One-way valves are particularly common in the veins of the extremities. why?

Blood is pumped from heart with high pressure and hence its flow is maintained unidirectional in the arteries. In contrast, when it is returning to heart from various body parts high pressure is not generated. In this case, blood can move back to the body part under the influence of gravity or muscle contraction specially in extremities like arms and legs. To make sure that blood does not flow back and goes only to heart, one way valves are present in veins of extremities. The valves are made of two flaps which are pushed opened when blood comes in contact with them. Once blood has crossed the valve, the valve closes and backward flow of blood is not possible.

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answer:

it could come from a variety of sources.

explanation:

..although it was most likely developed through zoonotic transmission, where an infectious disease which usually afflicts when animals jump to humans.