An Introduction to Cardiovascular Physiology
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An Introduction to Cardiovascular Physiology

J R Levick

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eBook - ePub

An Introduction to Cardiovascular Physiology

J R Levick

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About This Book

An Introduction to Cardiovascular Physiology is designed primarily for students of medicine and physiology. This introductory text is mostly didactic in teaching style and it attempts to show that knowledge of the circulatory system is derived from experimental observations. This book is organized into 15 chapters. The chapters provide a fuller account of microvascular physiology to reflect the explosion of microvascular research and include a discussion of the fundamental function of the cardiovascular system involving the transfer of nutrients from plasma to the tissue. They also cover major advances in cardiovascular physiology including biochemical events underlying Starling's law of the heart, nonadrenergic, non-cholinergic neurotransmission, the discovery of new vasoactive substances produced by endothelium and the novel concepts on the organization of the central nervous control of the circulation. This book is intended to medicine and physiology students.

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Chapter 1

Overview of the cardiovascular system

Publisher Summary

This chapter provides an overview of the cardiovascular system. The rate at which diffusional transport occurs is critically important because the supply of nutrients must keep up with cellular demand. The first and foremost function of cardiovascular system is the rapid convection of oxygen, glucose, amino acids, fatty acids, vitamins, drugs, and water to the tissues and the rapid washout of metabolic waste products like carbon dioxide, urea, and creatinine. The cardiac output is the volume of blood ejected by one ventricle during one minute, and this depends on both the volume ejected per contraction (the stroke volume) and the number of contractions per minute (heart rate). The behavior of the heart and the blood vessels has to be regulated to deal with varying environmental and internal stresses.
1.1 Diffusion: its virtues and limitations
1.2 Functions of the cardiovascular system
1.3 Circulation of blood
1.4 Cardiac output and its distribution
1.5 Introducing some hydraulic considerations: pressure and flow
1.6 Structure and functional classification of blood vessels
1.7 Plumbing of the vascular circuits
1.8 Central control of the cardiovascular system
The heart and blood vessels form a system for the rapid transport of oxygen, nutrients, waste products and heat around the body. Small primitive organisms lack such a system because their needs can be met by direct diffusion from the environment, and even in man diffusion remains the fundamental transport process between blood and cells. In order to appreciate fully the need for a cardiovascular system we must begin by considering some properties of the diffusion process.

1.1 Diffusion: its virtues and limitations

The ‘drunkard’s walk’ theory

Diffusion is a passive process in that it is not driven by metabolic energy but arises from the innate random thermal motion of molecules in a solution or gas. Although each individual movement of a solute molecule occurs in a random direction (the ‘drunkard’s walk’) this nevertheless produces a net movement of solute in the presence of a concentration gradient. Figure 1.1 illustrates how this happens. Notice that although the net transfer of solute is from compartment A into compartment B there is also a smaller backflux into compartment A. This can be proved by adding a trace of radiolabeled solute to compartment B; some labelled molecules appear in compartment A even though the net diffusion is from A to B.
image
Figure 1.1 Sketch illustrating how random molecular steps result in a net movement of solute down a concentration gradient. At time 1 (upper sketch) there are 8 molecules per unit volume in (A) and 2 in (B). At time 2 (lower sketch) each molecule has moved a unit step in a random direction. Because there was a greater density of molecules in A there was a greater probability of random movement from A to B, resulting in a net ‘downhill’ flux

The importance of diffusion distance

The rate at which diffusional transport occurs is critically important because the supply of nutrients must keep up with cellular demand. However, as Albert Einstein showed the time (t) that it takes a randomly jumping particle to move a distance x in one specific direction increases with the square of distance:
image
(1.1)
(see footnote to Table 1.1); and as a result diffusional transport is extremely slow over large distances. While diffusion across a short distance, such as the neuromuscular gap (0.1 μm) takes only 5 millionths of a second, diffusion across the heart wall (approximately 1cm) is hopelessly slow, taking over half a day (Table 1.1). Sadly, Nature often proves the validity of Einstein’s equation and Figure 1.2 is an example of this: it shows the heart of a patient who suffered a coronary thrombosis (obstruction of the blood supply to the heart wall). The pale area in the wall is muscle which has died from lack of oxygen even though the adjacent cavity (the left ventricle) was fully of richly oxygenated blood; the patient died simply because a distance of a few millimetres reduced diffusional transport to an inadequate rate.
Table 1.1
Time taken for a glucose molecule to diffuse a specified distance in one direction
Distance(x) Time(t)* Comparable distance in vivo
0.1 μm 0.000005 s Neuromuscular gap
1.0 μm 0.0005 s Capillary wall
10.0 μm 0.05 s Cell to capillary
1 mm 9.26 min Skin, artery wall
1 cm 15.4 h Ventricle wall
*Times are calculated by Einstein’s equation t = x2/2D. ‘D’ is the solute diffusion coefficient. For glucose in water at 37ºC, D is 0.9 × 10–5 cm2/s (Einstein, A. (1905) Theory of Brownian Movement (trans, and ed. by R. Furth and A. D. Cowper, 1956), Dover Publications, New York)
image
Figure 1.2 Section through the left ventricle of a human heart after a coronary thrombosis. The section is stained to show intracellular enzyme content. The pale area marked by asterisks is an infarct, an area of muscle severely damaged or killed by oxygen lack; the pallor is due to the intracellular enzyme having leaked out of the dying cells. The infarct was caused by a thrombus in a coronary artery, blocking the convectional delivery of oxygen. Diffusion of oxygen from the blood in the adjacent cavity of the left ventricle is unaffected yet only a thin rim of tissue (approximately 1 mm) can survive on this diffusional flux. (Courtesy of Professor M. Davies, St. George’s Hospital Medical School, London)

Convection for fast long-distance transport

Clearly then, for distances greater than approximately 0.1mm a faster transport system is needed and this is provided by the cardiovascular system (Figure 1.3). The cardiovascular system still relies on diffusion for the uptake of molecules at points of close proximity to the environment (e.g. oxygen uptake into lung capillaries) but it then transports them rapidly over large distances by sweeping them along in a stream of pumped fluid. This form of transport is called bulk flow or convective transport. Convective transport requires an energy input and this is provided by a pump, the heart. In man convection takes only 30 s to carry oxygen over a metre or more from the lungs to the smallest blood vessels of the limbs (capillaries). Over the final 10–20 microns separating the capillary from the cells, diffusion is again the main transport process.
image
Figure 1.3 Schematic diagram of the mammalian cardiovascular system to illustrate the roles of diffusion and convection in oxygen transport. L, left side of h...

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