Intravenous describes the injection of liquid directly into a vein. It is the most common method of introducing medicine, fluids, and transfused blood into the body. Generally, it is difficult to inject anything into an artery because the pressure in the artery will cause a leak in the injection site, even when the site is kept under pressure. Intramuscular injection, where the injection is made into a muscle, is necessary for some injections (such as vaccinations), but is avoided because the aftereffects are very painful to the patient.
Generally, a rubber tourniquet is applied to the arm in order to make a vein on the skin stand out under pressure. The site of the injection is sterilized with an antiseptic to avoid infection. At that point, a needle is injected into the vein and for an intravenous drip, left under the skin while the intravenous line is attached. The tourniquet is then removed.