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Catheters are long, flexible tubes that are inserted into the body for various purposes, either to remove an unwanted substance or to instill nourishment or medication.

A relatively large catheter can be passed through the nose, down the throat and into the stomach to remove the contents of the stomach; for example, if someone has consumed a poisonous substance, a catheter can be used to remove a small sample of the stomach contents for laboratory testing. A catheter may also be used to pass liquid nourishment into the digestive tract, as in the case of someone unable to swallow for some reason. This catheter is called a nasogastric tube.

A smaller tube can be passed through the urethra into the bladder to empty its urine. Oftentimes after surgery or trauma an individual is unable to void and must have the bladder emptied. Sometimes these urinary catheters must be left in place; a small balloon near the end of the catheter is inflated to hold the catheter in the bladder.

Very long catheters are often passed through an incision in the thigh into an artery and into the heart; a doctor can then inject contrast agents into the patient to outline the coronary arteries. The physician can watch as the agent, which is visible on x rays, is injected and courses through the hearts arterial system. In this way a doctor can see a blockage and take measures to bypass it or remove it. These catheters have now been fitted with devices to open clogged arteries. Small balloons mash obstructions out of the way, and laser tips or whirring blades cut stubborn blockages from the arterial passage. Since there are no pain receptors inside blood vessels, passing the cardiac catheter is done under local anesthetic with the patient fully awake.

Still other catheters can be inserted through the trachea into the lungs to remove fluid or mucus. Some two-channeled catheters are used to induce a chemical into an organ and remove the organs contents at the same time. Others can be used for wound drainage or for measuring blood pressure in any of the hearts four chambers. Most intravenous devices (IVs) have very thin catheters that remain in the blood vessel to deliver the intended medication or nutrition. As the body temperature warms these catheters, they become very soft and flexible for patient comfort.

Often, sterile catheters are imbedded or treated with antibacterial substances to minimize infection once they are inserted into the body.

See also Surgery.