When the muscle relaxes, valves in the vessels shut preventing the fluid from returning to the lower extremities.

During an accident where radioactive material may be released, these valves must shut to prevent the release of radioactive material or the loss of reactor coolant.

To do this, huge vertical valves, operating much like giant elevator doors, can selectively shut water off.

It cannot retract its siphons into the protection of its shell, so the two valves can never shut completely.

With variable timing, valves would stay open longer, or shut later, depending on engine speed.

The valve shut, the ramp retracted.

When the ventricles are full, the tricuspid and mitral valves shut.

When the ventricles finish contracting and begin to relax, the aortic and pulmonic valves shut.

A. In a hot-water system, the valves on all radiators should be open all the way, or shut all the way - never partly open.

There--an unmistakable airlock- She was prepared to cut her way in, but when she had identified the controls, the valves opened and shut for her.