The anorectal region represents the final segment of the digestive tract, encompassing the rectum and the anus. This area plays a role in the storage and elimination of waste from the body. Understanding the structure and function of this region is important for comprehending a fundamental bodily process.
Anatomy of the Anorectal Region
The rectum, measuring approximately 12 to 16 centimeters in length, serves as the terminal portion of the large intestine. It connects the sigmoid colon to the anal canal and functions as a temporary reservoir for feces before elimination. This lower flexure is maintained by the puborectalis muscle, which forms a sling around the anorectal junction.
Following the rectum is the anal canal, a short muscular tube measuring about 3 to 5 centimeters in length, which extends to the anal orifice. The anal canal is surrounded by two distinct rings of muscle known as sphincters: the internal anal sphincter and the external anal sphincter. The internal anal sphincter is an involuntary ring of smooth muscle, formed by a thickening of the circular muscle fibers of the rectum. This sphincter maintains a continuous state of contraction, contributing significantly to the resting pressure of the anal canal.
The external anal sphincter, in contrast, is an oval tube of skeletal muscle fibers, which means it is under voluntary control. This muscle is more substantial than the internal sphincter and plays a role in keeping the anal canal closed.
Key Functions of the Anorectal System
The anorectal system facilitates two primary functions: the controlled elimination of waste, known as defecation, and the ability to prevent involuntary passage of stool, referred to as fecal continence. These processes rely on the coordinated interaction of the rectum, anal canal, and both anal sphincters.
During defecation, the rectal muscles contract, and the internal anal sphincter involuntarily relaxes, allowing fecal matter to move into the anal canal. The external anal sphincter, which is under voluntary control, then relaxes to permit the passage of stool.
Fecal continence, the ability to hold stool, is maintained by a complex interplay of mechanisms. The internal anal sphincter contributes a significant portion of the anal canal’s resting pressure, acting involuntarily to prevent leakage of gases or liquid stool. The external anal sphincter provides additional, voluntary control, allowing individuals to consciously prevent defecation. The anorectal angle, formed by the puborectalis muscle, also helps to maintain continence by creating a kink that prevents stool from continuously entering the anal canal.
Common Conditions of the Anorectal Area
Several medical conditions can affect the anorectal region, leading to discomfort or other issues. Hemorrhoids are a common condition involving swollen veins in the anus and lower rectum, similar to varicose veins. They can be internal, forming inside the rectum, or external, developing under the skin around the anus. In their normal state, these vascular structures are cushions that help with stool control.
Anal fissures are small tears or cracks that occur in the skin lining the anal canal. These tears can cause pain, particularly during bowel movements, and may result in bright red bleeding. Fissures often develop when the anal mucosa is stretched beyond its capacity, such as during the passage of hard stools.
An anal fistula is an abnormal tunnel that forms, typically connecting the anal canal to the skin near the anus. Most anal fistulas arise from an infection in an anal gland, which can lead to a pus-filled collection called an abscess. If the abscess drains, it can leave behind this small channel.
A perianal abscess is a localized collection of pus that develops near the opening of the anus. This infection typically occurs when an anal gland becomes blocked, allowing bacteria to build up and form a painful, swollen lump. Perianal abscesses can be seen as a tender, red bump under the skin near the anus.