260 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



approach the heart the total bed of the stream is becoming continually 

 smaller, but never so small as in the corresponding arteries ; a vein 

 is always twice the size, often more than twice the size of the cor- 

 responding artery. Velocity of flow varies inversely with the bed 

 of the stream, the velocity is therefore greatest in the aorta, slows 

 down in the small arteries, and becomes slowest of all in the capil- 

 laries where the total bed is widest ; we may compare the combined 

 capillaries to a vast lake into which the arterial river flows. On 

 leaving the capillaries, the blood, in traversing the veins once more, 

 becomes accelerated because the bed of the stream becomes narrower, 

 but its speed in a vein is only about half that in the corresponding 

 artery because the bed is twice as great. 



In connection with the variation in the bed of the stream we 

 must also consider the question of resistance. If the increase in 

 sectional area took place without division of the stream into numerous 

 branches, the main effect would be to lower resistance to the flow of 

 fluid ; but the friction-lowering effect of increased area is much more 

 than counterbalanced by the increased surface of the numerous 

 branches, and there is increased friction on this account. The resist- 

 ance of the capillaries would be large even for a stream of water, 

 and when we consider that the blood is much more viscid than water, 

 we see the effect must be much greater. The resistance to the flow 

 of fluid along a small tube is in inverse proportion to the fourth 

 power of the diameter, i.e., if the diameter of the tube is halved, the 

 resistance is increased sixteen-fold. Between the arteries and the 

 capillaries are the small arteries or arterioles ; these vessels are 

 always in a state of moderate or tonic constriction ; they may roughly 

 be compared to narrow inlets into the wide capillary lake. The 

 main resistance to the passage of blood through the tissues is situated 

 in the arterioles, and not in the capillaries ; this is usually spoken 

 of as the peripheral resistance, and it is variable by alterations in the 

 calibre of the arterioles, their muscular tissue being under the control 

 of nerves which are termed vaso-motor. 



The main resistance is in the arterioles and not in the capillaries 

 for the following reason : each individual capillary is small, and its 

 resistance therefore great, but their number is so immense, and the 

 total bed so large that the resultant resistance offered is com- 

 paratively small. This is well brought out by a comparison of the 

 velocity in the two cases ; in the arterioles the velocity has to be 

 high in order to supply with blood the large capillary areas spring- 

 ing from them; in the capillaries, as we have already seen, the 

 velocity is low. 



After this general account of the main features of the circulation, 

 we can now pass to a detailed description of the various points 

 raised. 



