Changes in the blood volume will occur in highly compliant vessels, such as veins. Changes in the transmural pressure in veins will cause significant changes in the blood volume. So there will be a shift from central blood volume to the peripheral regions. So when you stand, there will be an increased volume in the peripheral veins, which shifts blood away from the central blood volume.
Venous return and cardiac output are normally in a steady state to compensate for this change. So Venous return will always equilibrate with Cardiac Output. Thus, increasing venous transmural pressure will cause an increase in leg venous volume, and a subsequent decrease in central blood volume. Cardiac output and stroke volume will decrease. The baroreceptor reflex, which are stretch receptors, will sense the decreased stretch per decrease in arterial pressure. These receptors are located in the carotid sinus and aortic arch. Decrease pressure causes decreased firing to the vasomotor centers in the brainstem. The response will be to withdraw parasympathetic tone to the heart, and increase sympathetic tone to both the heart and blood vessels. Thus, HR, contractility and stroke volume increase, arterioles will constrict, and veins will constrict. All of which work to increase cardiac output. This is what normally occurs. Failure to increase CO to normal can lead to decrease blood flow to vital areas, such as the brain, and cause syncope.
When you are laying down, lower gravity or in the water will result in less of the central blood volume being shifted away to the periphery. If you want to know more, google Starling's law of the heart.
