Page 110 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

PHYSICAL AND FLOW PROPERTIES OF BLOOD 87

FIGURE 3.13 Structure of the heart and course of blood flow through
the heart chambers. [From Guyton and Hall (1996) by permission.]

belong to the first type, whereas the pulmonic and aortic valves compose the second. Valves of the
same type are not only structurally similar, but also functionally alike. For this reason, conclusions
derived from studies on aortic and mitral valves are generally applicable to pulmonic and tricuspid
valves, respectively.
One-way passages of blood from the ventricles into the main arteries is due to the heart valves.
Though relatively simple in structure, heart valves are in their healthy state remarkably efficient,
opening smoothly with little resistance to flow and closing swiftly in response to a small pressure
difference with negligible regurgitation. Yet they are essentially passive devices that move in
reaction to fluid-dynamical forces imposed upon them.
The motions of heart valves have drawn a considerable amount of attention from physicians and
anatomists. One of the major reasons for this interest can be attributed to the frequent diagnosis of
valvular incompetence in association with cardiopulmonary dysfunctionings. The first study exploring
the nature of valve functioning, by Henderson and Johnson (1912), was a series of simple in vitro
experiments, which provided a fairly accurate description of the dynamics of valve closure. Then
came the Bellhouse and Bellhouse experiments (1969) and Bellhouse and Talbot (1969) analytical
solution, which showed that the forces responsible for valve closure, are directly related to the
stagnation pressure of the flow field behind the valve cusps.
Computational fluid dynamics models were developed over the years that include the effects of
leaflet motion and its interaction with the flowing blood (Bellhouse et al., 1973; Mazumdar, 1992).
Several finite-element structural models for heart valves were also developed in which issues such
as material and geometric nonlinearities, leaflet structural dynamics, stent deformation, and leaflet
coaptation for closed valve configurations were effectively dealt with (Bluestein and Einav, 1993;