Why straining redirects bloodflow upwards?
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Blood flows from the heart through the body by the physical pressure exerted by the heart. Once the blood passes through the smaller vessels and capillaries it has barely any pressure behind it and it has to flow back to the heart through the veins. Simply explained, it’s the contraction and tension of the leg muscles that compress the veins of the legs that push the blood back upwards to the heart. The veins that go towards the heart have valve-like structures that prevent blood to flow back down to the legs.
There’s other forces at work that make the blood flow upwards like the negative (suction) provided by the right side of the heart, but the above is mostly why exertion and muscle tension causes an improvement of bloodflow back to the heart. It’s also the reason why jet fighter uniforms are pressurized. The added pressure makes the blood able to flow back to the heart and brain even under high G loads.
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