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糞便の形成と排便
大腸で 3 ～ 10 時間過ごした後、糜粥は多くの水分を失い、消化の最終産物である糞便になります。糞便は、セルロース、粘液、剥離した上皮細胞、微生物などの未消化の食物繊維で構成されています。下行結腸と S 状結腸は糞便を貯蔵し、収縮を利用して乾燥させますが、半固形の質感を与えるのに十分な水分を保持し…
大腸で3〜10時間後、粥はかなりの水分損失を受け、消化の最終産物である糞便を形成します。
セルロース、粘液、脱落した上皮細胞、微生物などの未消化の食物繊維で構成されています。
下行結腸とS状結腸は糞便を貯蔵し、糞便は最終的な乾燥のために軟膜収縮を受け、半固体のテクスチャーに十分な水分を保持します。
次に、集団蠕動運動により、通常は空の直腸に糞便が押し込まれ、直腸壁が膨張して伸張受容体が刺激されます。
その結果、排便反射、つまり下部排便に対する不随意の反応が引き起こされます。
それは腸神経叢から始まり、短い反射を引き起こしてS状結腸と直腸の収縮を刺激します。
同時に、仙骨領域の副交感神経運動ニューロンは、腫瘤の蠕動運動を強化するための長い反射を引き起こします。
集団運動により糞便が肛門管に移動し、排便反射が引き起こされ、内部の肛門括約筋が弛緩します。
最後に、体性神経系の陰部神経は、外肛門括約筋の骨格筋を阻害し、糞便の自発的な排出のための弛緩を可能にします。
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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.