The Digestive System Lecture 5 - PowerPoint Presentation

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The Digestive System

Lecture 5

Slide2

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.

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Slide3

Relations

Anterior 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

.

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Slide4

Membranous urethra

Anal canal

Perineal body

Anterior relations of anal canal (male)

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Slide5

Anal canal

Perineal body

Vagina

4

Anterior relations of anal canal (female)

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Lateral relations of anal canal

Levator ani

Obturator internus

Ischiorectal fossa

Anal canal

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Slide7

Anococcygeal Raphe

Levator ani

Posterior

relations of anal canal

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Slide8

The

upper half

of the anal canal is

lined by columnar epithelium.

The mucous membrane in this region exhibits

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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.

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Slide9

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

.

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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 Sphincter

Consist three parts:

Subcutaneous part:

Encircles the lower end of the anal canal beneath the skin at the anal orifice, and has no bony attachments

.

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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.

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Slide13

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Internal anal sphincter

Internal Anal Sphincter

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External

Anal Sphincter

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Slide15

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.

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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

.

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Slide17

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

.

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Slide18

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.

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Slide19

Blood Supply

Arterial 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.17

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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.

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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.

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d. Median sacral artery:

Supplies the posterior wall of anorectal junction, and of the anal canal.

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Superior rectal

Middle rectal

Inferior rectal

Inferior mesenteric

Internal iliac

Median sacral

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Slide24

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.22

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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

. 23

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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

.

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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

. 26

Slide29

Nerve Supply

A. 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.

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Slide30

Pain fibers

Accompany

both sympathetic and parasympathetic supplies.

Sensation of distension

I

s

conveyed by parasympathetic afferents.

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Slide31

B. Anal Canal

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.

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Slide32

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.

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Slide33

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.

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Slide34

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

). 32

Slide35

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.

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Slide36

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

.

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Internal Hemorrhoids (Piles)

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Internal Hemorrhoids (Piles)

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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.

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Slide40

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.

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Slide41

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.

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Slide42

The fissure occurs most commonly in the midline posteriorly, or less commonly, anteriorly

.

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