As you put on weight, you gain mostly fatty tissue. Just like other parts of the body, this tissue relies on oxygen and nutrients in your blood to survive. As demand for oxygen and nutrients increases, the amount of blood circulating through your body also increases. More blood traveling through your arteries means added pressure on your artery walls. Weight gain also typically increases the level of insulin, a blood-sugar-controlling hormone, in your blood. The increase in insulin is associated with retention of sodium and water, which increases blood volume. In addition, excess weight often is associated with an increase in your heart rate and a reduction in the capacity of your blood vessels to transport blood. All of these factors can increase blood pressure.