Ambulance is a vehicle designed to transport sick or injured people. Many types of vehicles serve as ambulances. Ambulances for ground transport are specially built versions of trucks or vans. They are equipped with flashing lights and sirens for alerting drivers and pedestrians of their approach. Helicopters or airplanes serve as fast transport for the seriously ill or injured. Modern ambulances carry a variety of emergency equipment, including bandages, splints, heart monitors and defibrillators (devices to correct abnormal heart rhythms), oxygen machines, and medicines. Highly trained professionals working in ambulances can respond to a wide variety of situations involving serious illness or trauma (injury). Ambulances and the professionals that staff them are essential parts of a health care system.