Anus

Overview

The anus is the terminal opening of the digestive tract through which feces are expelled from the body. It marks the end of the anal canal and plays a critical role in the voluntary and involuntary control of defecation. Despite its small size, the anus is a complex structure composed of skin, muscle, vascular tissue, and nerve supply specialized for maintaining continence and coordinating bowel elimination.

Location

The anus is located in the perineal region, specifically in the anal triangle of the pelvic floor. Key anatomical landmarks include:

• Anteriorly: Perineal body (in males, near the scrotum; in females, near the posterior vaginal wall)

• Posteriorly: Coccyx

• Superiorly: Continuous with the anal canal

• Inferiorly: Opens externally as the anal orifice

Structure

The anus is approximately 2–4 cm in length and consists of the following components:

• External anal sphincter: A voluntary skeletal muscle encircling the anus, under conscious control

• Internal anal sphincter: An involuntary smooth muscle layer that maintains baseline tone

• Perianal skin: The outer part of the anus is lined by keratinized stratified squamous epithelium, similar to normal skin and containing hair follicles, sebaceous glands, and sweat glands

• Vascular plexus: Subcutaneous hemorrhoidal veins contribute to the hemorrhoidal cushions involved in fine control of continence

The mucocutaneous junction — also called the white line of Hilton — marks the transition from non-keratinized to keratinized epithelium.

Function

The primary functions of the anus include:

• Regulating defecation: The anus provides the final control mechanism for the release of feces

• Maintaining continence: Through tonic contraction of the internal sphincter and voluntary control of the external sphincter

• Protecting underlying structures: The anal skin serves as a barrier against mechanical trauma, microbial invasion, and irritants

Physiological Role(s)

The anus supports several vital physiological processes:

• Voluntary and involuntary muscle coordination: The internal and external anal sphincters work together to maintain continence and permit timely defecation

• Sensory discrimination: Richly innervated to distinguish between gas, liquid, and solid stool, enabling appropriate reflex responses

• Support of pelvic floor function: Works with the levator ani and puborectalis muscles to maintain anorectal angle and continence

These roles are tightly regulated by both autonomic (involuntary) and somatic (voluntary) nervous systems.

Clinical Significance

The anus is involved in numerous common and important medical conditions:

• Anal fissures: Painful tears in the anoderm, often caused by hard stools or straining

• Hemorrhoids: Engorgement of venous plexuses; external hemorrhoids occur below the dentate line and are often painful

• Perianal abscess: Infection of anal glands may result in pus accumulation, requiring surgical drainage

• Anal fistula: Abnormal tract connecting the anal canal to the skin, typically a complication of abscesses

• Anal cancer: Most commonly squamous cell carcinoma, associated with HPV infection and more common in immunocompromised individuals

• Fecal incontinence: Results from damage to sphincters, nerves, or pelvic floor; may be caused by childbirth, surgery, trauma, or neurological disease

• Proctologic examination: Includes inspection, digital rectal examination (DRE), anoscopy, and proctoscopy to evaluate anorectal disorders

Understanding anal anatomy and physiology is essential in managing conditions affecting defecation, continence, and perianal health.