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And you might say, well does this just go on endlessly?
大家可能要問，會這樣一直沒有止境地繼續下去嗎？
You know, vessels with vessels on them
難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，這樣一直循環下去嗎？
Actually no. It's just the large vessels that have this.
當然不是的，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.
這叫血管滋養管（營養血管）
And this is kind of a fancy name for it.
可能聽起來很復雜
But they're little blood vessels on blood vessels, which I always thought was kind of a cool thing.
它們就是血管壁上的小血管，我覺得這樣的構造是很神奇的
So the tunica externa has the blood vessels and it also has nerve endings.
外膜上可能存在小血管，同時也有神經末梢
And I'm not going to actually draw that in because it'd be kind of hard to show that.
這里我就不畫了，神經末梢是很難表示的
But nerve endings actually can be in that layer.
一般神經末梢就在這一層
In that tunica externa layer.
在外膜這一層
So you've got three layers, each of them has some pretty cool things in them.
好，血管實際分為三層，每一層物質都有所不同
And now what I thought we'd do is we'd go and think about each vessel type,
下面我們要做的是看各種不同的血管類型
and how this could look for that vessel.
看它們的層次結構有何不同
So let's start with the veins. And I'll do the veins over here.
先來看靜脈，我寫在這
So let's say you have a vein.
假設這是一條靜脈
On the inside it has that tunica intima layer -
首先是內膜層
and I'm not going to draw all the cells now
大家知道內皮細胞這層的意思了
because now you know what that represents.
細胞我就不畫出來了
That purple line represents the endothelial cells and the base membrane.
紫線表示的是內皮細胞以及基膜
And you also have a smooth muscle layer- so kind of a red line to represent tunica media.
然后有一層平滑肌細胞，用紅線表示
And veins have a tunica externa- a third layer.
靜脈也有第三層，外膜
So veins kind of follow this general pattern.
靜脈的構成跟血管的一般構成是一致的
They have three layers and they're pretty straightforward in terms of following exactly what I just talked about.
靜脈血管分三層，跟一般血管結構吻合，相對簡單
Fair enough. Now arteries.
好，下面看動脈
Let me do a different color for arteries. Let's say red.
我用不同的顏色畫動脈
I think of arteries in two different groups.
動脈要分兩類考慮
So I think of large and middle sized arteries
一類是大中型動脈
a little differently than I think about small arteries or arterioles.
這一類動脈跟小動脈、微動脈有所區分
And you'll see that the small arteries and arterioles actually look quite similar to one another.
大家可以看到小動脈和微動脈是非常相似的
So let's start with the large and middle sized arteries.
首先我們介紹大中型動脈
So there you have, let's say, an inside.
當然，要有內膜
And then on the outside of that, on the tunica media layer- you start seeing a difference from the vein.
在此基礎上，是中膜。這里跟靜脈就有不同了
You see that it's much larger, much larger.
這一層要厚得多
And in the vein you just had kind of a thin layer of smooth muscle,
靜脈中，中膜只是一層薄薄的平滑肌細胞
here you have a nice large layer of smooth muscle.
而這里是很厚的一層平滑肌細胞
And in addition to having these smooth muscle cells
除了這些
in the large and middle arteries you have an interesting new thing.
大中型動脈中還有一種新的物質
You have something called elastin protein.
叫彈性蛋白
And elastin is gonna be in here and this is a protein that helps make these very elastic.
就在這里，彈性蛋白就是提升彈性的
Right because they're going to have such high pressures
因為動脈血管壓力太大
that they have to be able to not break when these pressures are going through.
要確保當血壓過高時，血管不會爆裂
So this elastin (this green elastin protein) helps make the arteries more elastic.
綠色畫的彈性蛋白會增大血管的彈性
More elastic for high pressures.
血壓很高時，保持血管的彈性
And I'll write "P" for pressures.
p代表“pressure（壓力，血壓）”
So that's what the middle layer looks like.
這是中膜
And of course you still have your outer layer as before.
當然，還有外層
