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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



extent of contractile activity of the smooth muscle 

 cells which encircle the terminal arterioles, and there- 

 fore local conditions can be responsible for controlling 

 capillary flow to meet the requirements of the tissues 

 in the immediate environment. 



Microcirculation in the Rabbit Ear 



Collection of new and important data on mam- 

 malian small blood vessels began in 1924, following 

 the introduction of the transparent chamber technique 

 by Sandison (104). He reported (106) observations on 

 circulation in the rabbit ear primarily concerned with 

 contractility of small blood vessels. Local control of 

 blood flow was seen to reside in the smooth muscle 

 cells which developed on newly formed capillaries as 

 they were transformed into arterioles. In observing 

 circulation of blood through the vessels which formed 

 in the chamber, Sandison saw an axial stream of cells 

 surrounded by a narrow, clear plasma layer. Leuko- 

 cytes were thrown into the peripheral layer of plasma 

 and slowly rolled along the vessel wall. An uneven 

 mixture of blood cells and plasma was observed during 

 sluggish or irregular flow through capillary nets, this 

 type of flow resulting from the aphasic and independ- 

 ent contraction of arterioles which causes blood to be 

 fed to the veins through capillaries and venules in a 

 broken stream. "Plasma skimming" was seen mainly 

 in partially contracted vessels or in capillaries con- 

 necting two vessels and in which there was no circula- 

 tion due to equal pressure at each end of the con- 

 necting capillary. In a capillary loop, the two ends of 

 which were connected to a larger vessel, plasma flow 

 (indicated by the passage of blood platelets) would 

 continue in the absence of circulation of blood cells. 

 An increase in the blood supply to the larger, parent 

 vessel often caused red blood cells and leukocytes to 

 be forced through the capillary loop. The blood cells 

 were often seen to take long narrow shapes as they 

 were forced through the constricted entrance to the 

 capillary loop. 



Although capillary circulation was almost entirely 

 regulated by contraction of the arterial vessels sup- 

 plying the capillary plexus, flow was seen to be slowed 

 or even stopped by a single leukocyte caught in a 

 constricted portion of a vessel. One of the most 

 favorable places for plugging by a leukocyte was 

 found to be at the origin of the small arterioles from 

 their arteries. This region was normally partly con- 

 stricted because of the bulging of endothelial cells into 

 the lumen of the vessel. This site bears a close re- 

 semblance to the Indian Club structure described by 



Cap. 



fig. 4. Camera-lucida drawing of a precapillary branch of 

 an artery. Muse. = muscle cells; End. nucl. = endothelial 

 nucleus; Adv. = adventitial cell; Cap. = capillary. [From 

 Sandison (106).] 



Nicoll & Webb (89) (see fig. 4). The leukocytes were 

 dislodged by an increase in force of the blood stream 

 or by the ameboid activity of the leukocytes. A similar 

 occurrence was seen by Nicoll & Webb (88) in blood 

 flow through comparable vessels in the bat wing. 



Clark & Clark (29), in the same year, reported on 

 the behavior of microscopic vessels seen in the rabbit 

 ear using a "preformed-tissue" chamber, one in which 

 the original structures were present as opposed to 

 newly formed vessels and nerves seen in the first 

 studies using the transparent chamber. The Clarks 

 were impressed with the contraction of arteries and 

 arterioles; spontaneous rhythmical contractions as 

 well as contractions in response to artificial stimulation 

 (mechanical, tactile, or auditory). Spontaneous rhyth- 

 mical contraction was seen to play an important role 

 in regulation of blood flow, causing changes in the 

 distribution of blood to different capillary areas and 

 causing continuous alterations in the direction of flow. 

 Contractions of arterial vessels were found to be 

 varied. Contraction of the main artery reduced the 

 blood flow to the whole area, but the distribution of 

 blood to different portions was dependent on contrac- 

 tions of different arterial branches, each at a different 

 tempo and independent of the contraction of the main 

 artery and of each other. An arteriole might contract 

 to complete closure and thus cut off blood to the 

 capillaries it supplied while an adjacent vessel, a 

 branch from the same artery, would remain open to 

 allow rapid passage of blood. Arteriovenous anasto- 

 moses were seen to contract actively and so influence 

 the distribution of blood. Contractions were seen to 

 decrease in animals that were asleep or anesthetized. 



Further studies on the activity of arterial vessels, 



