-ending
pouch situated in the right iliac fossa. Converging at the base of the caecum
on the appendix are Taenia Coli. These are flattened bands of muscle which pass
from the caecum to the rectosigmoid region. These taenia are shorter than the
rest of the bowel, hence giving the large bowel its characteristic sacculated
appearance. The bulk of the caecum lies just inferior to the ileocaecal valve.
Anterior to the caecum is the abdominal wall, greater omentum and ileal coils.
The iliacus and psoas muscles lie posteriorly with the caecum usually completely
covered by peritoneum. The appendix usually arises approximately 2.5 cm below
the ileocaecal valve on the posteromedial aspect of the caecum.
Micro Anatomy
The colonic wall is made up
of (from the lumen outwards) the mucosa, submucosa, muscular layer and serosa.
The muscular layer is made up of the circular muscle and 3 longitudinal muscle
bands known as the Taenia coli. Note that the taenia coli are absent in the
rectum.
The muscular wall is thick, ideal for powerful peristaltic movements. In contrast to the small intestine there are no villi or permanent circular folds in the mucosal layer. The mucosal layer consists of simple columnar epithelium, lamina propria and muscularis mucosae. The cell types are mostly absorptive and goblet cells. These are consistent with the primary functions of the bowel which are the absorption of water from the faeces, and the lubrication of the faeces before expulsion.
Blood Supply
The ileocolic artery is a
branch of the superior mesenteric artery and supplies the caecum and initial
ascending colon.
Appendix
Development
As discussed previously, the
caudal limb of the mid-gut loop elongates to form the caecum and the appendix.
Surface anatomy
The appendix usually lies
one-third along a line from the anterior superior iliac spine to the umbilicus.
This is known as McBurney’s point.
Macro-anatomy
Also known as the Vermiform
process (worm-like) the appendix is attached to the posteromedial surface of
the caecum. It is thick walled and varied in length from 5-10cm. The appendix
has a lumen which narrows with increasing age. The position of the appendix can
be variable but it usually lies retrocaecally but can be pelvic or in rare instances
anterior or posterior to the terminal ileum. The taenia coli describe earlier
converge at the root of the appendix. The appendix has a mesentery extending
from the ileal mesentary with the attachment in continuity with the appendiceal
serosal layer. This enables a degree of mobility. The mesoappendix contains
within it the appendicular artery.
Micro-Anatomy
The histology generally
conforms to that of the rest of the large intestine but one characteristic
feature is the presence of prominent lymphoid tissue in the mucosal and
submucosal layers.
Blood supply
The appendicular artery, a
branch of the ileocolic artery, is an end artery, and descends behind the terminal
ileum and runs within the mesoappendix to supply the appendix.
Ascending colon
Development
The ascending colon develops
from the caudal limb of the mid-gut loop.
Surface anatomy
From the caecum, the
ascending colon passes upwards through the right lumbar region. It is lateral
to the right lateral line. Between the intersection of the subcostal and right
lateral line lies the right colic (hepatic) flexure.
Macro-anatomy
Beginning at the level of the
iliocaecal valve the ascending colon runs vertically upwards and takes a left
turn under the liver to become the transverse colon. As stated earlier the
ascending colon is usually a retroperitoneal structure. The ileal loops,
greater omentum and abdominal wall lie anteriorly to it. Posteriorly its
relations include the iliacus, transversus abdominis and quadratus lumborum muscles,
as well as the perirenal fascia over the lateral aspect of the kidney.
Micro-Anatomy
The micro anatomy of the
large bowel has been described earlier.
The puckered appearance of the large colon comes from contraction of the
longitudinal bands (Taeniae coli) and the resultant formation of a series of
pouches is termed "haustra".
Blood supply
The ascending colon is
supplied by the ascending and descending branches of the right colic artery (a
branch of the superior mesenteric artery).
Transverse Colon
Development
The caudal limb of the mid-gut loop forms the proximal two-thirds of the transverse colon.
The distal third of the
transverse colon is formed from the hind-gut.
