Blood vessels are part of the cardiovascular system. They function to transport blood throughout the body. There are three types of blood vessels： arteries， veins and capillaries. The capillaries are the most important vessels in the system. They enable the actual exchange of water and chemicals between the blood and the tissues. Arteries carry blood away from the heart to the capillary bed while veins return blood to the heart from the capillary bed.

　　血管是心血管系統(tǒng)的一部分。其作用是向全身輸送血液。血管分三類：動脈、靜脈和毛細血管。毛細血管是系統(tǒng)中最重要的血管，它實現(xiàn)了血液和組織之間水和化學物質(zhì)的實際交換。動脈將血液由心臟帶到毛細血管床，靜脈將血液由毛細血管床回流到心臟。

　　All blood vessels have the same basic structure. There are three layers， from inside to outside：

　　所有血管都有相同的基本結構，由內(nèi)而外由三層組成：

　　1.Tunica intima （the thinnest layer）： a single layer of simple squamous endothelial cells.

　　1.內(nèi)膜層（最薄層）：單層鱗狀內(nèi)皮細胞。

　　2.Tunica media （the thickest layer）： circularly arranged elastic fiber， connective tissue， polysaccharide substances， the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may （especially in arteries） be rich in vascular smooth muscle， which controls the caliber of the vessel.

　　2.中膜層（最厚層）：環(huán)形分布的彈性纖維、結締組織、多糖物質(zhì)，其第二和三層由另一稱為外彈性膜的厚彈性帶分隔。中膜（尤其是動脈）富含血管平滑肌，血管呂徑由血管平滑肌控制。

　　3. Tunica adventitia： entirely made of connective tissue. It also contains nerves that supply the muscular layer， as well as nutrient capillaries （vasa vasorum） in the larger blood vessels.

　　3.外膜：完全由結締組織構成。并含有支配肌層的神經(jīng)。在較大血管中含有營養(yǎng)毛細血管（血管滋養(yǎng)管）。

　　When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis （pl. anastomoses）。 Anastomoses provide critical alternative routes for blood to flow in case of blockages.

　　血管聯(lián)結構成一彌散性維管聯(lián)結區(qū)，即并接。并接為血管堵塞時的血液流動提供了關鍵的選擇途徑。

　　Laid end to end， the blood vessels in an average human body will stretch approximately 62，000 miles （100，000kms）——3 times around the earth.

　　端端相聯(lián)，全身血管長度可達100，000 km，可繞地球三圈。

　　Damage， due to trauma or spontaneously， may lead to haemorrhage. In contrast， occlusion of the blood vessel （e.g. by a ruptured atherosclerotic plaque， by an embolised blood clot or a foreign body） leads to downstream ischemia （insufficient blood supply） and necrosis （tissue breakdown）。

　　自然性或外傷性損傷可導致出血。相反，血管閉塞（如粥樣硬化斑塊剝落、血塊栓塞、異物等引起的）可導致下游局部缺血（血液供應不足）和壞死（組織衰竭）。