the digestive system lecture 6
TRANSCRIPT
The Digestive SystemLecture 7
5. Descending Colon▪ Is about 25 cm long.
▪ It begins at the left colic flexure, and
descends vertically through left
hypochondriac, lumbar (between left psoas
major and quadratus lumborum), and then
crosses the iliac crest, and turns medially
and downwards in contact with the iliacus
and psoas major.
▪ Finally it ends at the inlet of the lesser
pelvis (true pelvic cavity) where it becomes
continuous with sigmoid colon. 1
Descending
colon
2
Transverse colon
Descending
(2nd part)
of
duodenum
3
Relations
Anterior Relations:
From before backwards, the anterior
abdominal wall, greater omentum, and coils of
small intestine.
Posterior Relations:
▪ Lateral border of left kidney, quadratus
lumborum, psoas major, and iliacus.
▪ The subcostal nerve, iliohypogastic nerve,
ilioinguinal nerve, lateral cutaneous nerve
of thigh, and femoral nerve cross behind it.
4
Descending
colon
5
Descending colon
Lateral cutaneous
nerve of thigh
Psoas major
Iliacus
Iliohypogastric
nerve
Ilioinguinal
nerve
Subcostal nerve
Quadratus
lumborum
Posterior relations of Descending colon
6
Blood Supply
▪ Left colic and sigmoid branches of the
inferior mesenteric artery.
▪ Veins follow the arteries and drain into the
inferior mesenteric vein.
Nerve SupplySympathetic and parasympathetic (pelvic
splanchnic) nerve fibers enter the descending
colon via inferior mesenteric nerve plexus.
7
8
6. Sigmoid Colon▪ Lies in lesser pelvis, coiled in front of
rectum, on peritoneal (superior) surface of
bladder in male, and uterus in female.
▪ It varies in length, (15 – 80 cm, usually
approximately 30 cm), and extends from
descending colon at pelvic brim (inlet of
pelvis), to pelvic surface of 3rd piece of
sacrum.
▪ Here, it turns backwards in the median
plane to reach the 3rd piece of sacrum
where it curves downwards and ends in
rectum. 9
Sigmoid colon
10
11
▪ The sigmoid colon is completely
surrounded by peritoneum, which forms a
mesentery, the sigmoid mesocolon.
RelationsLateral Relations: Lateral pelvic wall, external
iliac vessels, obturator nerve, and vas
deferens in male, or ovary in female.
Posterior Relations: Internal iliac vessels,
ureter, piriformis, sacral plexus, and rectum.
Superior Relations: Terminal coils of ileum.
12
Obturator
internus
Lateral relations of sigmoid colon
13
Lateral relations of sigmoid colon (Male)
Vas deferens
External
iliac
artery
14
Round
ligament of
uterus
Obturator
nerve
Ovary
Lateral relations of sigmoid colon (Female)
15
Internal
iliac
artery
Ureter
Rectum
Posterior relations of sigmoid colon
16
Posterior relations of sigmoid colon
Sacral
plexus
periformis
17
Inferior Relations:
Rest on bladder in male, and on uterus and
bladder in female.
18
19
Inferior relations of sigmoid colon (male)
Urinary
bladder
Sigmoid
colon
20
Inferior relations of sigmoid colon (female)
Urinary
bladder
Uterus
Sigmoid
colon
Blood SupplySigmoid branch of inferior mesenteric artery.
Veins correspond to arteries and drain into
inferior mesenteric vein, which joins portal
venous system.
Lymph DrainageLymph vessels from sigmoid colon drain into
inferior mesenteric nodes via lymph nodes
situated along the course of sigmoid arteries.
21
22
Nerve SupplySympathetic and parasympathetic (pelvic
splanchnic) nerves enter sigmoid colon
through inferior hypogastric plexus.
23
Positions of taeniae coli on colon▪ The longitudinal muscle fibers is thickened
in three places of the colon to form three
band known as the taeniae coli.
▪ In the interval between these three band the
longitudinal coat is less than half the
thickness of the muscular coat.
▪ In the caecum, ascending colon,
descending colon, and sigmoid colon the
taeniae coli are placed: Anterioly,
posteromedially, and posterolaterally.
24
▪ In the transverse colon, the anterior is
placed inferiorly; the posteromedial is
placed posteriorly; and the posterolateral is
placed anterosuperiorly.
▪ These bands are said to be shorter than the
other coats of the intestine, and may
produce the sacculi or haustrations.
