With rising, a fall in blood pressure
When we stand up, blood tends to pool in the lower half of our bodies, filling veins in the liver, intestines, and other abdominal organs, as well as those in the legs. The downward flow means there's a danger of not enough blood reaching the brain, which can lead to a loss of consciousness. But adjustments occur that keep that from happening. Sensors in the aorta in the torso and in the carotid arteries in the neck trigger a response that revs up the "fight-or-flight" part of the nervous system and dials down the "rest-and-digest" part. The heart beats a little faster and stronger. Blood vessels constrict, squeezing blood into a tighter space. Blood flow and blood pressure stay more or less normal.
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