血管其实完全符合这样的定义，组织学上，血管由不同组织类型的同心层或“被膜”组成：
内膜包括内皮细胞层，以及和相对较薄的支持结缔组织层。中模包括中间的肌肉/弹性层，包含平滑肌和不同比例的弹性组织，外膜主要是外层的纤维结缔组织层。
虽然神经组织通常在血管中不明显，但用于调节平滑肌功能和调节痛觉。
因此在医学上，血管和胃、肠、膀胱、肾脏等内部具有较大空隙的器官，称为管状器官。
所以，最长的器官并不是小肠，而是血管。
参考资料
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