The venae cavae are the two largest veins in the body. These are the veins that return blood into the heart (singular, vena cava).
The superior vena cava transports blood from the upper portion of the body to the heart.
The inferior vena cava transports blood from the lower portion of the body to the heart.
Capillaries, which are only one epithelial cell in thickness,are the smallest blood vessels in the body. The capillaries form networks of expanded vascular beds that have the important role of delivering oxygen and nutrients to the cells of the tissues.
词汇：
venae cavae 腔静脉（复数）
vena cava 腔静脉（单数）
superior vena cava 上腔静脉
inferior vena cava 下腔静脉
Capillaries 毛细血管
epithelial cell 上皮细胞
vascular beds 血管床
oxygen 氧气
参考译文：
静脉腔是体内最大的两条静脉。这些是血液回流到心脏的静脉（单数，静脉）。
上腔静脉将血液从身体上部输送到心脏。
下腔静脉将血液从身体下部输送到心脏。
毛细血管厚度只有一个上皮细胞，是体内最小的血管。毛细血管形成扩张的血管床网络，具有向组织细胞输送氧气和营养的重要作用。
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