In Vivo Measurement of Dimensions of Veins with Implications Regarding Control of Venous Return

Control of the capacity of the venous system has been postulated as necessary to provide increased venous return to initiate an increase in cardiac output. In chronic animal experiments, we have studied the dimensions of the liver, spleen, and the superior vena cava as venous reservoirs in relation to fright, exercise, and shock. Two methods used were miniature mutual inductance coils and ultrasonic transit time technique; the former is a relatively simple and reliable, but nonquantifiable method. The sonar method is quantifiable, but to date has been unsatisfactory for narrow ranges. The spleen in the dog is a superb volume reservoir, but the liver is not. The superior vena cava, in at least some circumstances, demonstrates active constriction. With the onset of exercise, flow increased in the cava before either an increase in heart rate or an increase in stroke volume. With hemorrhagic shock, pooling of blood in the mesenteric veins was sometimes found, but there was no evidence of failure of the neural mechanisms of venoconstriction, evidenced by a marked, agonal contraction. With endotoxin shock, no portal pooling was observed; earlier reports of pooling based on weight changes were probably due to extravasated plasma secondary to endothelial damage and marked portal venoconstriction. Venous pooling in the legs in man in shock can be prevented by elevation of the legs without the disadvantages of the head-down (Trendelenburg) position.