THE TRANSPORTATION SYSTEM 



495 



Before studying the blood itself, let us ex- 

 amine the machinery of circulation. 



THE VESSELS 



These consist of a closed system of tubes, 

 large and small, which convey the blood 

 continuously within a circuit. The large 

 vessels carrying blood away from the heart 

 are known as arteries; the large vessels 

 bringing blood back to the heart, known as 

 veins. The tiny intermediate vessels are 

 capillaries. The last are the most important, 

 since it is through their walls that the real 

 work of the circulatory system goes on, and 

 fof this reason they will be considered first. 



The capillaries 



These tubules, often so small that blood 

 cells pass through single file, have walls 

 composed of a single layer of cells. These 

 same endothelial cells continue as the lining 

 of the larger vessels ( Fig. 19-4 ) . The capil- 

 laries form a network throughout all the 

 tissues of the body, so vast and complicated 

 that a pin prick anywhere usually punc- 

 tures one, causing blood to ooze out. Be- 

 cause of their thin walls, dissolved sub- 

 stances in the blood can readily pass into 

 the tissues, and conversely, waste sub- 

 stances in the tissues can readily diffuse into 

 the blood and be carried away. In an active 

 animal such as man it is essential that this 

 exchange be a rapid one. Even in the rela- 

 tively sluggish frog, whose capillaries can 

 be easily observed in the web of its foot, 

 blood cells race through the capillaries in 

 a fraction of a second and yet this is suf- 

 ficient time for the important processes of 

 exchange to occur. 



The arteries and veins 



On either end of the capillaries are larger 

 tubules, venules (little veins) at the end 

 toward the heart and arterioles (little ar- 

 teries) at the end coming from the heart. 

 These venules and arterioles become larger 



Fig. 19-5. Cross-section of the human umbilical cord to 

 show the construction of an artery and vein. The 

 largest opening is a vein. Note the thin wall as com- 

 pared to that of the two smaller arteries just above 

 it. Veins and arteries are similar in structure except 

 that the muscle layer in the latter is heavier. 



and larger as the heart is approached, 

 where they are designated as veins and 

 arteries. There is no structural difference 

 between the two except that the walls of 

 the arteries are thicker and stronger than 

 those of the veins (Fig. 19-5). Both are 

 composed of three layers of tissue. The 

 inner endothelium layer is the same tissue 

 that makes up the whole of the capillary 

 wall. As a matter of fact, it lines the entire 

 circulatory system. The outside layer of 

 both arteries and veins is made up of a 

 tough connective tissue so that it readily 

 stretches to permit an increase in diameter, 

 but does not easily rend. Between the two 

 is a smooth muscle layer which regulates 

 the diameter of the blood vessel, thereby 

 controlling the amount of blood flowing 

 through it. These muscles (vasoconstric- 

 tors) are under the influence of the auto- 

 nomic nervous system and the state of their 

 contraction depends on tlie need of various 

 tissues of the body for food and oxygen. For 

 instance, following a meal the muscles in 

 the walls of the blood vessels going to and 

 from the viscera relax, allowing more blood 

 to flow to and from these organs. On the 

 other hand, during violent exercise they 

 contract in this region but relax in the mus- 

 cles and respiratory system. By such regula- 

 tion the various parts of the body are sup- 

 plied with the proper amount of blood at 

 all times. 



