Figure 1. Experimental preparation. In the terminal branches of the diagonal coronary artery, 25-micro meter-diameter latex spheres (dots) occluded antegrade flow in the vessel and directed collateral flow from surrounding arteries retrograde. This retrograde flow (Fretro) was measured as a function of outflow pressure during steady hemodynamic conditions in which mean arterial pressure was held at 60 mmHg with a pressurized blood reservoir (not shown) and heart rate at 100 beats *symbol* min sup -1 by ventricular pacing after creation of an atrioventricular heart block. Outflow pressure was increased in steps by tightening an adjustable screw clamp. When flow stopped, outflow pressure provided an estimate of the stem pressure (Pstem) at the origin of the collateral vessels. The left circumflex coronary artery (LCx) was cannulated and perfused with arterial blood at a constant 60-mmHg pressure with a pump and a system controlled by servomechanism (not shown). Pressure in the LCx (Plcx) was measured at the cannula tip with a small internal tube (not shown). Stippled area = diagonal branch and supplying collateral vessels.