Complex sentence: It should be emphasized that this relationship applies only to the unit of tissue, as well as to skeletal muscle units (the unit being the individual muscle fiber) and to the heart (the unit being the entire auricles or the entire ventricles).
Split into simpler sentences: It should be emphasized that this relationship applies only to the unit of tissue. It applies also to skeletal muscle (the unit being the individual muscle fiber) and to heart (the unit being the entire auricles or the entire ventricles).