Semi-conductors can best be described as materials that are neither normal conductors (substances that conducts electricity such as copper) or normal insulators (substances that does not conduct electricity such as rubber), but have some of the properties of both. Semiconductors that do not contain impurities do not conduct electricity but the addition of some elements to them allows easy passage of electricity through them. Examples of semi-conductors are silicon, germanium, and selenium.
<h3><em>The circulatory system consists of three independent systems that work together: the heart (cardiovascular), lungs (pulmonary), and arteries, veins, coronary and portal vessels (systemic). The system is responsible for the flow of blood, nutrients, oxygen and other gases, and as well as hormones to and from cells.</em></h3>
