234 ESSENTIALS OF PHYSIOLOGY. 



compensating action of the vaso-motor centre for the effects of gravity 

 is also inefficient in anaesthetised persons. 



Owing to the influence of gravity, the arterial pressure in the 

 femoral artery of an individual in the erect position is much higher 

 than that in the brachial artery. The constriction of the arterioles of 

 the legs, however, is so great that the pressure in the capillaries and 

 veins of the leg and foot is no higher than that in the hands. The 

 flow of blood from the foot and leg back to the heart against the force 

 of gravity is greatly assisted, and indeed made possible, by muscular 

 movement : each muscular movement squeezes the blood along the 

 veins towards the heart, and the valves prevent any reflux. In persons 

 who are compelled to stand still for any length of time, or in whom 

 the valves are defective, the veins tend to become dilated and varicose. 



The Effect of Haemorrhage. Any considerable loss of blood from 

 the body lessens the amount present in the arterial system, and the 

 output of the heart at each beat decreases ; the arterial pressure falls, 

 and the supply of blood to the brain becomes inadequate. The vaso- 

 motor centre is at once stimulated, causing increased constriction of 

 the arterioles ; at the same time fluid passes from the tissues into the 

 blood, and the arterial pressure rapidly regains its normal level. 

 After a very severe haemorrhage these compensatory mechanisms are 

 inadequate, and the blood pressure remains low. 



The Influence of Adrenalin. The structure and functions of the 

 suprarenal glands are dealt with on p. 400, but it is necessary to 

 mention at this point their influence on the circulation. These glands 

 produce a substance, adrenalin, which can be extracted from them and 

 obtained in a pure form. A minute amount of adrenalin (e.g. O'Ol 

 or 0'02 mgr.), injected into a vein, stimulates the nerve endings of all 

 the fibres of the sympathetic system, including those which supply the 

 arterioles, and causes extreme vaso-constriction of all the arterioles 

 except the coronary vessels, which are dilated, and the cerebral vessels, 

 which are unaffected ; and if the vagus nerves have been divided a 

 huge rise of blood pressure is produced. The suprarenal glands receive 

 fibres from the splanchnic nerves, and when a splanchnic nerve is 

 stimulated, some of the adrenalin present in the suprarenal gland passes 

 into the suprarenal vein and so into the blood stream, and gives rise 

 to the effects just described. It is clear, therefore, that whenever a< 

 splanchnic nerve is stimulated the ensuing rise of blood pressure is 

 partly due to the increased peripheral resistance brought about by the 

 direct action of the splanchnic nerve on the abdominal blood-vessels, 

 and is partly caused by the constriction of arterioles all over the body 

 by the adrenalin set free into the blood stream. The influence of 



