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