Obesity affects blood flow in Many ways. First, with substantial fat deposits you have to grow essentially miles of extra blood vessels to supply the tissues with oxygen. Second, the added pressure of tissue against the vessels increases pressure like standing on a water hose. Third, with time and the added blood pressure, the walls of the heart thicken like any other muscle will do when challenged. These thicker walls do not allow the heart to pump as much blood with each beat. Thus leading to many many other secondary health problems.