A catheter is a thin, hollow, stiff but flexible tube that is used to thread through thin vessels in the body in order to either insert instruments or reach past sphincters or other obstructions. They are commonly used in cases of bladder infection to drain the bladder when urine cannot pass through the urethra. They can also be used to insert wires or other instruments through veins into body organs such as the heart or liver when it is impossible to reach those organs through other body tissue.