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The microcirculation is essential to many functions of the organism. Its central role in

the function of the cardiovascular system was emphasized by Carl J. Wiggers, the deanof

cardiac physiologists of the past century, who wrote: inour zeal to interpret the importance

of the heart and great vessels it should never be forgotten that the more obvious phenomena

of the circulation are but a means throughwhich the real object of maintaining an adequate

capillary flow is attained.

I.1.1 Functions of the microcirculationIn addition to delivering nutrients and removing waste products essential for moment

to moment function, the microcirculation plays an essential role in fluid exchange between

blood and tissue, delivery of hormones from endocrine glands to target organs, bulk delivery

between organs for storage or synthesis and providing a line of defense against pathogens. To

execute these functions satisfactorily, certain features are necessary in the microcirculation.

In the description that follows we provide an overview of these features, based in large part

on skeletal muscle, which constitutes 50% of body mass and has perhaps the largest

capability of any organ for altering blood flow according to need. On a practical level, it is

also more accessible for microcirculatory studies than most other organs. Certain specialized

features of the microcirculation of other organs are also described.

I.1.2 Definition of the microcirculation

As a first approximation, the microcirculation consists of those blood vessels too

small to be seen with the nakedeye. This limitation of visual acuity required Harvey in1628 to

postulate the existence of invisible pores of theflesh to support his hypothesis that blood

passes through microscopic channels in circulating from artery to vein. However, Harveys

critics suggested that such porositiesdid not exist but rather that blood moved through the

tissue by a general seepage. Development of the first single lens microscope enabled

Malpighi in 1661 to observediscrete capillaries connecting arteries and veins in the tortoise

lung. Van Leeuwenhoek in 1674. Was able to provide quantitative information on the size

and spatial density of microcirculatory vessels in the tail fin of the eel as well as measure the

velocity of red cells in these vessels. Both investigators provided critical support for

Harveyshypothesis. With further development of the microscope, the histology of the

vascular wall and the existence of acontinuous layer of endothelium lining the vessels

weredescribed. Subsequent studies led to an appreciation of the specialized structure and

topological organization of the smaller vessels located within organs and the manner in which

they differ from the larger conduit vessels that distribute blood flow to the organs.The

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rheological properties of blood in the microcir-culation differ from those in the large vessels

due to theFahraeus and Fahraeus-Lindqvist effects, which lead todiameter-dependent

reduction of hematocrit and effec-tive blood viscosity in these vessels. This feature

becomesincreasingly important in vessels less than 100m luminaldiameter.There is also

significant phase separation of red cells and plasma at bifurcations in the

microcirculatorynetwork as described by August Krogh. These phenom-ena are considered in

Section I, Chapter 1 of this volume.Microcirculatory studies most commonly involvedirect

observation under the microscope as in the examplesgiven above. However, studies of the

exchange process inthe microcirculation between blood and tissue have alsorelied to a great

extent on whole organ studies in which theextraction of diffusible indicators is measured and

com-pared under different conditions. Studies of the regulationof blood flow by the

microcirculatory vessels have alsobenefited considerably from determination of flow

andvascular resistance in individual organs and in the wholeorganism.

I.2 MICROCIRCULATORY ORGANIZATION AND STRUCTURE

The microcirculation is organized into three principal sections, arterioles, capillaries

and venules; each has unique structure and function. The arterioles are well invested with

vascular smooth muscle and are primarily responsible for delivery of blood to localized tissue

areas and regulation of the rate of delivery. The capillaries possess very thin walls and are

primarily responsible for exchange between blood and tissue. The venules drain blood from

the capillaries for return to the heart and generally parallel the arterioles in organization. They

are important for macromolecular exchange, postcapillary vascular resistance and

immunological defense.

I.2.1 Arterioles

I.2.1.1 Network organization

As we follow the distribution of blood from the heart and aorta through the major

arteries to the successively smaller and more numerous branches of the arterial system, a

region of the network is reached whose structure is recognizably different from the larger,

upstream vessels. Spalteholz, and later Krogh described the microcirculation as beginning

with an anastomosing network of vessels, the large arterioles, followed by a tree-type

network of smaller arterioles, an anastomosing network of capillaries and a network of

venules that is organized in a manner similar tothat of the arterioles. The various levels of the

arteriolar network differ inrespect to function as well as structure. To aid in analy-sis, several

schemes have been used to classify vessels.The simplest classification is by internal diameter.

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Thisclassification enables a particular function of the arteri-oles to be quantified and

compared as a function of rest-ing diameter. A limitation of this approach is that vesselsat the

same level in the vascular network, presumablyhaving very similar functions and

environment, may havesignificantly different resting diameters. To overcome thislimitation,

Wiedeman designated the large arterioles aris-ing from small arteries as first order vessels

and succes-sive,

smaller

branches as 2nd order, 3rd order etc.[9]. Inmost vascular beds five or six orders are identified

by thismethod. This system has an element of subjectivity asso-ciated with the assessment of

size in designating a branchas a new order rather than extension of the existing order.This

method, when applied consistently, does enable com-parison of arterioles at different levels

of the network andis useful in comparing findings from different laboratories.Alternatively,

the size criterion can be eliminated and gen-eration numbers assigned to each successive

segment of thebranching network [10] . This scheme preserves the great-est amount of

topological information. A third approach ispresented in the Horton-Strahler

method[11]which beginsat the capillary level and designates the immediate precap-illary

vessels as 1st order and the vessel feeding two 1storder vessels as 2nd order. Where a 2nd

order vessel meetsanother 2nd order-vessel the feeding vessel is designated3rd order etc. This

method is useful in comparing vesselsin the immediate vicinity of the capillary network but

losessome of the topological information and is less practical inclassifying vessels farther

upstream.Additional complexity is encountered in classifyingvessels in the arcade portion of

the network. In skeletalmuscle it has been shown that the loops formed in thisnetwork are

characterized by an ellipticity factor which issimilar among loops and an orientation which is

generallyparallel to the muscle fibers [12] .

I.2.1.2

Structure and dimensions

The diameter of vessels seen

in vivo

depends on the stateof vascular tone and the data presented here were generallyobtained

under control conditions. Under these conditionsthe diameter of vessels identified as large

arterioles or firstorder vessels by the Wiedeman system varies according to