Structural basis of ventricular stiffness. 
The morphologic component of ventricular stiffness has not been clearly delineated.  This stiffness factor varies over wide ranges with the degree of distension, being very low at low volumes and very high at high volumes.  It is constant in rats and hamsters from 30 days of age to 17 months, varying no more than 20%, with increases in mass of 4-fold.  These parameters alone suggest a complex structure or structures.  The ventricular stiffness factor of rats is about twice that of hamsters at all ages.  This naturally occurring variation in stiffness of normal hearts from the two species provides an excellent model to examine for morphologic differences that might explain the variation in stiffness of the two ventricles.  Hearts from each species from 1 to 7 months of age were examined by light microscopy and scanning and transmission electron microscopy.  There is a major difference between the two species in the amount of collagen in the form of 120- to 150-nm.  diameter bundles that form a weave around groups of myocytes.  This system is far more extensive in rats than hamsters and is the only morphologic difference that can easily explain the divergence in stiffness between the two species.