78 HUMAN PHYSIOLOGY 



muscles of the vessels in such a manner that, normally, each 

 part of the body contains the amount of blood that it needs. 

 The more active a certain part of the body is, trie more 

 dilated are its blood vessels and the greater the quantity of 

 blood they contain. Simultaneously with the dilation in an 

 active part, there is a constriction in the resting parts of the 

 body. 



When the body is at rest, the vessels in the abdomen and 

 the thorax contain more than one-half of all the blood. 

 During digestion, the quantity of blood in the intestine is 

 larger than during fasting. During work, the blood vessels 

 of the muscles are better filled with blood than during rest, 

 simultaneously the abdominal vessels innervated by the 

 splanchnic are constricted. 



In the suprarenal glands a substance is formed which increases 

 the tonus of the muscles of the vessels. It acts directly upon the 

 muscle fibres (see Chapter XI). 



Small losses of blood are compensated by general con- 

 striction of the vessels. But for great loss of blood, amount- 

 ing to over one-half of all the blood, this compensation is 

 not sufficient; the blood pressure sinks much, the valves do 

 not close completely and the circulation ceases. Death by 

 loss of blood in such cases does not take place because of 

 lack of any constituent of the blood, e.g. the haemoglobin, 

 but because the vessels are not sufficiently filled and this 

 entails disturbances in the circulation. If the lost blood is 

 replaced by an indifferent fluid (0.9^ NaCl), the circulation 

 is resumed [transfusion]. If the loss of blood amounts to 

 more than two-thirds of all the blood, the haemoglobin 

 present is not sufficient for respiration, and, notwithstanding 

 that the circulation may be repaired, death takes place 

 because of lack of oxygen. In such a case, life can be saved 

 only by the transfusion of human blood. Blood from other 

 animals cannot be used because of globucidal action. 



