The physical laws of conservation of mass and energy, the first and second laws of thermodynamics, and the Newton's laws are universally accepted. Quantuum mechanics, relativity theory, and the concept of an array of genes in the chromosomes for heredity burst onto the scientific stage almost simultaneously about a century ago. In physics, there are two major activities.

One is to show that a minimal set of basic laws exist that are necessary and sufficient to explain everything observable. The other activity is to show that all observable events are solutions of boundary-value problems based on the basic laws. Use of such a physical approach often simplifies scientific investigations. The study of HPH should be no exception.

Since the lung contains a huge number ofparticles (atoms, molecules), it may be considered as a continuum. The concept of a continuum was introduced into mechanics a long time ago to simplify the application of basic laws (8, 15). By stating that a matter is a continuum means that the particles involved can be identified as isomorphic with the real number system in mathematics. Thus the molecules in our body are considered to be as densely distributed as the real numbers.

Classical continuum mechanics has been applied to the problems of pulmonary physiology in the last 30-40 years. In classical continuum mechanics, a set ofuniversally accepted hypotheses is proposed. These hypotheses are called axioms. Axioms and boundary conditions define specific problems for investigation.

The axioms of the classical continuum mechanics are listed in Table 1. This includes the laws of conservation of mass and energy, the Newton's laws of motion, the immutable constitutive equations of the materials, the invariability of the zero-stress state of solids, the constancy of mechanical properties of solids, and the constancy of material composition of solids. Classically, these were considered as universal truth. Much of our modern civilization was developed on the basis of these axioms. Airplanes, ships, telescopes, microscopes, artificial heart valves, and kidney dialysis machines were designed on the basis of these classical axioms.

Table 1. Axioms in Classical Mechanics and Biomechanics

Classical Mechanics Biomechanics

Conservation of mass Conservation of atom, not molecules

Newton's law Newton's law

Conservation of kinetic + potential energy Sum of all energy categories conserved

Zero-stress state is invariable Zero-stress state remodels

Constancy of mechanical properties Mechanical properties remodels

Constancy of material composition Chemical composition remodels

With the advancement of biomechanics, the truth of the axioms of classical continuum mechanics began to be doubted. When the problems of atherosclerosis and hypoxic pulmonary hypertension came into focus, a revolution in the system of axioms of mechanics became inevitable. Some ofthe reasons will be explained in detail in Section 8 infra. In fact, it appears that a new system of axioms has to be introduced to replace the old system in biomechanics. Our new system is listed in the right-hand column of Table 1 (10).

In this system, the atoms are conserved, but the molecules are not. The molecular composition may change with time. The zero-stress state of the solid part can change, and the mechanical properties ofthe solids may change with time. Thus, the study of HPH is indeed a major fundamental event that will change the course of biomechanics.

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