Fluid buildup can occur due to vasodilation, low blood pressure, and salt retention as the body attempts to compensate for decreased blood volume. In some cases, this fluid accumulation may be a result of heart failure, where the heart pumps excessively to meet increased oxygen demands.Fluid buildup can occur due to vasodilation, low blood pressure, and salt retention as the body attempts to compensate for decreased blood volume. In some cases, this fluid accumulation may be a result of heart failure, where the heart pumps excessively to meet increased oxygen demands.