PRESSURE AND VELOCITY OF BLOOD IN VEINS 227 



The valves in the veins allow the blood to be forced only towards 

 the heart. The pumping action of walking can be observed on the 

 veins of the back of the foot. After standing still for a time, the 

 veins become prominent. The pressure is considerable, as can be 

 .gauged from the feel. After taking a few steps, they are emptied, 

 squeezed between the skin and muscle. 



Numerous anastomoses exist between the veins, so that, if the 

 flow of blood be obstructed in one direction, it readily finds a passage 

 in another. 



The venous circulation is impeded by (1) a lessening of the heart 

 power; (2) cardiac valvular effects, such as incompetence or narrowing 

 of the valvular orifices; (3) obstruction to the filling of the heart, as 

 in cases of pericardial effusion; (4) obstruction of the pulmonary 

 circulation, as by coughing and by pleuritic effusion. The results of 

 venous congestion are less efficient circulation, a dusky appearance 

 of the skin, a fall of cutaneous temperature, and an effusion of the 

 fluid into the tissue spaces, producing oedema or dropsy. This last 

 effect is not due, as has been supposed, to increased capillary pres- 

 sure producing increased transudation, for no such increase in venous 

 and capillary pressure is found under the conditions. It is due to 

 the altered nutrition of the tissues, which results from the deficient 

 circulation. The products of katabolism which collect within them 

 increase the osmotic properties of the tissues. 



If for any reason the left ventricle fail to maintain its full systolic 

 output, it ceases to receive the full auricular input, and in consequence 

 the pulmonary vessels congest. This tells back on the right heart, 

 and the right ventricle is unable to empty itself into the congested 

 pulmonary vessels, and this in its turn leads to venous congestion. 

 The final result of any obstruction thus is a pooling of the blood in 

 the venous cistern. Dyspnoea results from cardiac insufficiency. It 

 is excited by the increased venosity of the blood acting on the respira- 

 tory centre. Both the excess of carbon dioxide and deficiency of 

 oxygen increase the acidity of the blood, and this excites the centre. 

 The increased respiratory movements aid the circulation. 



The vascular system is so constructed that considerable changes 

 of pressure may be brought about on the arterial side without any 

 (or scarcely any) alteration of the pressures in the venous or pulmonary 

 sections of the circulatory system. A high-pressure main (the arteries) 

 runs to all the organs, and this is supplied with taps; for by means 

 of the vaso-motor nerves, which control the diameter of the arterioles, 

 the stream can be turned on here or there and any part flushed with 

 the blood, while the supply to the remaining parts is kept under control. 

 Normally, the sum of the resistances which at any moment oppose 

 the outflow through the capillaries is maintained at the same value, 

 for the vascular system is so co-ordinated by the nervous system that 

 the dilatation of the arterioles in any one organ is compensated for 

 by constriction in another. Thus the arterial pressure remains con- 

 stant, except at times of great muscular activity. 



The great splanchnic system of arterioles acts as " the rejis^ance 



