A catheter is a tube that is inserted into a hollow organ of the body in order to drain or introduce fluids. A urinary catheter is inserted into the |bladder| through the |urethra| to relieve obstruction to the flow of ~urine~. Cardiac catheters are used to measure ~blood~ pressure in the |heart|. Similar catheters are used to inject radio-opaque substances into ~blood~ vessels for X-ray examination.
