Food is broken apart during digestion, and it is then moved from the gastrointestinal tract into circulation and on to the cells (Insel, Turner, & Ross, 2010). Insel et al., (2010) states that many of the nutrients, vitamins, minerals, and water do not need to be digested before they are absorbed but the energy-yielding nutrients such as carbohydrate, fat, and protein are too huge to be absorbed and should be digested first (Seeley, Stephens, & Tate, 2006). Seeley et al. (2006) further described that the stomach works as an intermediate between the behavioural act of eating and the physiological events of digestion and absorption in the intestine. Food has been processed into a thick liquid called chyme by the time it leaves the stomach (The Nemours Foundation, n.
d.). Integrated signals and feedback loops between the intestine and stomach regulate the rate at which chyme enters the duodenum and ensures that the intestine is not overwhelmed (Seeley et al., 2006). Chyme is kept by a muscular tube called Pylorus in the stomach until it reaches the right consistency to pass into the small intestine where digestion of food continues so the body can absorb the nutrients into the bloodstream (The Nemours Foundation, n.d.).Nagel (2000) stated that the chyme becomes compacted into faeces as water is absorbed gradually during the time it passes through the large intestine by peristalsis.
The peristaltic movement is slow and normally needs about 12 to 24 hours for material to travel the along of the organ (Insel, Ross, McMahon, & Bernstein, 2010). Rectum is a place where faeces are stored while anus is a sphincter that controlling defecation (Keshav, 2004). The rectal wall stretches as soon as the powerful peristaltic contractions push the faeces from the sigmoid colon into the rectum, causing the defecation reflex to be initiated (Sullivan, 2004). According to Sullivan (2004), the defecation reflex causes the first sphincter at the beginning of the canal to open, so the faeces is able to enter the anal canal while the outer sphincter is controlled voluntarily and can stay closed.
Sullivan explained that, at this moment, local nerves send a signal to the brain regarding the need to defecate. Once this nerve receive the signals to defecate, the walls of the sigmoid colon and rectum contracts and the anal sphincters relaxes to allow the elimination of faeces through the anus, mentioned by Nagel.