VENOUS RETURN 



I I 23 



In quiet standing, blood from the legs returns to 

 the heart only with great difficulty, and the pressures 

 in the veins of the lower limb rise to values equal to 

 the weight of blood between the lower limbs and the 

 heart, that is, to as much as 90 mm Hg. However, 

 during walking, the venous return from the lower 

 limbs will be so satisfactory that venous pressures in 

 the feet may be as low as 20 to 25 mm Hg (156). In 

 the absence of an active venous pump, a person can 

 develop such high pressures in the lower part of the 

 body when he stands that he actually loses as much 

 as 15 to 20 per cent of his blood volume in less than 

 one-half hour, thereby in many instances provoking 

 fainting. 



EFFECT OF VENOUS COLLAPSE ON VENOUS RETURN. The 



phenomenon of "venous collapse" is based on the 

 simple fact that it is impossible to suck fluid through 

 a collapsible tube. Since the heart is located in the 

 thoracic cavity where the pressure is normally ap- 

 proximately — 5 mm Hg and since the right atrial 

 pressure often is also in the range of —2 to —3 mm 

 Hg, suction frequently is applied to the central veins. 

 This is particularly true of the veins entering from 

 above downward when a person is in an upright po- 

 sition, because, under these conditions, the negative 

 hydrostatic pressure of the blood flowing downward 

 toward the heart adds to the negative pressures al- 



ready in the chest, thus causing essentially complete 

 collapse of the veins in the neck. However, this, too, 

 is a very old story known by almost every student of 

 physiology (34, 54-58, 112, 114, 170), and it can be 

 summarized by simply saying that any factor which 

 makes the right atrial pressure more negative than 

 normal does not cause a significant increase in venous 

 return. That is, the venous return will be as great 

 when the right atrial pressure is approximately —2 

 mm Hg as it will be should the right atrial pressure 

 fall to as low as — 1 5 mm Hg. For instance, when a 

 person breathes air from a chamber that is under 

 negative pressure, this negative pressure is trans- 

 mitted through his lungs to the chambers of his heart. 

 Yet, breathing against the negative pressure does not 

 increase the venous return to values above normal — 

 all because the veins collapse any time there is an 

 attempt to suck blood from the periphery. 



Venous collapse also occurs whenever pressure is 

 applied to the outside of the veins. This very fre- 

 quently occurs in the case of elevated abdominal 

 pressure (24, 88). Figure 28 illustrates the effect of 

 intra-abdominal pressure on the pressure along the 

 inside of the inferior vena cava. In these studies, a 

 catheter was introduced upward from the femoral 

 vein until it entered the right atrium, showing that 

 the venous pressure all along the extent of the intra- 

 abdominal veins was always slightly greater than 



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fig. 28. Effect of increased abdomi- 

 nal pressure (AP) on pressures meas- 

 ured from the tip of a catheter inserted 

 up the femoral vein and along the vena 

 cava until it entered the right atrium. 

 [From Guyton & Adkins (88).] 



2 6 10 14 18 22 26 30 34 38 42 46 50 

 LENGTH OF CATHETER INTRODUCED IN CM 



