The heart of fish are the simplest vertebrate hearts. They are made up of two main chambers: a thin walled atrium, and a more muscular ventricle. The atrium pumps the blood into the ventricle, which in turn pumps the blood into the conus, an elastic compartment which does not pump, but has the ability to stretch and squeeze. From the conus the blood travels straight to the gills where it is enriched with oxygen. The oxygenated blood then flows through the body and returns to the atrium. The fish therefore has a single-curcuit circulation through which blood is driven by a simple two-chambered heart.