Although our digestive system absorbs most of the nutrients from the food we consume, there are substances that are impossible to digest, like for example cellulose from foods that contain fiber. Under normal conditions, this type of compound mixed with a surplus amount of gastric juices, remains of bile and intestinal lining cells makes up feces or fecal matter.
An important part of this waste material is also bacteria that, whether dead or debilitated, settle in the intestinal wall and travel alongside feces. These microorganisms make up nearly one third of their dry weight. Escherichia coli stands out among them.
In simple words, defecation consists on the elimination of the feces formed in the large intestine through the anal orifice. It sounds like a simple task because all of the structures involved are anatomically adapted to ease the evacuation of solid waste. However, as in all bodily actions, a series of signals and stimuli are activated to unleash this important process.
Defecation, just like micturition, is a reflex and voluntary act at the same time. Although we feel the desire to defecate and can control the urge at times, a series of nerves spring into action in order to ease the exit of feces. There is a nerve control over defecation, whose primary center is located in the spinal cord (sacral portion).
We already said the large intestine is the portion of the digestive tube in charge of reabsorbing water and forming, with the wastes of digestion, feces. It is correct to point out then, that the contractions at all of its portions begin defecation, as the waste is driven towards the rectal and anal area through the peristaltic movements.
Once the feces is stocked up, a series of conjoined actions begin. A distension in the walls of the rectum is produced, which is picked up by some receptors. The information is sent to the spinal cord through an afferent or sensory way and the reply arrives through an effectory or motor way (in which the hypogastric, pelvic and pundendal nerves are involved), producing an increase in the contractions around the sigmoid colon, relaxation of the internal anal sphincter and the contraction of the abdominal muscles.
It is necessary to point out that the action of the external anal sphincter, as well as the puborectal muscle, depend on our conscience. They both have a high degree of voluntary nerve control, so we can keep it contracted until we choose to relax it, allowing defecation.