70 HUMAN PHYSIOLOGY 



from the aorta or pulmonary artery, through the arteries, 

 capillaries, and veins, to the heart. The blood must conse- 

 quently flow in this direction. By this movement of the 

 blood, the difference in pressure is normally not entirely 

 equalized, because by each succeeding ventricular systole 

 the difference in pressure is increased. 



In the aorta, the blood pressure undergoes variations of 

 about 150 mm Hg; in the larger arteries, about 1 10 120 mm ; 

 in the capillaries, 24-54 mm; in the veins, only a few mm, 

 indeed in the large veins in and near the thorax it may be a 

 negative pressure of a few mm, i.e. it is less than the atmos- 

 pheric pressure. On cutting such a large vein, no blood 

 flows from the vessel, but air enters it. The cause of this 

 negative pressure in the veins is the negative pressure exist- 

 ing in the thoracic cavity which is increased during inspira- 

 tion (see Chapter V). The blood pressure in the pulmonary 

 artery is about 50 mm Hg. 



The blood pressure in the arteries undergoes periodic 

 variations caused by the action of the heart ; these variations 

 are called the pulse. Each time that the ventricular systole 

 sends blood into the aorta and pulmonary artery, the 

 pressure in these vessels is suddenly increased ; after this, 

 the streaming of the blood to the capillaries causes a diminu- 

 tion in pressure. This periodic variation in pressure spreads 

 itself as a wave throughout the whole arterial system. 



The pulsatory variations in pressure are the largest in the 

 aorta, where they amount to half of the average pressure ; 

 they become smaller in the peripheral arteries. In the 

 capillaries and the veins there is normally no pulse. 



The rate of the pulse wave (not to be confounded with the rate 

 of the blood flow) can be found by determining the time elapsing 

 between the beginning of the cardiac impulse and the appearance 

 of the pulse in a peripheral artery. The rate is about six metres 

 in one second; it depends upon the tension and the elasticity of 

 the arterial walls. The length of the pulse wave is about i. 5 m. 



If a lever is placed upon an artery so that it is moved by the 

 pulsating artery, and if this movement is transferred to and mag- 

 nified by a writing lever, a curve, the pulse curve, or sphygmogram, 



