See also: Defecation frequency - When it is normal and when it is not
Defecation consists in the emission of faeces from the body. Basically, it is a physiological reflex triggered by the distension of the terminal portion of the large intestine, called the rectum.
Mass contractions do not occur continuously like remixing ones, but occur on average three or four times a day. Their onset is often associated with the appearance of the defecation reflex. It usually occurs once a day, but a frequency between one discharge every two days and three a day is still considered physiological. It is possible to understand how long the stool remained in the colon by examining its appearance and comparing it with a scale ranging from liquid consistency (diarrhea, insufficient permanence) to goat (particularly hard pellets, excessive permanence), passing through the classic form a sausage, which becomes more or less knotty as you approach a picture of constipation.
Mass peristaltic movements typically occur in the moments following awakening; aided by the upright assumption and the first steps, they push the contents towards the rectum producing the stimulus. In some people it is a physiological impulse so strong that it pushes them to defecate urgently. Other subjects, on the other hand, need to wake up their intestines with a "hearty breakfast. For a mechanism defined as gastrocolic, the distension of the stomach sets the colon in motion, generating the long-awaited stimulus."
As we said, the defecation reflex is triggered by the passage of fecal material into the rectum. The internal anal sphincter is released, while the external one, which is voluntary and therefore controllable, contracts. If the situation is deemed appropriate, the external anal sphincter is released, as well as the levator ani muscle and defecation occurs.
The whole process is favored by voluntary abdominal contractions and forced expiration with the glottis closed (Valsalva maneuver). All this is aimed at increasing intra-abdominal pressure and promoting defecation. In reality, it would be much better to wait for it to start spontaneously and only then exert a slight pressure to facilitate emptying (preventive action on the development of hemorrhoids).
Defecation is a voluntary act and is based on two coordinated events: the relaxation of the pelvic floor and the "increase in intra-abdominal pressure. When the rectum is empty, there is no desire to evacuate." As the stool enters the rectum, the pressure exerted on the rectal wall causes a sensation of fullness. The further distension of the rectal wall induces the relaxation of the internal anal sphincter allowing the stools to come into contact with the sensory receptors present on the upper part of the anal canal; thus the evacuation sensation is felt, which also determines the relaxation of the external sphincter and the pelvic floor muscles. When, on the other hand, the pelvic floor muscles contract to maintain continence, the stool remains in the upper part of the rectum and no longer comes into contact with the anal mucosa. The accommodation of the muscle cells to the new content reduces the tension of the rectal wall and the desire to evacuate ceases.
Evacuation is favored by taking particular positions, such as crouching (squat), in which the abdomen is naturally compressed against the thighs.
Defecation is also influenced by the psychological state and dietary habits of the subject (see diet for constipation), which can favor a slowing or an increase in intestinal motility (see diarrhea and constipation).
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