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.
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结构，由内而外由三层组成：
1. Tunica intima (the thinnest layer): a single layer of simple squamous endothelial cells.
1. 内膜层（最薄层）：单层鳞状内皮细胞。
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.
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：完全由结缔组织构成。并含有支配肌层的神经。在较大血管中含有营养毛细血管（血管滋养管）。
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.
血管联结构成一弥散性维管联结区，即并接。并接为血管堵塞时的血液流动提供了关键的选择途径。
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.
端端相联，全身血管长度可达100,000 km，可绕地球三圈。
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).
