Original article
The vascular system: An overview of structure and function

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Abstract

It is the function of the vascular system, through a complex network of arteries, capillaries and veins, to maintain cellular homeostasis. As research scientists it is necessary to understand not only some of the basic properties of the blood vessel itself but also how these vessels differ in cellular and physiological function. This review provides an overview of the basic physiological and pharmacological tenets of blood vessels. It also briefly describes in vivo and in vitro methods used in the measurement of blood flow and blood vessel function. It is hoped that this review will provide readers of this focussed issue of the Journal of Pharmacological & Toxicological Methods with an appreciation of the many mechanical, electrical and biochemical methodologies described within this issue.

Introduction

The vascular system of the human body is comprised of a large number of vessels that play an integral role in the movement of blood throughout the circulatory system. It is only through this extensive network of arteries, capillaries, and veins that cellular function (growth and development), absorption of essential nutrients (such as vitamins and minerals), and removal of cellular and metabolic waste products — the cell homeostasis — are maintained. In order for such a balance to occur, it is a necessity that the vessels and related structural elements involved exhibit some homology of cellular structure and composition, yet be different enough in their cellular properties so that they can fulfill their physiological roles. For example, blood vessels in the heart share a structural similarity to those found in the nephron of the kidney, and yet, comparatively, their physiological roles are diverse. Heart vessels are necessary to maintain required oxygen to the heart muscle while those in the nephron are required for the selective filtration of blood constituents. The complexity of vessel organization within the cardiovascular system begins with the recognition that there are essentially two components in the circulatory system: the cardiovascular and lymphatic systems. In the broadest sense, the cardiovascular system can be thought of as consisting of the heart, vessels, and blood (and all cellular components within the blood) while the lymphatic circulatory system consists of lymphatic microvessels (capillaries) and larger lymph vessels.

William Harvey is the first physiologist credited with describing the existence of the circulation of blood in large blood vessels (for an in-depth historical overview, see Bowman & Rand, 1980). It was the rapid improvement in scientific methodology that allowed Malpighi into visualize the interconnecting capillary network that was first proposed by Harvey some 30 years earlier. Since that time, our level of understanding of many aspects of vascular function has increased dramatically. Improved methods in cell and molecular biology, along with advances in equipment and therefore techniques, provide the modern researcher with a more complete, but not absolute, understanding of cellular structure and organization. This has led to a marked advance in drug development for vascular-related pathophysiological disease states. A recent National Library of Medicine (PubMed) search reveals that there were 2321 papers published in 1990 that examined the role of blood vessels in disease. A similar search in 1999 reveals that 3293 papers (a 30% increase) had been published. The burgeoning numbers of papers involving blood vessel research suggest that there is a growing sense of importance of the blood vessel in the pathology of many disease states which, like all other areas of medical research today, are fueled by improved pharmacological, cellular. and molecular methodologies.

Section snippets

Some basic structural features of blood vessels

The development of research techniques for use in the cardiovascular system requires an understanding of some of the basic physiological properties of the system at the cellular level. The vascular system, despite some complex regional and organ differences, does share a common histological organization that will be briefly overviewed.

In the general case, most blood vessels usually consist of three histologically distinct regions (Borysenko & Beringer, 1984). Each region contains variable

The arterial and venous components of the cardiovascular system

Oxygenated blood returning from the lungs is ejected from the left ventricle of the heart into a large network of arteries. Arteries, based upon differences in cell histology, can be classified into two distinct types: elastic and muscular (for a complete review of the functional histology of blood vessels, see Borysenko & Beringer, 1984).

Elastic (or conducting) arteries are present near the heart and other organs that are associated with the movement of large volumes of blood. These types of

Some simple physical properties of blood vessels

Another series of properties, which will be briefly considered in this mini-review, is that which discusses the physical properties (such as fluid dynamics) involved in the movement of blood through small and large blood vessels. We will also briefly discuss the properties of resistance to flow observed in the elastic walls of blood vessels. These properties are important in many methods discussed in this focussed issue of the Journal that are used to examine flow in arteries and veins, organs,

Some methods used in the measurement of blood flow and vessel function

A large number of techniques can be used alone, or in combination, to study the function of blood vessels. Table 1 lists a number of methods, many of which are discussed in this edition of the Journal. These methods have dramatically changed our understanding and approach to the study of vascular and cardiovascular disease. One recent example is the use of vascular imaging techniques to identify the development of sub-clinical atherosclerotic lesions in patients. Early detection of such lesions

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