The Mechanics of the Circulation

by C.G. Caro, T.J. Pedley,
R.C. Schroter and W.A. Seed

Oxford University Press, 1978

ISBN 0-19-263323-6

ISBN 0-19-261171-2 Pbk

Preface

In 1808 Thomas Young introduced his Croonian lecture to the Royal Society on the function of the heart and arteries with the words: The mechanical motions, which take place in an animal body, are regulated by the same general laws as the motions of inanimate bodies ... and it is obvious that the inquiry, in what manner and in what degree, the circulation of the blood depends on the muscular and elastic powers of the heart and of the arteries, supposing the nature of those powers to be known, must become simply a question belonging to the most refined departments of the theory of hydraulics.

For Young this was a natural approach to physiology; like many other scientists in the nineteenth century, he paid scant attention to the distinction between biological and physical science. Indeed, during his lifetime he was both a practising physician and a professor of physics; and, although he is remembered today mainly for his work on the wave theory of light and because the elastic modulus of materials is named after him, he also wrote authoritatively about optic mechanisms, colour vision, and the blood circulation, including wave propagation in arteries.

This polymath tradition seems to have been particularly strong among the early students of the circulation, as names like Borelli, Hales, Bernoulli, Euler, Poiseuille, Helmholtz, Fick, and Frank testify; but, as science developed, so did specialization and the study of the cardiovascular system became separated from physical science. This process was not, of course, complete because collaborative work between scientists from different disciplines has always gone on. However, its scale was quite limited, and many medical and physiological workers found it difficult to comprehend because of their inadequate background in mathematics and mechanics, just as physical scientists found the complexity and empiricism of physiological studies, as well as the terminology, forbidding.

The separation caused by specialization has now assumed new importance. Over about the last twenty years physical scientists and engineers have made considerable contributions to the understanding of the mechanics of the circulation. These have strongly stimulated collaborative research, but at the same time have made the field increasingly difficult for those with a limited training in physics and mathematics. Several recent reviews and monographs bear witness to the importance of this interdisciplinary work, but do little to help the medical reader, since they invariably assume an understanding of mechanics and are often quite mathematical in format.

This book is an attempt to alleviate the problem. It is intended as an introductory text on the mechanics of the circulation which, so far as is practicable, avoids mathematical formulations and presents mechanics in readily comprehensible terms. Our experience in teaching students of physiology and medicine, and cardiological physicians and surgeons, suggests that this approach is helpful, and it is to such a readership that the book is primarily directed. In addition, we think that the book will prove useful to physical scientists, mathematicians, and engineers interested in the field, since it provides the relevant anatomical and physiological background to the mechanics, and gives definitions of terms and numerical data wherever possible.

The book is divided into two parts. The first part, 'Background mechanics', provides a non-mathematical outline of the physical processes and mechanisms which have general importance in the circulation. Thus it forms a physical introduction to the later material, though since it is self-contained and deals in a general rather than a specific way with solid and fluid mechanics and mass transport, it may also prove useful as a background to the study of systems other than the circulation.

The second part, 'The mechanics of the circulation', examines in some detail the physiological events that occur in the circulation and the physical mechanisms that underlie them. It deals first with the relevant properties of blood and then considers the circulation systematically, starting with the heart and moving forward chapter by chapter through the circulation. No attempt is made to deal in detail with active physiological mechanisms such as reflexes, but the resulting changes in the physical properties of the system are studied. In each chapter the relevant anatomical and physiological background is presented first, followed by a discussion of the mechanics. There is extensive cross-referencing to physical processes already examined earlier in the book; more specialized physical processes, relevant to the mechanics of a part of the circulation, are introduced as they arise.

We have attempted to cover all the mechanical features of the circulation which are currently considered important. However, the book is not intended as a research review, and we have therefore largely avoided citing original research references in the text. Instead, we have provided a reading list for each chapter in the second part of the book, chosen to guide readers unfamiliar with the literature to suitable reviews and sources. In addition, we have, wherever possible, taken our illustrations from important sources, in many cases the original research literature, so that the references given in the Figure captions supplement the reading list.

A temptation in writing an interdisciplinary book of this kind is to oversimplify, usually at the expense of one of the disciplines; we have tried hard to avoid doing this. We have also tried, wherever possible, to supply numerical data; for convenience the more important measured and derived values, which'are referred to repeatedly throughout the book, are collected together in a table reproduced on the end-pages. The units used are those of the Systeme Internationale, though with quantities such as pressure, where confusion might arise, we have added the traditional units. Since physical scale is important to so much of mechanics, and the dog is the only species for which anything like a comprehensive range of reliable measurements is available, we have given values from this animal throughout the book. Even so, we have had to turn to other species in describing the microcirculation, though this is a region where inter-species differences in scale appear to be relatively slight. Finally we have referred specifically to the human circulation wherever the mechanics appears to be different, or where we believe that it has relevance to a circulatory disease process.

July 1977

C.G.C.

T.J.P.

R.C.S.

W.A.S.

Acknowledgments

We owe debts of gratitude to many colleagues for their advice and help. In particular we would like to thank Dr Laurence Smaje, who contributed a major part of the physiological material contained in the chapter on the microcirculation, and without whose guidance we could not have surveyed that topic.

Mr Paul Minton of Imperial College, Dr Giorgio Gabella of University College, and Drs Graham Miller and Derek Gibson of the Brompton Hospital all made available to us original data or material for figures. Dr Michael Sudlow contributed greatly to early discussions on the scope and form of the book, and provided material for the chapter on veins; and Prof. Ilsley Ingram and Drs Julien Hoffmann and Michael Hughes made valuable comments and suggestions about the chapters on the blood, heart, and pulmonary circulation respectively. In addition, we owe our thanks to all the authors and journals cited in the Figure captions for permission to reproduce Figures.

Finally, we give our special thanks to Miss Evelyn Edwards, whose editorial precision helped (and chastened) us throughout the preparation of the book.