Surface anatomy
The transverse colon runs
within the region of the umbilical and epigastric regions. Its lower border is
at a level just above the umbilicus and its upper border just below the greater
curvature of the stomach. It is however important to remember that the large
intestine is a mobile structure.
Macro-anatomy
The transverse colon extends
from the right (hepatic flexure) to the left colic (splenic flexure) and is
suspended by the transverse mesocolon. Its middle portion hangs down and across
the umbilical region. At the splenic flexure the transverse colon takes an
inferior turn just below the splenic hilum to become the descending colon. The
superior relations of the transverse colon are the liver, gallbladder, greater
curvature of stomach and spleen. Inferiorly lie coils of the small intestine.
Anteriorly lies the greater omentum and posteriorly the kidneys, pancreas,
second part of the duodenum and small intestine.
Micro-Anatomy
As described for the large
intestine.
Blood supply
The middle colic artery (a
branch of the superior mesenteric artery) divides into right and left branches
in the mesentery and supplies the proximal two-thirds of the transverse colon.
The left colic artery (a
branch of the inferior mesenteric artery) divides into ascending and descending
branches. The ascending branch supplies the distal one-third of the transverse
colon including the left colic flexure.
Descending Colon
Development
The hind-gut gives rise to
the descending colon.
Surface anatomy
The left colic flexure is
situated in the upper left angle of where the left lateral and transpyloric
lines intersect. The descending colon runs down the left lumbar region lateral
to the left lateral line and around the iliac crest.
Macro-anatomy
As discussed above, the
descending colon runs from the left splenic flexure to the iliac crest then
turns medially to continue as the sigmoid colon. It is covered anterolaterally
by peritoneum.
Posterior to the descending
colon lies the left kidney, quadratus lumborum and iliacus. Anteriorly the
upper portion lies covered by coils of jejunum whereas the lower part lies
adjacent to the abdominal wall where it can sometimes be palpable.
Micro-Anatomy
This is as for the rest of
the large bowel.
Blood supply
The first branch of the
inferior mesenteric artery, the left colic artery runs to the left giving
ascending and descending branches. The descending branch supplies the
descending colon and the initial segment of the sigmoid colon.
Sigmoid Colon
Development
The developing hindgut gives
rise to the sigmoid colon.
Surface anatomy
Slightly lateral to the left
lateral line the descending colon turns medially to at a point just beyond the
left lateral line as the sigmoid colon.
Macro-anatomy
The sigmoid colon ends just
anterior to the third sacral vertebra at the rectosigmoid junction. It has a
variable length and it is mobile on its mesentery. During its initial course it
crosses the external iliac vessels and left ureter. In males the sigmoid loop
rests on the bladder and in females it is related to the uterus and posterior
fornix of the vagina.
Micro-Anatomy
This is as for the rest of
the large intestine.
Blood supply
The sigmoid branches run
directly from the inferior mesenteric artery and travel along sigmoid mesentery
to supply the sigmoid colon.
Rectum
Development
The rectum is formed from the
hindgut.
Surface anatomy
The commencement of the
rectum lies anterior to the third sacral vertebra. It ends 2.5 cm anterior to
the coccyx. It is a midline structure.
Macro-anatomy
Positioned in the posterior
part of the pelvic cavity, the rectum is approximately 12 cm long. After its
course anterior to the coccyx, the rectum turns sharply backwards and downwards
piercing the pelvic floor and terminating at the anorectal junction. The upper
two-thirds has some peritoneal covering however the lower third is
extraperitoneal.
Anteriorly in the male is the
rectovesical pouch, base of the bladder and prostate. In the female this is named
the rectouterine pouch and posterior wall of the vagina. Posterior to the
rectum is the sacrum, coccyx, sacral nerves and middle sacral artery. Laterally
below the peritoneal reflections lie the levator ani and coccygeus muscles.
Micro-Anatomy
The histology and cell types
are as that for the rest of the large intestine. An interesting point is that
the rectum is devoid of taeniae coli and appendices epiploicae.
