2l6 ANATOMY AND PHYSIOLOGY 



vaso-motor nerves. By vaso-dilators the arterioles are enlarged, 

 allowing a free passage of blood ; by the vaso-constrictors they are 

 made smaller, thus cutting down the quantity of blood in a given 

 area. A certain balance of tone in the blood-vessel system is neces- 

 sary to proper action of the heart and to the process of osmosis. 



The moving blood current exerts a certain amount of pressure 

 upon the vessel walls; this (in health) is normal blood pressure 

 estimated by the experienced touch, but far more accurately by 

 instruments designed for the purpose, whereby the pressure in an 

 accessible artery is recorded upon a graduated scale. The instru- 

 ment is the sphygmomanometer of which there are various designs. 



Blood pressure is increased by an increase in the effort of the 

 heart, or in the resistance in front of it, or in the volume of the 

 blood. On the other hand, pressure is diminished by diminished 

 heart force, diminished resistance, diminished volume. 



Arterial pressureis illustrated by the force of the stream spurting 

 from the mouth of a severed artery. In a small vessel this is an in- 

 terrupted force, giving the appearance of throbbing or beating in the 

 stream. Venous pressure causes the blood to well up in a wound 

 rapidly but with a steady flow. Capillary blood simply oozes. 



The cause of blood pressure is the resistance in front of the 

 stream resulting from the constantly diminishing size of the arter- 

 ies, reacting to the attempt to drive the blood through them. 



Clinical note. In the processes which lead to arterio-sclerosis the middle 

 coat of the artery is affected, the loss of elasticity being the first element of 

 failure. Long-continued high pressure is a common cause of arterio-sclerosis 

 since it calls for increasing action of the muscle and elastic fibers in the 

 tunica media and at last tires them out. The elasticity of the vessel wall is 

 gone; it can no longer preserve normal tone; connective tissue thickening 

 follows and stiffens the artery. Later, a uniform hardness may be caused and 

 brittleness, of which a common result is rupture and hemorrhage. This may 

 occur at any place, often in the brain, often in the muscles of the lower 

 extremities. 



Arterio-sclerosis also interferes with the interchange between the blood 

 and lymph and the normal metabolism in the tissues. 



A physiologic or normal high pressure is caused temporarily by 

 a quickening of the circulation, as in vigorous muscle exercise: 

 by nervous excitement, as fright, anger, joy, etc. 



Pathologic or abnormal high pressure may be caused by poisons, 

 either swallowed, or retained in the system from tissue waste, as 



