Assertion: circulatory system's vein | /r/HasAspect | valve
Facets:
value: in them
type: LOCATION
value: because of this low pressure
type: CAUSE
Source sentences:
text: Because of this low pressure, veins have valves to prevent backflow of blood.
text: The veins have valves that prevent back-flow of blood, as shown in Figure 9.
text: That's why veins have valves in them.
text: Veins have valves that allow blood to flow only in one direction.
text: Veins have valves that keep blood flowing forward.