▪ In the rectum the longitudinal muscle fibers
are thicker on the anterior and posterior
surfaces forming two bands.
25
7. Rectum▪ Is about 12 cm long.
▪ Begins on pelvic surface of S3 vertebra as
continuation of sigmoid colon.
▪ It follows the concavity of sacrum and
coccyx, and passes through pelvic
diaphragm to become continuous with anal
canal.
▪ The anorectal junction lies 2 – 3 cm beyond
tip of coccyx, which in the male is opposite
apex of prostate. 26
▪ The lower part of rectum is dilated to form
rectal ampulla.
▪ The teniae coli form two wide muscular
bands which descend, one in the anterior
and the other in the posterior wall of
rectum.
▪ The peritoneum is related only to upper
two-thirds of rectum, covering at first its
front and sides, but lower down its front
only.
27
▪ From the latter it is reflected on to the bladder
in male, forming the rectovesical pouch of
peritoneum, and on to the posterior wall of
vagina and uterus in female, forming the
rectouterine pouch (pouch of Douglas).
▪ In empty state of the rectum, the mucous
membrane lining its lower part presents a
number of longitudinal folds which are
effected by distension of the rectum.
▪ Beside these, there are 3 permanent
transverse folds (valves of Houston) of a
semilunar shape, which are most marked
when the rectum is distended.
28
Rectovesical
pouch
29
Rectouterine
pouch
(of Douglas)
30
Superior
Middle
Inferior
Transverse folds of
rectum
(valves of Houston)
31
RelationsAnterior RelationsUpper two-thirds:
▪ In the male to rectovesical pouch.
▪ In the female to rectouterine pouch.
▪ In both sex the pouch contains terminal
coils of ileum and sigmoid colon
Lower third:
▪ In the male to posterior surface of bladder,
vas deferens, seminal vesicles, terminal
part of the ureter, and prostate. 32
▪ In the female to vagina.
33
Anterior relations of rectum (male)
Rectovesical pouch
Urinary bladder
Seminal vesicle
Rectovesical Fascia (septum)
Prostate
Ureter
34
Rectouterine pouch
(of Douglas)
Rectum
Vagina
Anterior relations of rectum (female)
35
Posterior Relations▪ In the median plane, it is related to lower
three sacral vertebrae, coccyx, median
sacral vessels, ganglion impar, and
branches of superior rectal vessels.
▪ On each side of the median plane, it is
related to piriformis, anterior rami of lower
three sacral and coccygeal nerves,
sympathetic trunk, right and left coccygeus
and right and left levator ani.
36
Posterior relations of rectum
37
Ventral rami of
Sacral nerves
periformis
Sympathetic trunk
Coccygeus
Levator aniGanglion
impar
Lateral RelationsSigmoid colon or distal part of ileum, pelvic
sympathetic plexuses, coccygei, and
levatores ani.
38
8. Anal Canal▪ Is about 4 cm in length.
▪ Passes downwards and backwards from
the rectal ampulla (at the level of the
prostate, in the males) to the anus.
▪ Except during defecation, its lateral walls
are maintained in position by levatores ani
muscles and anal sphincters.
39
RelationsAnterior relations
• In the male, it is separated from
membranous urethra by the perineal body.
• In the female, it is related to perineal body
and lower part of vagina.
Lateral relations
It is separated from the fat of ischiorectal
fossae by levator ani and external anal
sphincter muscles.
Posterior relations
Related to anococcygeal raphe. 40
Membranous
urethra
Anal canal
Perineal body
Anterior relations of anal canal (male)
41
Anal canal
Perineal body
Vagina
42
Anterior relations of anal canal (female)
Lateral relations of anal canal
Obturator
internus Levator ani
Ischiorectal fossa
Anal canal
43
Anococcygeal
Raphe
Levator ani
Posterior relations of anal canal
44
▪ The upper half of the anal canal is lined by
columnar epithelium.
▪ The mucous membrane in this region
exhibits 5 to 10 vertical folds, the anal
columns, which are joined together at their
lower ends by small semilunar folds called
anal valves.
▪ The interval between a valve and the anal
wall is called an anal sinus.
▪ The function of the anal column and valves
is not known.
45
▪ The site of attachment of the valves forms
the pectinate line, which indicates the level
where the upper half of the anal canal joins
the lower half.
▪ The lower half of the canal is lined by
stratified squamous epithelium, which
gradually merges at the anus with the
perianal epidermis.
▪ In this region the mucous membrane has
no vertical folds.
