A cannula is a plastic or metal tube with a smooth, unsharpened tip for inserting into a blood vessel, lymphatic vessel, or body cavity in order to introduce or withdraw fluids.

 

To insert a cannula, the doctor first punctures the site with a long, thin needle, slides the cannula over it, and then withdraws the needle. Alternatively, he or she may insert a trocar (sharp-pointed rod) inside the cannula and remove it once the vessel has been entered.

 

Cannulas are frequently used for load transfusions and for draining pleural effusions. In certain circumstances, such as when blood is required for testing over a period of time, the cannula may be left in place for several days.