As anus (also anus) the excretory opening of the gastrointestinal tract or the rectum is called.
As an integral part of the digestive system, the anus ensures controlled defecation (defecation) and, as a rectal closing organ, ensures the continence of the stool.
The anus is primarily modeled by an internal and external circular muscle, whose muscular activity, in cooperation with other structures of the digestive tract, controls the defecation.
The anus is mainly made up of two major circular muscles. The autonomous, ie independent of the human will, functioning internal sphincter muscle, the so-called sphincter ani internus muscle, consists of smooth musculature, which proceeds seamlessly from the rectal tunica muscularis (smooth muscle layer of the rectum).
It is separated from the outer sphincter muscle of the anus, the sphincter ani externus muscle, by a longitudinal muscle stripe. The striated external sphincter muscle has looped fibers and is subdivided into the subcutaneous, pars superficial and pars profunda.
The outer sphincter muscle is innervated at will via the pudendal nerve and, via its relaxation, initiates the discharge of the stool via the anus. The end segment of the rectum, which is formed by the sphincter muscles and is about 4 cm long, is called the anal canal (anal canal) and forms the lumen of the anus. The anal canal is lined with pocket-like anal crypts, which, as ducts of the glands, secrete analgesic mucous secretions in the anal canal.
Posteriorly (to the back), the anus is connected to the coccyx vertebrae by a connective tissue ligament (anococcygeal ligament), while the anterior (abdominal) urethra (male) or vagina (female) directly adjoins the anus.
The two ring-shaped sphincters control the defecation process in conjunction with the levator ani muscle and act together as a rectal closure apparatus. The inner sphincter prevents the involuntary discharge of the stools.
If the intestinal contents penetrate into the rectum during its passage through the gastrointestinal tract, expansion receptors are stimulated on its wall, which cause an expansion (dilation) of the inner sphincter while the external sphincter muscle contracts and continence via a permanent tension condition (continuous tone) of the musculature maintains.
In addition, the external anal sphincter causes an enlargement of the so-called Plexus venosus rectalis (rectal venous plexus), which additionally seals the anus and prevents leakage of intestinal gases. In addition, the contractions of the internal circular muscle ensure that the blood can not flow back. As a result, the hemorrhoids are filled and ensure a fine sealing of the rectum. As the filling volume increases, the urge to stool builds up.
The voluntary relaxation of the external circular muscle initiates the defecation, with the inner sphincter also relaxing. The defecation is supported by the reflex contraction activity of the rectum and sigmoid colon (rectosigmoid) as well as by the anus being raised by the levator ani muscle. By the so-called abdominal press, the simultaneous contraction of diaphragm and abdominal and pelvic floor muscles, the stool can be accelerated through the anus additionally.
The anus can be affected by a variety of ailments and disorders, which are, however, harmless in most cases. One of the clinically most relevant diseases of the anus is, for example, perianal thrombosis (also called anal thrombosis), in which there is a venous thrombosis (blood clot) in the outer area of the anus or in the venous plexus due to long sitting (stasis) or permanent pressure. A perianal thrombosis should be differentiated from a perianal abscess, which is characterized by a purulent inflammation in the tissue structures around the anus and is usually caused by a bacterial infection.
In addition, following a conservative treatment of perianal thrombosis, so-called skin tags (skin folds) may develop which, if insufficiently anal-friendly, are susceptible to infections and can lead to dermatitis and pruritus (itching). In addition, a reduced blood flow to the anal canal, increased sphincter tone (tension of the circular musculature) and / or constipation present (constipation) can cause the skin of the anal canal to rupture (anal fissure).
A very common disorder of the anal area is hemorrhoidal disease associated with enlargement of the hemorrhoidal blood pads, the annular arterovenous vascular cushions that ensure fine continency. In rare cases, anal cancer (malignant tumors) may manifest. In addition, genetic abnormalities of the anus such as an atresia ani (closed anus) can also be observed.