46
47
Internal Anal Sphincter▪ Consists of a thickening of the circular
muscle of the gut wall which encloses the
upper two-thirds of the anal canal.
▪ It is enclosed by a layer of striped muscle
that forms the voluntary external anal
sphincter.
External Anal SphincterConsist three parts:
i. Subcutaneous part:
▪ Encircles the lower end of the anal canal
beneath the skin at the anal orifice, and
has no bony attachments. 48
▪ Lies below the lower border of the internal
anal sphincter and of the superficial part of
the external anal sphincter.
ii. Superficial part:
▪ Is attached anteriorly to perineal body, and
posteriorly to coccyx.
▪ Lies deep to the subcutaneous part.
iii. Deep part:
Encloses the upper end of anal canal and has
no bony attachments.
49
50
Internal anal
sphincter
Internal Anal Sphincter
External Anal Sphincter
51
▪ In addition to the sphincter, the lower part
of rectum and upper part of anal canal are
supported by puborectalis muscle, which
passes around their lateral and posterior
sides like a sling.
▪ Contraction of puborectalis muscle causes
the angle between rectum and anal canal to
become more acute.
▪ Thus its contraction is an important factor
in preventing passage of feces from rectum
to anal canal.
52
▪ Tonic contractions of external and internal
sphincters keep the anus and anal canal
closed, and are inhibited during defecation.
▪ The contractions can, however, be
overcome by strong contractions of the
rectum.
▪ The external sphincter is stronger than the
internal, which appears to be unimportant
for normal fecal continence since surgical
division of internal sphincter does not
result in incontinence.
53
▪ If the external sphincter is paralyzed,
sphincter control is lost.
▪ At the anorectal junction, the internal
sphincter, deep part of external sphincter, and
puborectalis muscles form a distinct ring,
called the anorectal ring, which can be felt on
rectal examination.
▪ The longitudinal smooth muscle layer of anal
canal is continuous above with that of rectum.
▪ It forms a continuous layer around the canal
and descends between internal and external
sphincters. 54
▪ Some of the fibers are attached to the lining
of the canal, while others pass laterally
deep to the subcutaneous part of the
external sphincter to become continuous
with the septum of the ischiorectal fossa.
▪ The attachment of the longitudinal fibers to
the anal canal separates the internal rectal
venous plexus from the external rectal
venous plexus.
55
Blood SupplyArterial supply: Derived principally from
superior rectal artery with contributions from
middle and inferior rectal and median sacral
arteries.
a. Superior rectal artery: ▪ Is direct continuation of inferior mesenteric
artery.
▪ Descends in root of sigmoid mesocolon.
▪ At the level of S3 vertebra (where the rectum
commences) it divides into right and left
branches, which descend on each side of
rectum and subdivides into smaller branches.
56
▪ These branches pierce the muscular wall
and supply the whole thickness of the
rectal wall including the mucous
membrane.
▪ They continue in the submucosa of the
rectum and thence in the anal columns and
end in a dense capillary plexus at the level
of the anal valves, which anastomose with
branches of the inferior rectal artery.
57
b. Middle rectal artery:
▪ Is a branch of internal iliac artery.
▪ It is present in only one in five people.
▪ It supplies only muscle of middle and lower
portions of rectum.
c. Inferior rectal artery: ▪ Is a branch of internal pudendal artery, in the
perineum.
▪ It supplies the internal and external anal
sphincters, portion of anal canal below anal
valves (lower half of the canal), and perineal
skin. 58
d. Median sacral artery:
▪ Branch of descending abdominal aorta.
▪ Supplies the posterior wall of anorectal
junction, and of the anal canal.
59
Superior rectal
Middle rectal
Inferior rectal
Inferior mesenteric
Internal iliac
Median sacral
60
Venous Drainage:▪ The upper half of the canal drains by
superior rectal veins (about 6 in number),
which begin in the internal rectal venous
plexus (in the submucosa) and continues
upwards in the submucosa.
▪ On the surface of the rectum they unite to
form a superior rectal vein, which is
continuous as the inferior mesenteric vein,
a tributary of the portal circulation.
61
▪ The lower half of the canal drains by
inferior rectal veins, which, on each side,
arises from the external rectal venous
plexus (lies immediately underneath the
skin of the anal canal) and drains into
internal pudendal vein (systemic tributary).
▪ Communicating veins connect the external
and internal plexuses, and so form an
important connection between the systemic
and portal circulations.
