CHAPTER XIV 



THE CIRCULATION OF THE BLOOD (Continued) 

 THE VASCULAR APPARATUS : ITS STRUCTURE AND FUNCTIONS 



The Vascular Apparatus in its entirety consists of a closed system of 

 vessels which not only contain the blood, but under the driving power of the 

 heart, transmit it to and from all regions of the body. It is usually divided 

 into a systemic and a pulmonic portion. 



The Systemic Vascular Apparatus. This portion of the general vas- 

 cular apparatus includes all the vessels extending from the left ventricle to 

 the right auricle: viz., the arteries, capillaries, and veins. Though serving 

 as a whole to transmit blood from the one side of the heart to the other, 

 each one of these three divisions has separate but related functions, which 

 are dependent partly on differences in structure and physiologic properties, 

 and partly on their relation to the heart and its physiologic activities. 



The Structure, Properties and Functions of the Arteries. The 

 arteries serve to transmit the blood ejected from the heart to the capillaries; 

 that this may be accomplished they divide and subdivide again and again 

 and ultimately penetrate all the areas of the body. Their repeated 

 division is attended by a diminution in size, a decrease in the thickness and 

 a change in the structure of their walls. 



A typical artery consists of three coats: an internal, the tunica intima; a 

 middle, the tunica media; an external, the tunica adventitia. 



The internal coat consists of a structureless elastic basement membrane, 

 the inner surface of which is covered by a layer of elongated spindle-shaped 

 endothelial cells. The middle coat consists of several layers of circularly 

 arranged, non-striated muscle-fibers, between which are networks of elastic 

 fibers. The external coat consists of bundles of connective tissue of the 

 white fibrous and yellow elastic varieties. Between the external and middle 

 coats there is an additional elastic membrane. In the small arteries there is 

 but a single layer of muscle-fibers. In the large arteries the elastic tissue 

 is very abundant, exceeding largely in amount the muscle-tissue. It is also 

 more closely and compactly arranged. The external coat is well developed 

 in the large arteries (Fig. 146). 



The presence in their walls of both elastic and contractile elements, 

 endows the arteries with the two properties of elasticity and contractility. 



Elasticity. The elasticity is best developed in the large arteries, though 

 it is also present in arteries of relatively small size. By virtue of the elas- 

 ticity, the arteries are capable of being distended and elongated and when 

 the distending force is removed of recoiling or retracting and returning to 

 their former condition. Thus the capacity of the aorta and carotid artery 

 of the rabbit can be increased four times and six times respectively by raising 

 the intra-arterial pressure from o to 200 mm. of mercury. The elasticity 

 permits of a wide variation in the amount of blood the arterial system can 

 hold between its minimum and maximum distension. The arteries thus 



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