When you stand up, the normal pull of Earth’s gravity causes blood to pool in your legs. The veins there stretch out slightly, and rather than fighting against gravity to get back to your heart, a significant amount of blood just stays in the newly expanded veins.
Less blood is available to circulate, blood pressure drops, and less blood is pumped to the brain.
Your body restores normal circulation with a pressure receptor in the major artery leading to the brain. It senses the drop in blood pressure and sparks a reflex response that speeds up your heart rate and constricts your blood vessels – so that your blood pressure returns back to normal.
It’s just that sometimes this response doesn’t kick in immediately – especially if you’re hungry, hot, or tired. And that’s when – if you stand up suddenly – you might feel like you’re on the verge of blacking out.
This might make you feel as though your body wants to fall down. If so, it’s because it’s easier for your blood to flow when you’re no longer standing.