HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 15. The relationship of the 

 abundance of transverse (circular) 

 muscles in human veins {smooth line) 

 to the venous pressure in the erect 

 posture {dotted line). The numbers 

 indicate the same veins as in fig. 14. 

 [v. Kiigelgen (48).] 



ICO 

 % 

 SO 



150 

 cm 



flachenanteil der Quermuskulalur 

 Wahrer Innendruck 



Obere Extremitat { Thorax i- 



16 18 20 

 Bauchhohle 



£. tT- ZU CO ./I 



-I iUntcre Extremiiat 



abdomen, and neck are not under this hydrostatic 

 stress and have less circular muscle tissue. 



The mechanical properties of the veins are similar 

 to those of the arteries. Smaller veins show different 

 pressure-volume diagrams, depending upon the state 

 of contraction of their smooth muscles, as do those of 

 muscular arteries [Alexander (2)]. The only differ- 

 ence between the diagrams of the muscular arteries 

 and the veins is that the diagram of the veins is 

 located at much lower pressures. This indicates that 

 the elasticity of small veins depends to a high degree 

 on smooth muscle. The large veins, like the vena cava, 

 give a pressure-volume diagram more like that of 

 elastic arteries [Blomer (14)]. It shows an S-shape, 

 like the aorta, with an inflexion point at the low 

 pressure of about 7 mm Hg. This S-shape does not 

 depend on the activity of the smooth muscles. The 

 main support of the wall tension of the large veins, 

 at least above 7 mm Hg, is the collagen tissue rather 

 than the elastic tissue, as is the case for elastic arteries, 

 since the amount of collagen exceeds by far the 

 amount of elastic tissue in the vein wall. The relatively 

 high distensibility of the vena cava, in spite of the 

 collagen fibers, may depend on a gradual recruit- 

 ment of these fibers, as shown in figure 9, phases 3 

 and 4, or it may be due to a reorientation of the net- 

 work formed by the collagen fibers in the venous 

 wall [see v. Kiigelgen (49)]. 



The most striking difference between arteries and 

 veins are that the veins possess valves and are securely 

 embedded in the surrounding tissue (33, 51, 53, 78), 



whereas the arteries never have valves and they are 

 loosely connected with the surrounding tissue. 



The valves of the veins are folds of the intima. They 

 consist of collagen and elastic fibers but not of smooth 

 muscles. Around the vein at the base of the valve is a 

 thickened band of collagen fiber (51). Usually two 

 valves face each other [Bardeleben (8)]. The leg veins 

 are best guarded by valves. Very small veins are said 

 to be free of valves [Klotz (45)], as are the venae 

 cavae. The valves minimize postural hydrostatic 

 pressure changes in the leg veins, protecting the 

 capillaries and the veins themselves from unphysio- 

 logical pressures. 



The action of the skeletal muscles, compressing, 

 stretching, and releasing the veins, and even arterial 

 pulsation (53, 78), cause periodic changes in venous 

 capacity. Since the valves open toward the heart, 

 these movements cause the veins to act as pumps, 

 promoting return of blood to the heart and main- 

 taining low capillary pressures (for further details 

 see Chapter 32). 



The numbers of valves in the veins depend very 

 much on the age of the individual. Many valves 

 degenerate with aging. Bardeleben (8) has ascer- 

 tained, for example, that the greater saphenous vein 

 of a child has, on the average, 1 3.6 valves, whereas 

 that of an adult has only 10.7 valves. Klotz (45) has 

 even found up to 70 per cent of atrophied valves at 

 age 70. The first sign of degeneration is a functional 

 insufficiency of the valves permitting leaking at higher 

 pressures when the vein is distended, although they 



