If the cardiac output stays the same (hypothetically, although obviously it doesn't stay the same), then why does higher venous volume increase right atrial pressure? Shouldn't the flow rate be conserved (with a constant cardiac output) so that the venous return to the right atrium is no different than before, and therefore the pressure is no different?
Imagine the circulatory system is an amusement park ride. The ride is the left ventricle and the arterial system. After riding, people go around and get back in line to go again. The line and the building housing it are the venous system. Maybe the lungs are some sort of staging area where you get ready to get on the ride.
The right atrium is the end of the venous system. Venous pressures are right atrial pressure.
When all is well the line is moderate and people wait a short time before riding again. But now 100 more people arrive. If the park workers can speed up the ride or loading rate to accommodate the extra riders then great. But if the ride cannot go any faster then the line backs up. If you imagine that it is raining out, all the people in line bunch into the building housing it. Venous pressure increases. It will stay increased unless the number of riders (blood volume) decreases or the heart improves its output to accommodate the load.
Congestive heart failure is a useful example. The ride speed (cardiac output) cannot increase because the heart is weak. If riders increase (blood volume) it backs up into the waiting line or venous system. This increase in volume occurs if someone with CHF eats a salty meal – water follows salt and blood volume increases, which backs up in the venous system because the heart cannot increase its output to accommodate the increased volume. These sick folks are said to be volume overloaded. You can see increased venous pressure as the signs of CHF – for example dilated external jugular veins.
Among many adaptations to deal with increased venous pressure is atrial natriuretic peptide. When increased venous pressure stretches the right atrium this peptide hormone is secreted which is a cue to the kidney to dump sodium, and with it fluid volume.