380 TEXT-BOOK OF PHYSIOLOGY 



veins, as seen on the back of the hand, will become distended with 

 blood. If now the arm be raised, the blood will flow rapidly toward 

 the heart, as shown by the rapid emptying of the veins. 

 Work Done by the Heart. The work which the left ventricle per- 

 forms at each contraction when it discharges its contained volume of blood 

 into the aorta is : 



1. To overcome the total resistance of the systemic vascular apparatus 



2. To impart velocity to the blood. 



The resistance may be expressed in terms of aortic pressure. The 

 pressure in the aorta has not been absolutely determined, though for many 

 reasons it may be assumed to be about 150 mm. Hg., or its equivalent, a 

 column of blood 1.93 meters in height. If the volume of blood which the 

 heart discharges is assumed to be 83 grams, the work done may be calculated 

 by multiplying the weight by the height to which it is raised: viz., 

 0.083 X 1.93 = 0.16019 kilogrammeter. 



The velocity of the blood in the aorta has been approximately estimated 

 at 0.5 meter per second. The work done in imparting this velocity to 83 

 grams is estimated by squaring the velocity and dividing by the accelerating 

 force of gravity ( 2 -j' 8i ) and multiplying the quotient by 0.083. The 

 value of the fraction given above represents the distance a body would have 

 to fall to acquire this velocity: viz., 0.0127 meter. The work done is there- 

 fore 0.083X0.0127, or 0.01054 kilogrammeter. 



The entire work of the left ventricle is the sum of these two amounts, 

 or 0.17073 kilogrammeter. Assuming that the heart beats 72 times per 

 minute, the work done daily would be 0.17073X72X60X24, or 17701.3 

 kilogrammeters. The right ventricle approximately performs one-third of 

 this amount of work in overcoming the resistance offered by the pulmonary 

 system and in imparting velocity to its contained volume of blood. The 

 work of the entire heart would therefore' be for the twenty-four hours about 

 23,600 kilogrammeters. 



THE NERVE MECHANISM OF THE VASCULAR APPARATUS 



By this expression is meant a combination of nerves and nerve-centers 

 by which the rate and force of the heart contractions and the contraction 

 of the arteriole muscles are maintained. It includes the cardiac nerves 

 (the cardio-accelerator and the cardio-inhibitor) and the vascular or vaso- 

 motor nerves (the vaso-augmentor or constrictor and the vaso-inhibitor or 

 dilatator nerves). The function of this mechanism is to maintain the 

 high blood-pressure characteristic of the arterial system, and to regulate 

 from moment to moment, the quantity of blood flowing into and out of organs 

 in accordance with their functional activities. The cardiac nerves have 

 been considered on pages 313-315. 



Arterial Tonus. The arteries, especially those in the peripheral region 

 of the arterial system, possess a well-defined layer of non-striated muscle- 

 fibers arranged in a circular direction or at right angles to the long axis of 

 the vessel. In the physiologic condition these arterioles are distended 

 beyond the natural condition by the side pressure of the blood flowing 

 through them, at the same time the muscle-fibers are in a state of tonic con- 

 traction, thus giving to the arteries a certain average caliber which permits 

 a definite volume of blood to flow through them in a given unit of time. To 

 this condition of the arterial wall the term tonus is applied. 



