Anatomy of the Rectum and Anal Canal

Question 1

At what vertebral level does the rectum begin?

Answer 1

S3

Question 2

Which 2 flexures are found within the rectum?

Answer 2

Sacral flexure

Rectoanal flexure - Formed by the puborectalis muscle

Question 3

Function of the rectal ampulla

Answer 3

Acts as a temporary stores of faeces before defecation. This structure is continuous with the anal canal.

Question 4

Outline the arterial supply to the rectum

Answer 4

Upper 2/3: Supplied by the super rectal artery, from the IMA
Middle 1/3: Supplied by the middle rectal artery, from the internal iliac artery
Lower 1/3: Supplied by the inferior rectal artery, from the internal pudendal artery

Question 5

Outline the venous drainage of the rectum

Answer 5

Superior rectal artery drains into the portal venous system.
The middle and inferior rectal arteries drain into the systemic venous system.
*Anastomoses between the portal and systemic venous system are located within the wall of the anal canal.

Question 6

Outline the innervation of the rectum

Answer 6

Sympathetic: Lumbar splanchnic nerves and inferior/superior hypogastric plexuses
Parasympathetic: S2-S4 from pelvic splanchnic nerves
Visceral afferents: Follow the parasympathetic nerves

Question 7

Outline the lymphatic drainage of the rectum

Answer 7

Pararectal lymph nodes –> inferior mesenteric nodes

Question 8

Describe the sphincters of the anal canal

Answer 8

Internal sphincter: Formed from involuntary smooth muscle and is seen at the upper 2/3 of the anal canal.
External sphincter: Formed from voluntary muscle and forms the lower 2/3 of the anal canal. The sphincter merges with the puborectalis muscle of the pelvic floor.

Question 9

What structure ‘divides’ the anal canal?

Answer 9

The pectinate line - formed by anal valves

Question 10

Outline the embryological origin of the anal canal both above and below the pectinate line.

Answer 10

Above: Derived from embryonic hindgut (endoderm)
Below: Derived from ectoderm

Question 11

Outline the arterial supply of the anal canal

Answer 11

Above the pectinate line: Superior anal artery, from IMA

Below the pectinate line: Inferior anal artery, from the internal pudendal artery

Question 12

Outline the innervation of the anal canal

Answer 12

Above the pectinate line: Visceral innervation from the inferior hypogastric plexus
Sensitive to stretch
Below the pectinate line: Somatic innervation from the inferior anal nerves (from the pudendal nerve)
Sensitive to pain, temperature, touch and pressure

Question 13

Outline the venous drainage of the anal canal

Answer 13

Above the pectinate line: Superior anal vein (into inferior mesenteric vein)
Below the pectinate line: Inferior anal vein (into internal pudendal vein)

Question 14

Pathological haemorrhoids: Causes, symptoms

Answer 14

Haemorrhoids are vascular cushions found within the wall of the anal canal of healthy individuals - help maintain faecal continence.
Pathological haemorrhoids describe these vascular cushions when they become swollen and distended.
Causes: Prolonged straining (e.g. constipation), raised intraabdominal pressure (ascites, pregnancy)
Symptoms: Itching and bleeding

Question 15

Which muscles form the anorectal ring?

Answer 15

The muscles of the internal and external anal sphincter and puborectalis muscle