Blood supply
The superior rectal artery, a
continuation of the inferior mesenteric artery, supplies the rectum. The
internal iliac arteries may also give rise to the middle rectal branches which
can supplement the blood supply to the rectum.
Anal Canal and Sphincters
Development
The cloaca is the
terminal part of the gut tube. It is related to a cloacal membrane,
which is made up of the endoderm of the cloaca and the ectoderm of the
proctodeum - the anal pit. Mesenchyme develops into the cloaca establishing
itself as the urorectal septum. This creates a division of the cloaca into the bladder
anteriorly and rectum posteriorly. The urorectal septum reaches the cloacal
membrane and divides it into anal and urogenital membranes. Therefore
the superior two-thirds of the anal canal are formed from the hindgut whereas
the inferior third from the surface ectoderm (proctodeum).
Surface anatomy
The anal canal is a midline
structure.
Macro-anatomy
The anal canal is the
terminal end of the large intestine and is approximately 4cm long. From the
anorectal junction it courses postero-inferiorly as far as the anus. The
superior part bears several longitudinal ridges known as anal columns. The
lower part is lined by skin. The smooth mucle of the anal canal thickens to
form the internal anal sphincter. This is met on its outer wall by striated
fibres of the levator ani muscle. The lower two-thirds of the anal canal has
within it the external anal sphincter made up of 3 parts. The upper part blends
with the levator ani, the superficial part is attached to the coccyx with an
anterior attachment to the perineal body and lastly a subcutaneous part. The
deep part of the external sphincter with its attachment to from levator ani is
known as the anorectal ring, which is palpable during digital rectal examination.
Micro-Anatomy
A distinctive feature of the
anal canal are the longitudinal anal columns or Columns of Morgagni that line its midpoint. Propulsion here aided
by circumanal mucus-producing glands prevents clogging. The upper half of the
canal is lined by columnar epithelium whereas the lower half is lined by stratified
squamous epithelium. At the lower end of the external sphincter the mucosa
makes a gradual transition from stratified epithelium to skin consisting of a
large number of sebaceous and apocrine sweat glands.
Blood supply
The blood supply to the
superior part of the anal canal comes from the superior rectal artery, a branch
of the inferior mesenteric artery. This may be supplemented in some cases by
the middle rectal artery, a branch off the internal iliac artery. The majority of the canal is supplied by the
inferior rectal artery, a branch of the internal pudendal artery. Branches of
the superior rectal artery anastomose with those of the inferior rectal artery.
The position of these explains the occurrence of haemorrhoids at the 3, 7 and
11 o clock areas when the patient is in the lithotomy position.
The upper half of the blood
supply drains back into the portal venous system whereas the lower half drains
into the systemic venous system. This is an important site of portocaval
anastomosis.
Nerve Supply
Sympathetic and
parasympathetic innervation to the caecum, ascending colon and most of the
transverse colon comes from the Superior Mesenteric plexus and in the latter areas
also the inferior Mesenteric Plexus.
Parasympathetic innervation
in the latter part of the transverse colon comes from the Vagus nerve and
Pelvic Splanchnic Nerves.
In the descending colon, sympathetic
innervation is from the lumbar splanchnic nerves and inferior mesenteric
plexus. Parasympathetic nerve supply arises from the pelvic splanchnic nerves.
The lower part of the anal
canal also has somatic innervation from the inferior rectal nerve. The external
sphincter is innervated by the inferior rectal branch, pudendal nerve (S2,S3) and
perineal branch of S4.
Physiology
As discussed previously the
functions of the large intestine are mass peristalsis, the action of colonic
flora, absorption (water, ion, vitamins) and defecation.
Chyme (partially digested
liquid stool) passes into the caecum following a gastroileal reflex after a
meal. This process initiates colonic movement. A characteristic feature of the
large intestine is haustral churning. When haustra distend from accumulation of
chyme they are in a relaxed state. At a certain level of distension, the wall
contracts and contents are squeezed into the next haustrum. Peristalsis also
occurs but a much slower rate than in the small intestine. Food in the stomach
initiates the gastrocolic reflex which causes another type of movement - mass
peristalsis. This occurs from the midpoint of the transverse colon and forces
the contents quickly into the rectum.
