Rectum and Anal Canal

Development

• Rectum and superior anal canal: derived from endoderm of the hindgut from the primitive gut tube
• Lower portion of the anal canal (including the external anal sphincter (EAS)): derived from ectoderm
• Junction of the upper and lower anal canal resulting from different embryologic origins is called the pectinate line (also known as the dentate line).

Gross anatomy

Rectum

• A straight, tubular structure in the pelvic cavity
• Located between the sigmoid colon and anal canal
• Length: approximately 15 cm
• Relation of the rectum with the peritoneum:
  • Upper 3rd: covered anteriorly and laterally by the peritoneum
  • Middle 3rd: covered anteriorly (only) by the peritoneum
  • Lower 3rd: not covered by the peritoneum (completely below the pelvic cavity)
• Rectosigmoid junction:
  • Point between the sigmoid colon and rectum
  • Defined by loss of taenia coli of the colon
• The rectum has 2 flexures:
  1. Sacral flexure: a dorsal concave bend as the rectum courses anterior to the sacrum
  2. Anorectal flexure: 
     • A ventral convex bend representing the transition between the rectum above and the anal canal below
     • Formed by the puborectalis muscle (part of the levator ani complex) wrapping like a sling from the pubic bones anteriorly around the posterior anorectum
• Rectal folds (valves of Houston):
  • 3 transverse submucosal folds of tissue protruding into the lumen:
    1. Superior
    2. Intermediate
    3. Inferior
  • To create “shelves” that help hold feces
  • Help separate solid feces from gas
• Rectal ampulla:
  • Terminal portion of the rectum
  • Present on top of the pelvic diaphragm
  • Capable of expanding to serve as a reservoir for feces

Anal canal

• A straight tubular structure at the end of the GI tract
• Connects the rectum to the anus (external opening)
• Length: approximately 3‒4 cm
• Completely extraperitoneal (known as “subperitoneal”)
• Anal columns (also called columns of Morgagni):
  • Approximately 6‒10 vertical folds of the mucosa, submucosa, and circular muscle layer
  • Found in the upper half of the lumen of the anal canal
• Anal sinuses (rectal sinuses):
  • Furrows in the anal canal that separate the anal columns from each other 
  • End in small valve-like folds (anal valves)
• Anal valves:
  • Small valve-like folds at the lower ends of the anal sinuses in the rectum
  • Join together at the lower ends of the anal columns
  • Valves and sinuses form the pectinate line.
• Pectinate line (also known as the dentate line):
  • Divides the anal canal into upper ⅔ and lower ⅓
  • Multiple neurovascular and histologic differences above versus below the line
• The anal canal has 2 anal sphincters:
  • Internal anal sphincter (IAS):
    • Under involuntary control
    • Surrounds the upper ⅔ of the anal canal
    • Formed from a thickening of the circular smooth muscle in the bowel wall
  • EAS:
    • Under voluntary control
    • Surrounds the lower ⅔ of the anal canal
    • Consists of several parts: subcutaneous, superficial, and deep

Anatomic relations

The rectum is the most posterior visceral organ in the pelvic cavity.

• Anterior to the rectum and anal canal are:
  • In males:
    • Bladder
    • Seminal vesicles
    • Prostate gland
  • In females:
    • Uterus
    • Cervix
    • Vagina
• Posterior to the rectum and anal canal:
  • Lower sacrum: S3‒S5
  • Coccyx (joined to the rectum by the anococcygeal ligament)

Microscopic Anatomy

Similar to other segments of the GI tract, the layers of the anorectal wall (from the inner lumen outward) are mucosa → submucosa → muscular layer → serosa. There are no villi or circular folds in the anorectal wall like those in the small intestine.

Mucosa

• Consists of 3 sublayers:
  1. Epithelium (innermost lining):
     • Above the pectinate line: columnar epithelium
     • Below the pectinate line: nonkeratinized, stratified squamous epithelium
  2. Lamina propria (contains small vasculature)
  3. Muscularis mucosa (thin layer of smooth muscle)
• Intestinal crypts of Liberkühn/anal mucous glands:
  • Pores that open into tubular glands
  • Similar to crypts in the small intestines, but deeper
  • Contain several mucus-secreting goblet cells that help lubricate feces and ease the passage of stool

Submucosa

• Loose connective tissue
• Contains larger vessels

Muscular layer

Made up of 2 layers of smooth muscle:

• Circular layer (inner layer)
• Longitudinal layer (outer layer): Taeniae coli of the colon coalesce to form a continuous outer layer.
• Myenteric (Auerbach’s) plexus:
  • Ganglia of the ANS that control the muscular layer
  • Located between the 2 layers of smooth muscle

Serosa

• Made up of connective tissue
• Joins with the peritoneum anteriorly in the upper regions of the rectum
• Becomes a thicker fibrous layer in the subperitoneal regions

Neurovasculature

Arterial blood supply

• Above the pectinate line (including the rectum and upper anal canal): superior rectal artery (branch of the inferior mesenteric artery)
• Below the pectinate line:
  • Middle rectal artery (branch of the internal iliac artery)
  • Inferior rectal artery (branch of the internal pudendal artery)

