Why does the skin on your arm around the area blood is taken from bruise after a blood test?

Bruise: The blood vessel is punctured when blood is withdrawn during a blood test. If the vessel is not compressed long enough, blood will leak out of the vessel giving a bruise. Holding direct pressure with a cotton ball for 10 or 15 minutes is usually enough. However if a person is on platlet inhibitors such as asprin longer might be necessary.