Anus plays the major role to expel out the solid and semisolid waste from the body. When food has passed through the digestive tract, nutrients and water are removed leaving behind the indigestible waste called faeces. As the faeces builds up in the rectum, increased pressure causes a series of muscular contractions. These contractions move the faeces along the rectum and relax the anus, which opens, expelling it from the body. This process is called as defecation.
Disorders like piles or haemorrhoids, anal fissure, anal abscess, anal fistula, crohns disease etc. these are the disorders of anus.
The vibration of the anal opening is the cause for the noise when gas gets passed. There is a faecal incontinence present in the babies which means lack of regular control of bowels. The average human beings produce up to 1.5 litre of gas throughout the day.