And finally, the outermost layer of your blood vessels is the tunica externa.

最后，你的血管的最外层是外膜。

These arteries invest in additional smooth muscle tissue, and proportionally, have the thickest tunica media of any blood vessel.

这些动脉覆盖着额外的平滑肌组织，并按比例，拥有任何血管中最厚的中膜。

The tunica externa also has nerve fibers, lymphatic vessels, and in the biggest vessels, the protein elastin.

外膜也有神经纤维、淋巴管，在最大的血管中也有弹性蛋白。

Surrounding the tunica intima is the middle layer, the tunica media, made of smooth muscle cells and sheets of the protein elastin.

围绕内膜的是中间层，中膜，由平滑肌细胞和蛋白质弹性蛋白片组成。

Both the cells of the tunica intima and tunica media generally get the nutrients they need from the blood in the lumen.

内膜和中膜的细胞，都能从管腔内的血液中获取所需的营养物质。

Next there's the tunica media or middle tunic which is mostly made of smooth muscle cells and sheets of elastin protein.

接下来是血管中膜，主要由平滑肌细胞和弹性蛋白组成。

To make things really wild, some huge vessels have a tunica externa that's so thick that it needs its own blood supply.

神奇的是，一些大血管拥有非常厚的外膜，以至需要有自己的血液供应。

The innermost tunic is called the tunica intima, which includes the endothelial cells, which create a slick surface that minimizes friction for blood moving through it.

最内层者称为内膜，有内皮细胞构成；它们形成了光滑表面，使血液流经时的摩擦降到最低。

The aorta also undergoes cystic medial necrosis, which is where there is degeneration of the tunica media, which is the central portion of the aortic wall.

除此之外，主动脉还会有囊性中层坏死，即主动脉的中膜发生退行性变；中膜是主动脉壁的中心部分。

Finally, there's the tunica externa or outside tunic which is made up of loosely woven fibers of collagen protein that protect and reinforce the blood vessel and anchor it in place.

最后的外膜，由疏松结缔组织组成，含弹性纤维和胶原纤维，可保护和加固血管并使其固定。