The other responses are good. I would just add that there are a few conditions that cause fluid retention (eg congestive heart failure) which can increase the pressure in the cardiovascular system. This increased hydrostatic pressure can be measured at the heart using echocardiography. Blood vessels are generally not thick and sturdy like heart muscle, and so the increased pressure causes fluid to leak out of the vessels ("extravasate") and leads to swelling ("edema"), typically of the lower extremities due to gravity.

So there are conditions that lead to overproduction of the cellular components of blood (polycythemia, essential thrombocytosis, leukemia), and different conditions that lead to retention of the liquid component -- but the liquid just leaks out into the soft tissue.