Chemical digestion is aided
by luminal bacteria and mucus. This produces gases leading to flatus. Proteins
are converted to amino acids by the bacteria and decomposes bilirubin to give
faeces its characteristic brown colour. B vitamins and vitamin K are absorbed
in the large bowel.
The colon is the final place
for water reabsorption. Sodium and water and reabsorbed by osmosis.
Defecation occurs when the
rectum fills to a certain level where the urge is initiated. Sympathetic,
parasympathetic and somatic nervous input controls the process. The reflex arc
consists of:
rectal distension - Stretch receptors in the rectum send signals via parasympathetic
fibres
Concious awareness - Ascending sensory pathways allow the distinction between solid faecal
matter and flatus
Parasympathetic impulse: Increase in colonic tone and relaxation of the
internal sphincter
Inconvenient to defecate: The external sphincter contracts via descending somatic
impulses.
Convenient to defecate: External sphincter relaxes and allows faeces through
the anus.
Clinical Anatomy
Appendicitis is
inflammation of the appendix. It is caused by obstruction of the appendicular
lumen by chyme, a foreign body or caecal tumours. Typically the pain is central
and thereafter localises to the right lower quadrant of the abdomen. There is
usually a loss of appetite accompanied by nausea and vomiting. The patient may
have a fever and raised white cell count on serology. Progression of the pathology
can lead to gangrene and perforation and so early appendicectomy is
recommended.
Polyps are growths
that can be sessile or pedunculated and benign or malignant. Sessile polyps are
more likely to become malignant.
Colorectal
cancer
has one of the highest incidence rates and mortality after lung cancer. Most
cancers develop from adenomas. Its aetiology can be genetic (familial
adenomatous polyposis), environmental (low fibre/high fat diet), due to
inflammatory bowel disease, polyps and exposure to radiation. Staging systems
used are Dukes and TNM 9tumour, noes and metastases). Complications of
colorectal cancers include obstruction, perforation, fistula formation and
compressive symptoms.
Inflammatory
bowel conditions that can affect the large bowel include Crohn’s
and Ulcerative colitis. The aetiology of Crohns is unclear. Characteristically
it can form skip lesions and a firming of the bowel wall. Ulcerative colitis
involves the rectum as well as other parts of the colon. The speculated
aetiologies include an abnormal immune response and genetic factors. Varying lengths
of colitis may include proctitis and terminal ileitis. There is mucosal
reddening and contact bleeding. Both inflammatory conditions can cause
bleeding, obstruction, perforation but only Crohn’s causes fistulation.
Diverticular
disease is an outpouching of the colonic mucosa through a muscular
defect. It is commonly attributed to a low-fibre diet. The pathophysiology is
said to be increased luminal pressure and 90% of cases involve the sigmoid
colon. It can lead to inflammation, perforation, bleeding and obstruction.
Diarrhoea - This is frequent
passage of loose/liquid stool. The pathophysiology can be classified as osmotic,
secretory, inflammatory or abnormal motility.
Constipation - This is
infrequent of difficult passage of stool which can be slightly harder/firm. The
causes are:
|
Medical |
Surgical |
|
Diet |
Anal pathology - fissures, painful haemorrhoids |
|
Hypercalcaemia |
Cancer |
|
Hypothyroidism |
Neuropathy |
|
Drugs such as opiates |
Foreign body |
Quick Anatomy
Key Facts
Develops from mid and hindgut
structures
Consists of the caecum/appendix,
ascending, transverse and descending colon, rectum and anus.
Longitudinal muscle bands
line the colon - Taeniae coli. They are absent in the rectum.
Blood supply arises from the superior
and inferior mesenteric arteries with some distal supply from branches of the internal
iliac artery.
Aide-Memoire
Summary
The main physiological
functions are: Haustral churning, peristalsis, bacterial degradation and
synthesis of vitamins, absorption of water and ions, and defecation.