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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
大家知道内皮细胞这层的意思了
because now you know what that represents.
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
