All Direct Subsidized and Unsubsidized Loans disbursed between July 1, 2014 and June 30, 2015 carry a fixed interest rate of 4.66% for undergraduates. Graduate or professional students receiving a Direct Unsubsidized Loan disbursed between July 1, 2014 and June 30, 2015 will have a fixed interest rate of 6.21%.
There is a problem when the valve that controls what blood is good and what blood is used, no longer is strong enough to control this flow. Heart valves get damaged or simply wear out. I think you are asking what causes blood to flow in the wrong direction. My guess is that it would be a faulty heart valve.
Veins have valves to stop blood flowing in the wrong direction. So the blood returns to the heart though both the superior and inferior vena cava and is then carried to the lungs where it is re-oxygenated, carried back to the heart and again pumped out the aorta. All in all the heart makes blood flow.