19. The correct answer is D. In fully compensated aortic coarctation, blood flow is normal in the lower and upper body (choices A and B) despite an increased arterial pressure (about 50% higher) in the upper body (choice C) compared with the pressure in the lower body. Because resistance = pressure/blood flow, it is clear that resistance must be lower in the lower portions of the body. The mechanism of this decrease in resistance below the constriction (and increased resistance above the constriction) is autoregulation of blood flow. The small arteries and arterioles dilate (or constrict) in accordance with the metabolic needs of the tissues, ensuring that each tissue receive an adequate amount of blood flow. Thus, the increase in blood pressure in the upper body leads to constriction of the arterioles, which increases vascular resistance (choice E), and the lower pressure below the coarctation leads to dilation of the arterioles, which decreases vascular resistance.