What is the anus?
The anus, also called the anal canal, is the lowest end of the rectum. It is divided into three areas from top to bottom:
Zona columnaris: the mucosa here has six to eight longitudinal anal columns with indentations in between. Beneath the mucosa lies a vascular cushion (corpus cavernosum recti), which, by filling with blood, causes the anal columns to lie against each other. This closes the anal canal, thus contributing to continence. In the case of a pathological, knot-like expansion of the vascular cushion, one speaks of “internal hemorrhoids”.
Zona intermedia: It is also called zona alba because of the light-appearing mucosa. The venous plexus located here forms the so-called “external hemorrhoids” in the case of pathological dilatation.
Zona cutanea: It borders on the external sphincter and resembles the skin due to stronger pigmentation as well as sebaceous and sweat glands.
The sphincters at the anus
The two sphincters together have a strong tone that can be felt when trying to insert a suppository or when the doctor wants to perform a rectal examination. Complementing the muscular closure is the venous plexus of the zona haemorrhoidalis, the filling of which guarantees tight closure.
What is the function of the anus?
The anal canal serves for bowel evacuation (defecation), which is a complex process. The peristalsis of the muscular wall of the colon (the upstream section of the large intestine) transports feces into the rectum. Filling of the rectum leads to relaxation of the internal sphincter and increased contraction of the external sphincter. The urge to defecate is noticeable, but can be suppressed by consciously holding the external sphincter in contraction.
What problems can the anus cause?
Perianal thrombosis is a vein thrombosis at the outer edge of the anus. The veins of the sphincter are affected, and the thrombus causes a reddened lump to form at the edge of the anus.
Hemorrhoids (more precisely, a hemorrhoidal condition) can be noticeable, for example, by bright red traces of blood on the feces or toilet paper.
An anal abscess is an acute, encapsulated, purulent inflammation around the anus caused by bacterial infections of the anal glands. In an anal fistula, the inflammation has made a tubular passage from the anal canal through the body to the outside, through which secretions and pus drain.
In an anal fissure, the skin in the anal canal is torn, for example, due to hard stool or frequent diarrhea.
In anal prolapse, the anal canal bulges outward when the patient pushes hard during defecation.
Malignant tumors (carcinomas) rarely occur in the anal area.