228 A TEXTBOOK OF PHYSIOLOGY 



box " of the arterial system. By the constriction of these arterioles 

 during mental or muscular activity, the blood-current through the 

 abdominal organs is diminished, and increased through the brain and 

 muscles; while by dilatation during rest and digestion the contrary 

 effect is produced. The constriction of the splanchnic vessels does 

 not sensibly diminish the capacity of the total vascular system, for 

 the veins relax. Thus big variations of arterial pressure, brought 

 about by constriction or dilatation of the arterial system, produce 

 little or 110 effect on the pressure in the great veins or pulmonary 

 circuit. On the other hand, the contraction of the abdominal 

 muscles, as we have seen, influences the diastolic or filling pressure of 

 the heart. 



Haemorrhage and Transfusion. The circulation may be aided by 

 the transfusion of salt solution (0-8 per cent.) or blood after severe 

 haemorrhage, or in states of surgical shock. Only the blood of man 

 must be used. The direct giving of blood by connecting the radial 

 artery of a relation to the median vein of a patient has been used as 

 a means of effecting restoration. Blood may be withdrawn from the 

 system slowly to the extent of 4 per cent., rapidly to the extent of 

 2 per cent., of the body weight without lowering the arterial pressure, 

 owing to the compensatory contraction of the anerioles and the rapid 

 absorption from the tissues into the blood. The beneficial effects 

 of the old treatment by bleeding were probably due to this latter 

 effect. The immune properties of the blood may be increased by the 

 passage of tissue juices into it. The withdrawal of the tissue lymph 

 excites extreme thirst and a great need for water after severe haemor- 

 rhage. About 75 per cent, by weight of the tissues, excluding fat 

 and bone, consists of water. The volume of tissue lymph is unknown, 

 but it must be considerable perhaps greater than that of the blood. 

 The lymphatics drain off the excess of fluid which transudes from 

 the capillaries, and finally return it to the vascular system. The 

 interchange between tissue, blood, and lymph, depends upon the 

 forces of the living cells, which are as yet far from complete elucidation. 



The vascular system confines the red corpuscles to its channels, 

 but cannot be regarded as a closed system ; for the fluid of the blood- 

 plasma transudes through the capillary wall into the tissue spaces, 

 and enters the lymphatics. Thus, if large quantities of Ringer's 

 solution be transfused into the circulatory system, it not only collects 

 in the capacious reservoirs of the veins and capillaries, especially in 

 the lungs, liver, and abdominal organs, but in the tissue spaces. Hence 

 the pressure in the vascular system cannot be raised by the injection 

 of enormous quantities of fluid. If the fluid part of blood be increased, 

 capillary transudation becomes greater, and the excess of fluid is 

 excreted from the kidneys and glands of the alimentary canal. If 

 the fluid part of the blood diminish, then fluid passes from the tissue 

 spaces into the blood, and the sensation of thirst arises, and more 

 drink is taken. 



