Organs are collections of tissues with a similar function. Humans have five vital organs that are essential for survival—the brain, heart, kidneys, liver, and lungs. Functionally related organs often cooperate to form whole organ systems. Organs are composed of parenchyma and stroma. Parenchyma is the main tissue which is unique for the specific organ, such as the myocardium, which is the main tissue of the heart. Stroma are sporadic tissues that include the nerves, blood vessels, and connective tissues.