The lining of the heart is the pericardium.
Natural Selection is a process in which certain traits become more common or not. This is determined by the necessity of certain traits and the environment the species finds itself in. Natural selection only acts on existing traits because they are the traits active in a given species in a given environment.
The cell membrane is a lipid bilayer that prevents that passage of water and ions. This allows cells to maintain a higher concentration of sodium ions out the outside of the cell.
