[Digestive system's large and changing needs of blood supply]

Doppler utrasonography has made it possible to record blood flow to the digestive tract (the superior mesenteric artery) directly and continuously in unanaesthetized, healthy humans. Several research groups have demonstrated how blood flow to the tract increases gradually and markedly after a meal, and more so after a big meal than after a small one. The increase in blood flow reaches its maximum after 20-40 minutes and lasts for 1.5-2 hours. In the postprandial period there is a parallel and similar increase in cardiac output; the meal thus imposes an increased work load on the heart. Carbohydrate meals, as well as meals of protein or fat all release increases in local blood flow as well as in cardiac output. Surprisingly, during physical exercise of relatively high intensity, there is no reduction in blood flow to the digestive tract in humans. This is in contrast to the exercise-caused flow reduction observed in several animal species. In the postprandial state the large increase in cardiac output caused by muscular exercise is actually added to the increase already established by the meal. This course of events helps to explain why patients with angina pectoris are more prone to chest pains after a meal.