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Uitwerpselvorming en ontlasting
Na 3 tot 10 uur in de dikke darm verliest chymus veel water en wordt het ontlasting, het eindproduct van de spijsvertering. Ontlasting bestaat uit onv…
After 3 to 10 hours in the large intestine, chyme undergoes considerable water loss to form feces, the end product of digestion.
It comprises undigested dietary fiber like cellulose, mucus, sloughed-off epithelial cells, and microbes.
The descending and sigmoid colon stores feces, which undergoes haustral contractions for final drying, retaining enough water for a semisolid texture.
Next, mass peristalsis propels the feces into the usually empty rectum, distending the rectal walls to stimulate the stretch receptors.
As a result, the defecation reflex, an involuntary response for lower bowel movement, is triggered.
It begins with the myenteric plexus, triggering the short reflex to stimulate contractions in the sigmoid colon and rectum.
At the same time, parasympathetic motor neurons in the sacral region trigger the long reflex to enhance mass peristalsis.
The mass movements move the feces into the anal canal, triggering the defecation reflex which relaxes the internal anal sphincter.
Finally, the pudendal nerves of the somatic nervous system inhibit the skeletal muscles of the external anal sphincter, allowing relaxation for the voluntary expulsion of feces.
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Q1: What is feces composed of?
Feces, the end product of digestion, comprises undigested dietary fiber like cellulose, mucus, sloughed-off epithelial cells, and microbes. After 3 to 10 hours in the large intestine, chyme loses considerable water to form this semisolid waste material that the descending and sigmoid colon store before elimination.
Q2: How does the large intestine prepare feces for elimination?
The descending and sigmoid colon store feces and use haustral contractions to dry it out while retaining enough water for a semisolid texture. Mass peristalsis then propels the feces into the rectum, where rectal wall distention stimulates stretch receptors that trigger the defecation reflex.
Q3: What triggers the defecation reflex?
The defecation reflex is triggered when mass peristalsis moves feces into the rectum, distending the rectal walls and stimulating stretch receptors. This involuntary response initiates a coordinated series of neural events involving both the myenteric plexus and parasympathetic motor neurons to facilitate bowel movement.
Q4: How do the short and long reflexes work during defecation?
The myenteric plexus triggers the short reflex to stimulate contractions in the sigmoid colon and rectum. Simultaneously, parasympathetic motor neurons in the sacral region activate the long reflex to enhance mass peristalsis, allowing both reflexes to work together to move feces into the anal canal.
Q5: What is the role of the anal sphincters in defecation?
The defecation reflex relaxes the internal anal sphincter involuntarily. The pudendal nerves of the somatic nervous system then inhibit the skeletal muscles of the external anal sphincter, allowing voluntary relaxation and enabling the expulsion of feces from the body during elimination.
Q6: Why is water retention important in feces formation?
Haustral contractions in the descending and sigmoid colon dry out feces while retaining enough water to maintain a semisolid texture. This balance prevents feces from becoming too dry and hard, which would make elimination difficult, while also ensuring adequate water reabsorption occurs.
Q7: How long does chyme remain in the large intestine?
Chyme spends 3 to 10 hours in the large intestine, during which it undergoes considerable water loss to form feces. This extended transit time allows the colon to absorb water and electrolytes while haustral contractions progressively dry and compact the remaining digestive material.