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本集视频介绍了血管壁的一般形态和层次结构，接着按静脉及大小动脉的区分方法，分别介绍了各种血管的层次组成，帮助观者了解血管壁的形态结构。
n. 增加，附加物，加法
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- And you might say, well does this just go on endlessly?
- 大家可能要问，会这样一直没有止境地继续下去吗？
- You know, vessels with vessels on them
- 难道大血管上有小血管
- and those little vessels have more vessels on them and so on and so forth.
- 小血管上又有小血管，这样一直循环下去吗？
- Actually no. It's just the large vessels that have this.
- 当然不是的，只有大血管壁上才可能有小血管
- So you SOMETIMES, not always, see what's called vasa vasorum.
- 这叫血管滋养管（营养血管）
- And this is kind of a fancy name for it.
- 可能听起来很复杂
- But they're little blood vessels on blood vessels, which I always thought was kind of a cool thing.
- 它们就是血管壁上的小血管，我觉得这样的构造是很神奇的
- So the tunica externa has the blood vessels and it also has nerve endings.
- 外膜上可能存在小血管，同时也有神经末梢
- And I'm not going to actually draw that in because it'd be kind of hard to show that.
- 这里我就不画了，神经末梢是很难表示的
- But nerve endings actually can be in that layer.
- 一般神经末梢就在这一层
- In that tunica externa layer.
- 在外膜这一层
- So you've got three layers, each of them has some pretty cool things in them.
- 好，血管实际分为三层，每一层物质都有所不同
- And now what I thought we'd do is we'd go and think about each vessel type,
- 下面我们要做的是看各种不同的血管类型
- and how this could look for that vessel.
- 看它们的层次结构有何不同
- So let's start with the veins. And I'll do the veins over here.
- 先来看静脉，我写在这
- So let's say you have a vein.
- 假设这是一条静脉
- On the inside it has that tunica intima layer -
- 首先是内膜层
- and I'm not going to draw all the cells now because now you know what that represents.
- 大家知道内皮细胞这层的意思了，细胞我就不画出来了
- That purple line represents the endothelial cells and the base membrane.
- 紫线表示的是内皮细胞以及基膜
- And you also have a smooth muscle layer- so kind of a red line to represent tunica media.
- 然后有一层平滑肌细胞，用红线表示
- And veins have a tunica externa- a third layer.
- 静脉也有第三层，外膜
- So veins kind of follow this general pattern.
- 静脉的构成跟血管的一般构成是一致的
- They have three layers and they're pretty straightforward in terms of following exactly what I just talked about.
- 静脉血管分三层，跟一般血管结构吻合，相对简单
- Fair enough. Now arteries.
- 好，下面看动脉
- Let me do a different color for arteries. Let's say red.
- 我用不同的颜色画动脉
- I think of arteries in two different groups.
- 动脉要分两类考虑
- So I think of large and middle sized arteries
- 一类是大中型动脉
- a little differently than I think about small arteries or arterioles.
- 这一类动脉跟小动脉、微动脉有所区分
- And you'll see that the small arteries and arterioles actually look quite similar to one another.
- 大家可以看到小动脉和微动脉是非常相似的
- So let's start with the large and middle sized arteries.
- 首先我们介绍大中型动脉
- So there you have, let's say, an inside.
- 当然，要有内膜
- And then on the outside of that, on the tunica media layer- you start seeing a difference from the vein.
- 在此基础上，是中膜。这里跟静脉就有不同了
- You see that it's much larger, much larger.
- 这一层要厚得多
- And in the vein you just had kind of a thin layer of smooth muscle,
- 静脉中，中膜只是一层薄薄的平滑肌细胞
- here you have a nice large layer of smooth muscle.
- 而这里是很厚的一层平滑肌细胞
- And in addition to having these smooth muscle cells
- 除了这些
- in the large and middle arteries you have an interesting new thing.
- 大中型动脉中还有一种新的物质
- You have something called elastin protein.
- 叫弹性蛋白
- And elastin is gonna be in here and this is a protein that helps make these very elastic.
- 就在这里，弹性蛋白就是提升弹性的
- Right because they're going to have such high pressures
- 因为动脉血管压力太大
- that they have to be able to not break when these pressures are going through.
- 要确保当血压过高时，血管不会爆裂
- So this elastin (this green elastin protein) helps make the arteries more elastic.
- 绿色画的弹性蛋白会增大血管的弹性
- More elastic for high pressures.
- 血压很高时，保持血管的弹性
- And I'll write "P" for pressures.
- p代表“pressure（压力，血压）”
- So that's what the middle layer looks like.
- 这是中膜
- And of course you still have your outer layer as before.
- 当然，还有外层
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