62
▪ Much of the blood from the external plexus
normally passes by these communicating
veins into the internal plexus, and, in
consequence of congestion or thrombosis
in the internal plexus, may result in similar
conditions in the external plexus.
63
64
Lymph Drainage:
▪ The upper half of anal canal drains into
pararectal lymph nodes and then
mesenteric lymph nodes.
▪ The lower half of anal canal drains into
medial group of superficial inguinal lymph
nodes.
65
Nerve SupplyA. Rectum
Sympathetic fibers:
▪ Are derived from inferior mesenteric
plexus, and accompanied inferior
mesenteric and superior rectal arteries.
Parasympathetic fibers:
▪ Are derived from S2, 3 and 4 by pelvic
splanchnic nerves via hypogastric plexus.
▪ They are motor to rectal muscle.
66
Pain fibers
Accompany both sympathetic and
parasympathetic supplies.
Sensation of distension
Is conveyed by parasympathetic afferents.
67
B. Anal Canala. Mucous membrane of upper half of the
canal:
Is sensitive to stretch and is supplied by
sensory fibers from hypogastric plexus.
b. Lower half of the canal:
Is sensitive to pain, temperature, touch, and
pressure and is innervated by inferior rectal
nerves.
c. The involuntary internal sphincter:
Is supplied by sympathetic fibers from
hypogastric plexuses. 68
d. The voluntary external sphincter:
Is supplied by inferior rectal nerve, a branch
of pudendal nerve, and by perineal branch of
the S4 nerve.
Portal-Systemic AnastomosisThe rectal veins form an important portal-
systemic anastomosis because the superior
rectal vein drains ultimately into the portal
vein and the inferior rectal vein drains into the
systemic system.
69
Internal Hemorrhoids (Piles)▪ Internal hemorrhoids are varicosities of the
tributaries of superior rectal (hemorrhoidal)
vein and are covered by mucous
membrane.
▪ The tributaries of the vein, which lie in the
anal column at the 3-, 7-, and 11- o'clock
positions when the patient is viewed in the
lithotomy position (commonly used for
pelvic examinations of the female), are
particularly liable to become varicosed.
70
▪ Anatomically, a hemorrhoid is therefore a fold
of mucous membrane and submucosa
containing a varicosed tributary of superior
rectal vein and a terminal branch of superior
rectal artery.
▪ Internal hemorrhoids are initially contained
within the anal canal (first degree).
▪ As they enlarge, they extrude from anal canal
on defecation but return at the end of the act
(second degree).
▪ With further elongation, they prolaps on
defecation and remain outside anus (third
degree). 71
▪ Since internal hemorrhoids occur in the upper
half of the anal canal where the mucous
membrane is innervated by autonomic afferent
nerves, they are painless and are sensitive
only to stretch.
▪ The causes of internal hemorrhoids are many.
▪ They frequently occur in members of the
same family, which suggests a congenital
weakness of the vein walls.
▪ Chronic constipation, associated with
prolonged straining at stool, is a common
predisposing factor.
72
▪ Pregnancy hemorroids are common owing
to pressure on the superior rectal veins by
the gravid uterus.
▪ Portal hypertension as a result of cirrhosis
of the liver can also cause hemorrhoids.
▪ The possibility that cancerous tumors of
the rectum are blocking the superior rectal
vein must never be overlooked.
73
74
Internal Hemorrhoids (Piles)
75
Internal Hemorrhoids (Piles)
External Hemorroids▪ External hemorrhoids are varicosities of
the tributaries of inferior rectal
(hemorrhoidal) vein as they run laterally
from anal margin.
▪ They are covered by skin and commonly
are associated with well-established
internal hemorrhoids.
▪ They are covered by mucous membrane of
the lower half of anal canal or skin, and
they are innervated by inferior rectal
nerves. 76
▪ They are sensitive to pain, temperatures,
touch, and pressure, which explains why
external hemorrhoids are painful.
▪ Its cause is unknown, although coughing or
straining may produce distention of the
hemorrhoid followed by stasis.
▪ The presence of a small, actually tender
swelling at anal margin is immediately
recognized by the patient.
77
Anal Fissure▪ The lower ends of the anal columns are
connected by small folds called anal valves.
▪ In people suffering from chronic
constipation, the anal valves may be torn
down to the anus as the result of the edge
of the fecal mass catching on the fold of
mucous membrane.
▪ The elongated ulcer so formed, known as
an anal fissure, is extremely painful.
78
▪ The fissure occurs most commonly in the
midline posteriorly, or less commonly,
anteriorly.
79