Rectum

The rectum is the terminal portion of the large intestine, serving as a temporary storage site for feces before defecation. It plays a critical role in maintaining continence and coordinating the complex process of defecation. Its anatomical and functional characteristics are essential for understanding both normal physiology and various clinical conditions.

Anatomy of the Rectum

Gross Anatomy

The rectum extends from the sigmoid colon to the anal canal and occupies the pelvic cavity. It measures approximately 12 to 15 centimeters in length and follows the curvature of the sacrum and coccyx. The rectum is traditionally divided into three parts: upper, middle, and lower rectum. Its posterior surface lies against the sacrum and coccyx, while anterior relations differ between sexes, including the bladder and prostate in males, and the vagina in females.

• Location and boundaries: Between the sigmoid colon and anal canal within the pelvis
• Length and divisions: Upper rectum, middle rectum, lower rectum
• Relation to surrounding structures: Pelvic organs, sacrum, bladder, prostate, and vagina

Microscopic Anatomy

On a microscopic level, the rectal wall consists of several layers that support its function in storage and controlled evacuation of feces. The mucosa contains a columnar epithelium with mucus-secreting cells. The submucosa is rich in blood vessels and lymphatics. The muscularis externa consists of an inner circular layer and an outer longitudinal layer, with specialized fibers forming the rectal sphincters. The outermost layer is either serosa or adventitia depending on the location.

• Mucosa: Columnar epithelium with mucus-secreting crypts
• Submucosa: Contains vascular and lymphatic networks
• Muscularis externa: Inner circular and outer longitudinal muscle layers
• Serosa/adventitia: Peritoneal covering varies along the rectum

Blood Supply, Lymphatics, and Innervation

Arterial Supply

The rectum receives a rich blood supply from multiple arteries, ensuring adequate perfusion for both normal function and healing after injury. These arteries originate from both the inferior mesenteric artery and internal iliac artery.

• Superior rectal artery: Terminal branch of the inferior mesenteric artery supplying the upper rectum
• Middle rectal artery: Branch of the internal iliac artery supplying the mid-rectum
• Inferior rectal artery: Branch of the internal pudendal artery supplying the lower rectum and anal canal

Venous Drainage

Venous drainage of the rectum involves both the portal and systemic circulations, which has clinical significance in conditions such as hemorrhoids.

• Superior rectal vein: Drains into the inferior mesenteric vein and portal system
• Middle and inferior rectal veins: Drain into the internal iliac veins and systemic circulation
• Portal-systemic connections: Facilitate potential portosystemic shunting

Lymphatic Drainage

The rectal lymphatic system is important for immune surveillance and the spread of malignancies. Lymph drains through several nodal groups depending on the rectal level.

• Pararectal lymph nodes: Located adjacent to the rectum and receive primary lymphatic drainage
• Internal iliac lymph nodes: Receive lymph from the middle and lower rectum
• Inguinal lymph nodes: Involved in drainage of the distal anal canal

Nerve Supply

Innervation of the rectum includes both autonomic and somatic components, allowing voluntary and involuntary control of defecation.

• Autonomic innervation: Sympathetic fibers from the inferior mesenteric plexus and parasympathetic fibers from the pelvic splanchnic nerves
• Sensory innervation: Provides awareness of rectal distension and pain, contributing to defecatory reflexes

Physiology of the Rectum

Storage and Transit Function

The rectum functions primarily as a storage reservoir for feces. It demonstrates high compliance to accommodate varying volumes and coordinates the timing of defecation.

• Fecal storage and rectal compliance: Allows temporary accumulation without significant increase in pressure
• Rectoanal inhibitory reflex: Relaxation of the internal anal sphincter in response to rectal distension
• Defecation mechanism: Coordination of rectal contraction and sphincter relaxation to expel feces

Neurophysiology

The rectum is regulated by a complex interaction between the enteric nervous system and central nervous system pathways, enabling controlled defecation and continence.

• Role of enteric nervous system: Local reflexes regulate motility and sphincter function
• Central nervous system integration: Brain and spinal cord centers modulate voluntary control
• Voluntary and involuntary control: Internal sphincter responds automatically, external sphincter allows conscious regulation

Clinical Considerations

Common Diseases and Disorders

The rectum is susceptible to a variety of diseases that can impact bowel function, cause pain, or result in serious complications. Early recognition and management are essential for optimal outcomes.

• Hemorrhoids: Dilated veins of the rectal plexus causing bleeding, pain, or prolapse
• Anal fissures: Linear tears in the anoderm resulting in pain and bleeding
• Rectal prolapse: Full or partial protrusion of the rectal wall through the anal canal
• Inflammatory bowel disease involvement: Ulcerative colitis and Crohn’s disease affecting rectal mucosa
• Rectal cancer: Malignant tumors arising from rectal epithelium

Diagnostic Techniques

Accurate diagnosis of rectal diseases relies on a combination of clinical examination and imaging studies. Endoscopic evaluation allows direct visualization and biopsy of lesions.

• Digital rectal examination: Initial assessment for masses, tenderness, and sphincter tone
• Endoscopy: Sigmoidoscopy and colonoscopy for direct visualization and tissue sampling
• Imaging: CT, MRI, and defecography for structural and functional assessment

Surgical and Therapeutic Approaches

Treatment of rectal disorders may include conservative measures, minimally invasive procedures, or open surgical interventions depending on severity and pathology.

• Medical management: Pharmacological therapies for inflammation, pain, or constipation
• Minimally invasive procedures: Rubber band ligation, sclerotherapy, or endoscopic resection
• Open surgical techniques: Resection, anastomosis, or repair of prolapse and malignancy

Development and Embryology

• Origin from the hindgut: Rectum develops from the terminal portion of the embryonic hindgut
• Development of anorectal canal: Formation of the rectum and anal canal with partitioning of cloaca
• Congenital anomalies: Imperforate anus, rectal atresia, and fistulas due to abnormal development

References

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