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In the standing position, blood in our body is also be under influence of gravity. As you lie down, the gravity effect is nullified. Thus more blood enters into central blood vessel (those that supply major organ especially the heart and brain) from peripheries. This change is momentarily. Within few seconds under the action of autonomic nervous system, blood gets redistributed to all parts inorder to maintain adequate blood supply to all tissue.

When you stretch your body or any action of our muscles, then due to the pumping effect, blood from muscles enters the main central blood vessel and hence more blood supply to brain.

I hope this explains your query. Feel free to ask if any related query and I will be glad to answer them.

Wish you good health.

Regards

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