328 - Multicellular Animals, Especially Man 



RIGHT COMMON 

 CAROTID ARTERY - 



RIGHT SUBCLAVIAN ft 



ARTERY— x H 



RIGHT SUPERIOR 

 VENA CAVA 



RIGHT AURICLE 



RIGHT VENTRICLE 



HEPATIC VEINS 



ADRENAL 

 GLAND 



INNOMINATE 

 ARTERY 



LEFT COMMON 

 CAROTID ARTERY 



LEFT SUBCLAVIAN 

 ARTERY 



ARCH OF AORTA 



PULMONARY 

 ARTERY 



PULMONARY VEINS 



LEFT AURICLE 



LEFT VENTRICLE 



Fig. 17-10. Principal blood vessels of man and their 

 connections with the heart chambers. Ventral view. 

 Be sure to differentiate between the systemic and the 

 pulmonary circuits (see text, p. 327). 



fairly frequent intervals within the veins, 

 but not within the arteries. 



The same three layers of tissue are present 

 in the walls of an artery and vein, although 

 these layers are thicker and more clearly de- 

 fined in the artery. The inner layer (Fig. 17- 

 11) is the endothelium, a smooth lining of 

 squamous epithelial cells, arranged more or 

 less like the stones of a pavement. This 

 smooth endothelial lining offers a minimal 

 resistance to the flow of blood through the 

 vessels. The middle layer is a sheath of vis- 

 ceral muscle, flanked both inside and out by 

 elastic connective tissue; and the outer layer 

 is composed of loose connective tissue. The 

 fibers of the muscle sheath tend to encircle 

 the artery, permitting the artery to change 

 its caliber according to the needs of the tis- 

 sues at the particular time. During exercise, 

 for example, the arteries to the muscles and 

 lungs dilate, thus providing a more abun- 

 dant blood supply to these parts during the 

 period of extra activity. 



Unless the arterial walls were elastic, the 

 circulation could not be maintained on an 

 efficient basis. Each time the ventricle con- 

 tracts, it quickly empties its full content of 

 blood into the arterial system. This blood 

 cannot flow instantaneously through the very 



ever, there are many exceptions to this gen- 

 eral rule (see later). 



Structure of the Blood Vessels. Arteries and 

 veins display a similar structure, although 

 the arterial wall is thicker and stronger (Fig. 

 17-1 1). Blood pressure is at a maximum as the 

 blood is forced from the heart; and the pres- 

 sure remains relatively high throughout the 

 whole arterial system. Consequently if any 

 artery is cut, the blood tends to spurt out. As 

 blood passes through the capillary network, 

 however, much of its pressure is dissipated, 

 owing to the frictional resistance offered by 

 these microscopic channels. Consequently 

 blood reaching the venous system is under a 

 relatively low pressure, and if a vein is cut, 

 the bleeding is less forceful. Valves, which 

 prevent a backflow of blood, are found at 



VISCERAL MUSCLE 



ENDOTHELIUM 



ELASTIC CONNECTIVE TISSUE 



LUMEN 



LOOSE CONNECTIVE 

 TISSUE 





Wmow I 



mw 







ARTERY 



VEIN 



Fig. 17-11. Microscopic structure of a small artery 

 and vein. Cross-sectional view, semidiagrammatic. 



