Problem: Sam has entered into an exercise physiology study. As part of the study, the researchers want to measure cardiac output at different workloads. One of these activities is a treadmill run. Sam has been running on the treadmill for several minutes and one of the researchers tells him that his end diastolic volume is high, making his stroke volume high. When Sam asks how this happens, the researcher tells him that it has a lot to do with venous return. Describe two aspects of the venous system that directly contribute to an increase in venous return.