It is about the blood pressure change 😉

which is regulated by the autonomic nervous system, if you wanted to look into it further…
ERS

when you stand up your blood pressure (BP) drops slightly as you move through the degrees from supine to standing. As the BP drops, the heart rate (HR) increases slightly to compensate for the reduced BP. There is a cascade for recovery–which is why (barring any health problems) your BP recovers and you don’t pass out every time you stand up.

It is a neat little process: Stand up, BP drops, HR increases slightly, BP then overshoots a little, HR adjusts to accomodate the overshoot, then there is a little ripple dance between the two as they balance out to normal operational rates/levels. This all occurs automatically and within seconds. However, there are extraneous factors that can affect the rate of recovery as well as the shape of the cascade curve.

You can mimic and exaggerate this response by taking a deep breath, and then blowing out between pursed lips for 15-20 seconds. You will feel lightheaded just as you may when standing up, and your HR/BP and autonomic system in general goes through the same response as if you were moving from supine to standing….. this is officially called the Valsalva maneuver (for the record)

Does this help or do you need more explaining?
ERS

Another important aspect or compensating mechanism is the increase in arterial resistance by smooth muscle stretch receptors in the lower limb resistance arteries. In response to standing blood pools in the lower limbs, constriction of resistance arteries reduces the pooling of blood and helps to maintain blood brain flow. Decreased brain blood flow results in dizziness or even passing out. The term for this is orthostatic hypotension and is more common in elderly.

ya if i go down the pub. 8)