CHAPTER XXVI 



THE PRESSURE AND VELOCITY OP THE BLOOD IN 

 THE VEINS 



THE most striking difference between the structure of an artery 

 and its venae comites is a decrease of elastic tissue in the veins, together 

 with an increase of white connective tissue. The veins are tubes with 

 muscular walls, which not only fall together, but contract when empty, 

 and under slight pressure expand to their full capacity. Beyond 

 this point, the walls, on account of the quantity of connective tissue 

 entering into their structure can extend but little. 



The resistance to a breaking-strain on the part of the veins is very 

 great. It requires a higher pressure to rupture a vein than the cor- 

 responding artery. 



If by external compression the various outlets be blocked, the 

 pressure rises in the vein to the full pressure in the artery. For this 

 reason the veins must be strong enough to bear any such increased 

 strain. There is, however, another need for strength of veins, and 

 that is that they may be able to bear the strain which may arise from 

 external violence. The superficial veins are endowed with more 

 muscular and elastic tissue than those deeper, while those veins which 

 run in the muscles and in the bones, and are protected from violence 

 and supported by firm structures, possess no muscular elements. 

 When exposed, a superficial vein contracts on mechanical stimu- 

 lation and on cooling, while it may be made to dilate on applying 

 warmth. 



Pressure in the Venous System. In the active animal the venous 

 pressure varies according to the hydrostatic pressure of the column 

 of blood above the point of measurement and with the action of 

 the muscles which express the blood onwards towards the heart. 

 In the horizontal position, when these factors are almost elimin- 

 ated, the pressure in the large veins is found to be equal to a few 

 centimetres of blood, or about 5 mm. Hg. When a cannula is 

 pushed down the jugular vein past the valve till its opening lies in 

 the vena cava superior, the lateral pressure of this vein is ob- 

 tained. It may become negative during inspiration. The negative 

 pressure which occurs in the right auricle on each cardiac oscillation 

 is estimated at 2 to 3 mm. Hg, and may become 5 to 8 mm. Hg 

 during inspiration. 



In the sheep, with the animal in the horizontal posture and im- 



225 15 



