I can see the ring possibly flopping open if it has a weak catch.
There's still another issue that needs to be resolved: questions of vein or artery not withstanding, that being the initial spurt is followed by a steady state flow. Regardless of vein or artery, when taken with reports that Kirk survived long enough to be transported to a medical facility, it means that his heart was pumping which, by extension, means that the blood flow couldn't be steady state; it would be pulsed. Also, as veins are equipped with backflow prevention, the veinous system is on the low-pressure, downstream side of the capillaries, arguments along the lines of it being like a garden hose deflating don't stand up. Again, because his heart was beating there would be the initial spurt followed by pulsed flow. One could make the argument that his heart rate shot up in response to the sudden drop in blood pressure but there would still be the pulsed flow problem.