Venous drainage

• Portosystemic anastomosis exists around the pectinate line.
• Above the pectinate line:
  • Drains into the portal system
  • Internal hemorrhoidal plexus → superior rectal vein → inferior mesenteric vein (IMV)
• Below the pectinate line: 
  • Drains into the systemic veins (inferior vena cava)
  • External hemorrhoidal plexus → middle and inferior rectal veins → internal pudendal vein → internal iliac vein

Lymphatic drainage

• Above the pectinate line: internal iliac lymph nodes
• Below the pectinate line: superficial inguinal lymph nodes

Innervation

• Above the pectinate line:
  • Autonomic/visceral innervation via the inferior hypogastric plexus (contains both sympathetic and parasympathetic fibers)
  • Parasympathetic fibers relax the IAS.
  • Sympathetic fibers maintain tonic contraction of the IAS.
  • Under involuntary control
• Below the pectinate line: somatic innervation
  • Inferior rectal branch of the pudendal nerve
  • Under voluntary control

Functions

General functions

The primary roles of the rectum and anal canal involve controlled defecation.

• The rectum is able to store feces until defecation is consciously desired.
• Anal sphincters are in a state of tonic contraction, preventing fecal expulsion.
  • Internal sphincter: smooth muscle under involuntary control
  • External sphincter: skeletal muscle under voluntary control
• Defecation requires:
  • Peristalsis of rectal muscles
  • Involuntary relaxation of the IAS
  • Voluntary relaxation of the EAS

Defecation reflex

The rectum contains stretch receptors that stimulate the defecation reflex when the rectum begins to fill with feces.

• Stretch receptors activated in rectum → 
• Sensory nerve fibers carry signals to the sacral spinal cord → 
• Synapses with parasympathetic motor fibers → 
• Sends signals for peristaltic waves to the myenteric nerve plexus in the muscular layers of:
  • Descending colon
  • Sigmoid colon
  • Rectum
  • IAS
• Peristaltic contractions in the colon and rectum move feces downward.
• IAS relaxes.
• Defecation occurs only if the EAS is voluntarily relaxed at the same time due to motor impulses from the cerebral cortex.
• If defecation is consciously suppressed, peristaltic contractions cease within a few minutes.

Clinical Relevance

• Fecal incontinence: involuntary leakage of solid or liquid stool. There are 2 major types of fecal incontinence: urge incontinence (the desire to defecate is felt, but stool cannot be voluntarily retained) and passive incontinence (there is no desire to defecate before stool is involuntarily passed). Fecal incontinence can result from dysfunction of anal sphincters, abnormal rectal compliance, decreased rectal sensation, altered stool consistency, or frequently, a combination of these factors.
• Hemorrhoids: swollen veins in the hemorrhoidal plexuses surrounding the anus and lower rectum that are similar to varicose veins. Hemorrhoids can develop inside the rectum (internal hemorrhoids) or under the skin around the anus (external hemorrhoids). The most important clinical manifestations include hematochezia, pain associated with a thrombosed hemorrhoid, and perianal pruritus. Risk factors include advancing age, diarrhea, pregnancy, pelvic tumors, prolonged sitting, straining, and chronic constipation.
• Perianal and perirectal abscesses: pus collection in the enclosed space near the perirectal tissues. Perianal and perirectal abscesses typically originate due to obstruction of the anal crypt glands. Affected individuals present with severe pain in the anal or rectal area and a fluctuant mass on physical exam. Management requires prompt surgical incision and drainage, which may be followed by a course of antibiotics. If untreated, abscesses can lead to the formation of fistulas.
• Anal fistula: abnormal communications between the anorectal lumen and the skin or another structure in the body. An anal fistula often occurs due to the extension of anal abscesses but is also associated with specific conditions such as Crohn disease. Symptoms include pain or irritation around the anus; abnormal discharge or purulent drainage; and swelling, redness, or fever, if an abscess is present.
• Anal fissures: painful, superficial tears in the epithelial lining (anoderm) of the anal canal that cause anal pain and bleeding. Anal fissures are common in infants and middle-aged individuals and are typically secondary to local trauma or irritation that results from constipation, diarrhea, or anal intercourse. Clinical manifestations include severe pain during bowel movements, bright-red blood in the stool, the presence of blood on toilet paper, a visible crack in the skin around the anus, and a small lump or skin tag on the skin near the anal fissure.
• Rectal prolapse: a condition in which the rectum protrudes from the anal orifice either spontaneously or after defecation. Rectal prolapse may involve all layers of the rectal wall or only the mucosa. Damage to the pelvic floor and loss of internal rectal support are some of the causes of rectal prolapse. Symptoms of rectal prolapse include the development of an anal mass, or abdominal pain, incomplete evacuation, altered bowel habits, and mucus/stool discharge. Rectal prolapse is most common in elderly women.
• Colorectal cancer (CRC): almost all cases of CRC are adenocarcinoma and the majority of lesions arise from the malignant transformation of an adenomatous polyp. The most important clinical manifestations of CRC include changes in bowel habits, a rectal mass that is palpable on digital exam, iron deficiency anemia, and rectal bleeding; however, most individuals are asymptomatic. A rectal exam, colonoscopy, and/or stool tests for screening are generally recommended in individuals ≥ 50 years of age.