Our goal for this lesson is to learn the 本节课的目的是学习
structure and function of the three 人体内三种主要血管
major types of blood vessels in the body. 的结构和功能。
They're part of the circulatory system， 它们是循环系统的一部分，
they are arteries 、veins and capillaries. 它们是动脉、静脉和毛细血管
So grab the other out, so let's start 所以，让我们
with arteries. Arteries are the largest 从动脉开始。动脉是最大
blood vessel, they have a thick outer 的血管,他们有一层
layer made of connective tissue ,they're 由结缔组织构成的厚厚的壁，它们
also made of smooth muscle and 也由平滑肌和
endothelial tissue. This thickness 内皮组织构成的。这种厚度
supports its purpose which is to deliver 帮忙起到将
highly pressurized oxygen-rich blood 高压力的富氧血液
from the heart to faraway parts of the 从心脏输送到身体的遥远部位
body ,it needs to be thicker to support 的作用，它需要通过变得更厚来满足
that kind of demand.So next let's take a 这种需求。接下来让我们
look at the vein,the vein on the right 看一下右边的静脉，屏幕右边
side of the screen ,so this... um... this 的静脉,这种
structure has three layers of tissue, but 这种结构有三层组织，但
the endothelial walls are thinner .So why 它的内皮细胞层会更薄。那么为什么
is it thinner?Because the pressure from 内皮细胞层会变薄呢？因为
the heart 心脏的压力
has lessened a great deal, I'll put lower 已经减轻了很多，我把血压降低
blood pressure and by the time blood has 血压，当血液
reached the veins, it,it has lost this 到达静脉时，它已经失去了
pressure and therefore actually has 这种压力，因此实际上
instead these valves inside to help 已经把这些内部的瓣膜，以帮助
catch blood, as it may, it may be fighting 捕捉血液，因为它可能，它可能是克服
gravity.So it can help continue moving 重力。因此，它有助于帮助血液继续
back to the heart.Something else that 返回心脏。还有一些
helps in addition to these valves to 协助这些瓣膜
help blood travel back to the heart is 帮助血液返回心脏的东西，
um...nearby skeletal muscles, and skeletal 嗯，比如，附近的骨骼肌，骨骼肌
muscles...um...helps the blood keep moving 通过收缩帮助血液继续流动。
by contracting.so at last but not least, 最后，
we have the smallest,um...the smallest 我们有最小的
structures, the smallest blood vessel 结构，最小的血管
structures called capillaries.So 结构是毛细血管
capillaries are important,because they 毛细血管很重要，因为它们
connect arteries 连接动脉
with veins, um...how do they do this well? 和静脉。它们是如何做到的？
These tiny vessels web together and they 这些微小的血管连接成网状，它们
meet where arteries and veins meet to 在动脉和静脉汇合的地方相遇，
exchange oxygen nutrients and wastes for 来交换全身所有细胞所需和排出的氧气、营养物质和废物
all body cells either to come in or out，
and these vessels are so small that red 这些血管小到只可
blood cells actually can move 容许红细胞
single-file through them and their walls 单行通过，它们的血管壁
are only just one cell thick,this makes 由单层细胞构成，这使得
it much easier to exchange materials, and 物质交换更加容易进行。
let's say your eyes、your kidneys、your 比方说，你的眼睛，你的肾脏，你的
bones、skin for example,so at any given 骨头，皮肤，在任何
moment,about 30 percent of your blood 特定的时刻，大约30%的血液
and circulation will be in your arteries, 都会在你的动脉中，
five percent will be in your capillaries, 5％会出现在你的毛细血管里，
and you got it 65 percent if you do the 通过计算，即65%
math in the veins.Now hopefully you can 在静脉中。现在希望你
teach somebody else about your blood. 可以把关于你的血管的知识交给别人。
vessels