From heart beat to beat the model calculates cardiac output (in l/min) by multiplying stroke volume (l) times 60 (s) times heart frequency ( which is the reciprocal of heart cyclus duration; 1/s). As output parameter CO is given in a box just below the simulation animation.
With all the model's reflexes turned off and only acceleration and automatic adaptation of respiratory rate turned on, the question arises how to increase CO. Most people would answer by increasing heart rate. But, changing heart rate from 69 to, for instance, 100 BPM increases CO only slightly. The reason for this is that cardiac filling time is reduced: increasing heart rate reduces the duration of diastole more than of systole. Therefore, in order to increase CO, the heart needs to be filled over a shorter period of time. This can be achieved by increasing venous pressure.
Decreasing or increasing venous capacitance directly translates to an increase or decrease in cardiac output. It can be seen as a forced transfer of venous blood volume into the arterial compartment, causing a rise in arterial blood pressure when the peripheral resistance does not change but allowing arterial blood pressure to remain within limits when peripheral resistance decreases, as is the case in physical activity.