Why does the decrease in thoracic cavity, create negative pressure in the vena cava and right atrium that leads to increased intake of blood?

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I thought the circulatory system was closed off from outside factors such as pressure from lungs and how if the cavity expanded with volume, since vessels are tubular walled off structures. I’m possibly looking wrong at this and heard I’ve asked this before in other places, but people give me a generic answer that the thorax expands and this causes the negative pressure at the vena cava but yes I knew that before but it still does not explain why the pressure itself changes such as the physiological details as to why this would be happening.

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Veins have thin walls so they aren’t well separated from the outside – imagine having walls made out of concete vs having walls made of fabric. The fabric walls will flex in the wind but the thicker alls of the arteries don’t.

So when you inhale, the negative pressure kind of stretches the IVC and then blood rushes into that vaccuum to fill the empty space.