242 THE CIRCULATION OF THE BLOOD 



sure is too slight, a very small part of the characters may be represented on 

 the tracing. 



THE PERIPHERAL REGULATION OF THE FLOW OF BLOOD. 



The flow of blood through the circulatory system depends on the inter- 

 action of several factors which have already been mentioned in another con- 

 nection: The rate and volume of the heart-beat, the elasticity of the blood 

 vessels, the resistance of the microscopic peripheral vessels, and the volume 

 of blood in the body. We have already learned, page 205, that both the 

 rate and volume of the contractions of the heart are under very minute 

 and intimate regulation and control through the cardiac nervous mechanism. 

 Also we have found that there is intimate co-ordination between the activity 

 of the heart and the activity of all other parts of the body, a co-ordination 

 secured through the nervous system. All regulation which affects the 

 heart must of necessity affect the general blood-pressure and, therefore, 

 not directly any particular part. 



The general elasticity of the blood vessels, and of the arteries in par- 

 ticular, which makes the general arterial pressure possible, is dependent 

 primarily on the presence of a large amount of elastic connective tissue in 

 the walls of the vessels. The elasticity of this tissue is a purely passive 

 property which can be utilized only by some positive source of energy, in 

 this instance the heart. 



The Variations in Peripheral Resistance. Certain arteries and 

 veins, especially the smallest ones, the arterioles and venules, are supplied 

 with muscular tissue in their walls. The activity of these muscles in the 

 vascular complex makes the peripheral regulation of the flow of blood 

 possible. This muscular tissue not only exhibits a passive elasticity 

 comparable to that of the yellow elastic connective tissue, but upon the 

 proper stimulation it actively contracts or relaxes, thus securing to the per- 

 ipheral resistance of the vessels an active adjustment to the ever- varying 

 dynamic conditions of the vascular apparatus. 



The muscular tissue in the vessels increases relatively in amount as 

 the vessels become smaller. In the arterioles it is developed out of all 

 proportion to the other elements. In fact, in passing from the arterioles 

 to the capillary vessels, made up as we have seen of endothelial cells with a 

 supporting ground substance only, the last change on the side of the arteries, 

 which occurs as the vessels become smaller, is the disappearance of 

 muscular fibers. 



The office of the muscular coat is to adjust the size of the arterioles and, 

 therefore, the flow of the blood. It is to regulate the quantity of blood to be 

 received by each part or organ, and to adjust this quantity to the requirements 

 of each, according to various circumstances, but chiefly according to the de- 



