Where is the sphincter

The anus is a very important organ. If there is a problem with our anus, our defecation will be affected. Severe patients will not only suffer from constipation and diarrhea, but may also cause rectal prolapse. The anus also has muscles. The internal anal sphincter mainly helps the body defecate by contracting and relaxing. So where is the internal anal sphincter?

The anal sphincter is made up of two overlapping layers of muscle. Its outer layer is the external anal sphincter, which is skeletal muscle and voluntary muscle. The inner layer is the internal anal sphincter, which is an involuntary smooth muscle portion. The internal anal sphincter is a continuation of the circular muscle layer of the rectum. It is a smooth muscle and is pearly white in color. Its upper boundary is level with the plane of the anorectal muscle ring, and its lower boundary reaches the intersphincteric groove, surrounding the upper 2/3 of the anal canal. The muscle bundles of the internal anal sphincter are oval in shape and arranged continuously in a shingle-like manner. The upper fibers are slanted inward and downward, the middle part gradually becomes horizontal, and some fibers in the lower part are slightly upward. The internal anal sphincter is thickest at the lower end, forming a clear annular free edge surrounded by elastic fibers of the combined longitudinal muscles. The external anal sphincter is the outermost muscle of the anal canal. It is a striated muscle innervated by receptor nerves and is a voluntary muscle. It can be divided into three layers, namely the subcutaneous layer, the superficial layer and the deep layer. The subcutaneous layer surrounds the lower end of the anal canal. The superficial layer is an oval muscle bundle that surrounds both sides of the anal canal, with one end ending at the tip of the coccyx and the other end connected to the superficial transverse perineal muscle of the perineum in front of the anus. The deep layer is a circular muscle bundle that surrounds the anal canal and ends at the ischial tuberosities on both sides of the anus.

1. The physiological function of the internal anal sphincter is mainly

It mainly closes the anus and assists in defecation. It is usually in a contracted state, closing the anus to prevent feces, liquid, and gas in the rectum from flowing out, and maintaining a certain tension in the rectum. This contracted state allows it to work continuously except during defecation and is not prone to fatigue. It opens automatically when the rectum is filled with stool, assisting in the passage of stool. The internal anal sphincter is prone to spasm, which is a state of sustained contraction. If stimulated by inflammation, the anus will be in a spasmodic state, causing narrowing of the anus, difficulty in defecation, and anal pain. Dry stools may cause rupture of the anal canal skin. If not treated in time, anal fissures may form.

2. External anal sphincter The external anal sphincter has the function of sphinctering the anus. The composition of the external anal sphincter is like three U-shaped rings, which make the anal canal tightly closed. When you feel the urge to defecate, if external conditions do not allow defecation, you can control defecation by contracting the external sphincter to close the anus. However, the external sphincter is easily fatigued, and continuous contraction can generally only be maintained for 55 seconds. If this time is exceeded, the stool cannot be controlled and is discharged from the body.