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食糜.
食糜(英语:chyme,语源来自于希腊语:χυμός,意思为汁液[1][2])是一种从胃离开,进入十二指肠的半流体物质,成分为部分消化的食物[3]。
食糜在英文中又称作“chymus”,是一种液体状物质,会在通过幽门进入十二指肠前的胃部发现到。食糜经由机械力和打断化学键结的的作用而形成,成分包含了部分消化的食物、水、盐酸与各式各样的消化酶等。食糜经由幽门的括约肌,缓慢通过并进入十二指肠,进行营养物质的萃取。根据摄入食物的质与量,胃将食物消化成食糜的时间,大约为40分钟到几个小时。
在pH值为2的情形下,从胃形成的食糜非常酸。为了要将pH值提升,十二指肠会分泌激素与胆囊收缩素,导致胆囊刺激并分泌出碱性的胆汁到十二指肠中。十二指肠还会分泌促胰液素,刺激胰脏分泌大量的碳酸氢钠,导致在进入空肠前就能将pH值提升到7。再加上十二指肠的肠壁有保护性的黏液,以及可以利用碳酸氢钠与胆汁进行中和反应,因此十二指肠是小肠部位当中,对于高酸性食糜不敏感的部位。
在pH值为7时,从胃而来的消化酶不再具有活性。这会导致进入十二指肠的厌氧细菌,会进行这些消化酶的进一步降解。这些细菌还同时合成维生素B与维生素K。(小学P4资料)
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