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粪便形成和排便
食糜在大肠中停留 3 到 10 小时后，会失去大量水分，变成粪便，即消化的最终产物。粪便由未消化的膳食纤维（如纤维素）、粘液、脱落的上皮细胞和微生物组成。降结肠和乙状结肠储存粪便，并使用结肠袋收缩使其干燥，但保留足够的水分使其呈半固体状。
然后，大量蠕动将粪便推入直肠，从而拉伸直肠壁以激活拉伸感受器…
食糜在大肠中停留 3 到 10 小时后，会大量失水，形成粪便，即消化过程的最终产物。
它包含未消化的膳食纤维（如纤维素）、黏液、脱落的上皮细胞以及微生物。
降结肠和乙状结肠储存粪便，粪便在此经历袋状收缩以完成最后的干燥过程，保留足够的水分以形成半固体状质地。
接着，集团蠕动将粪便推进通常为空的直肠，扩张直肠壁以刺激牵张感受器。
因此，会触发排便反射，这是一种针对下肠道运动的非自主性反应。
它起始于肌间神经丛，触发短反射以刺激乙状结肠和直肠的收缩。
与此同时，骶部的副交感运动神经元触发长反射，以增强集团蠕动。
质量运动将粪便推入肛管，触发排便反射，使肛门内括约肌松弛。
最后，躯体神经系统中的阴部神经抑制肛门外括约肌的骨骼肌，使其放松，从而主动排出粪便。
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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.