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Like the road system, the cardiovascular system has a left side and a right side. In the cardiovascular system, after the blood travels through the whole body, it comes back to where it started. A road system is similar, reaching the destination before returning to where it started. In the cardiovascular system, there are different parts, like the arteries, veins and others. In the road system, there are straight roads, curved roads, roundabouts and others. In the cardiovascular system, after the blood passes through a valve, there is no way it can go back, and it has to travel around the whole body it is at the valve again. In the road system, however, a car can make u-turns at certain places along the road and go back, instead of having to go around the whole system before it reaches the place again.