The anus or anus serves as the end segment of the digestive system for controlled defecation and ensures the continence of the rectum (rectum). Most of the complaints in the anal region are generally harmless, but are not clarified in many cases due to false shame.
What is the anus?
Schematic diagram showing the anatomy of the rectum or anus, as well as the sphincter muscles. Click to enlarge. The anus (also anus) is the excretory opening of the gastrointestinal tract or rectum. As part of the digestive system, the anus ensures controlled defecation (emptying of stool) and, as a rectal sphincter, ensures fecal continence. In this regard, the anus is primarily modeled by an internal and external ring muscle, whose muscle activity, in cooperation with other structures of the digestive tract, controls defecation.
Anatomy and structure
The anus is formed primarily by two major ring muscles. The inner ring muscle, known as the sphincter ani internus muscle, which functions autonomously, i.e., independently of human will, is composed of smooth muscle that extends seamlessly from the rectal tunica muscularis (smooth muscle layer of the rectum). This is separated from the outer ring muscle of the anus, the sphincter ani externus muscle, by a longitudinally striated muscle layer. The transversely striated sphincter ani externus muscle has fibers running in a loop and is divided into the pars subcutanea, pars superficialis, and pars profunda. The external sphincter muscle is voluntarily innervated via the pudendal nerve, and its relaxation initiates defecation via the anus. The terminal segment of the rectum, formed by the sphincter muscles and about 4 cm long, is called the anal canal (canalis analis) and forms the lumen of the anus. The anal canal is lined by pocket-shaped anal crypts, which are excretory ducts of the glandulae anales (anal glands) that secrete mucous secretions into the canalis analis. Posteriorly (to the back), the anus is connected to the coccygeal vertebrae (Os coccygis) by a connective tissue ligament (Ligamentum anococcygeum), while anteriorly (to the abdomen), the urethra (male) or vagina (female) lies directly adjacent to the anus.
Functions and duties
The two ring-shaped sphincters, in interaction with the levator ani muscle, control the defecation process and accordingly function together as the rectal sphincter. The internal sphincter prevents involuntary defecation. When the intestinal contents enter the rectum during their passage through the gastrointestinal tract, stretch receptors are stimulated on the wall of the rectum, causing dilation of the internal sphincter, while the external sphincter remains contracted and maintains continence via a permanent state of tension (permanent tone) of the musculature. In addition, the external anal sphincter causes dilatation of the so-called plexus venosus rectalis (rectal venous plexus), which additionally seals the anus and prevents leakage of intestinal gases. In addition, contractions of the internal ring muscle ensure that blood cannot flow back. This fills the hemorrhoids and provides a fine seal for the rectum. As the filling volume increases, the urge to defecate builds. Voluntary relaxation of the external ring muscle initiates defecation, with the internal sphincter also relaxing. Defecation is assisted by the reflex contraction activity of the section of the large intestine located between the rectum and the sigmoid (rectosigmoid) and by the elevation of the anus by the levator ani muscle. The so-called abdominal press, the simultaneous contraction of the diaphragm and the abdominal and pelvic floor muscles, can additionally accelerate defecation via the anus.
Diseases
The anus can be affected by various ailments and diseases, although in most cases they are harmless. For example, one of the most clinically relevant diseases of the anus is perianal thrombosis (also called anal thrombosis), in which a venous thrombosis (blood clot) is present in the external area of the anus or in the venous plexus as a result of prolonged sitting (stasis) or permanent pressing activities.Perianal thrombosis should be differentiated from a perianal abscess, which is characterized by purulent inflammation in the tissue structures around the anus and is usually caused by a bacterial infection. In addition, following conservative therapy for perianal thrombosis, so-called marisques (skin folds) can develop, which are susceptible to infection if anal hygiene is inadequate and can lead to dermatitis and pruritus (itching). In addition, the skin of the anal canal may tear (anal fissure) if blood flow to the anal canal skin is reduced, sphincter tone (state of tension of the ring muscles) is increased, and/or constipation (constipation) is present. A very common condition of the anal area is hemorrhoidal disease, which is associated with enlargement of the hemorrhoidal blood cushions, the annular arterovenous vascular cushions that ensure fine continence. In rare cases, anal carcinomas (malignant tumors) may manifest. In addition, genetic malformations of the anus such as atresia ani (occluded anus) may also be observed.
Typical and common diseases
- Anal discomfort (anal discomfort)
- Fecal incontinence
- Anal fissure (anal fissure)
- Anal fistula
- Anal itching (itching at the anus)
- Pain during defecation
