WORKING OF THE HEART AND BLOOD-VESSELS. 233 



is never so great as to obliterate their cavities; while the ven- 

 tricles contain no blood at the end of their systole. 



The auricles also govern to a certain extent the amount 

 of work done by the ventricles. These latter contract with 

 more than sufficient force to completely drive out all the 

 blood contained in them. If the auricles contract more 

 powerfully and empty themselves more completely at any 

 given time, the ventricles will contain more blood at the com- 

 mencement of their systole, and will have pumped out more 

 at its end. Now, as we shall see in Chapter XVIII, the con- 

 traction of the auricles is under the control of the nervous 

 system, and through the auricles the whole work of the 

 heart. In fact the ventricles represent the brute force con- 

 cerned in maintaining the circulation, while the auricles are 

 part of a highly-developed co-ordinating mechanism, by 

 which the rate of the blood-flow is governed according to 

 the needs of the whole Body at the time. 



The Work Done by the Heart. This can be calculated 

 with approximate correctness. At each systole each ven- 

 tricle sends out the same quantity of blood a^out 180 grams 

 (6.3 ounces); the pressure se^errfreffl by the blood in the aorta 

 against the semilunar jp^Tves, and which the ventricle has to 

 overcome, is about that which would be exerted on the same 

 surface by a column of mercury 200 millimeters (8 inches) 

 high. The left ventricle therefore drives out, seve-hty times 

 in a minute, 180 grams (6.3 ounces) of blood against this 

 pressure. Since the specific gravity of mercury is 12.5 and 

 that of blood may for practical purposes be taken as 1, the 

 work of each stroke of the ventricle is equivalent to raising 

 180 grams (6.3 ounces) of blood 200 X 12.5 = 2500 millim. 

 (8.2 feet); or one gram 450 meters (one ounce 51.66 feet); 

 or one kilogram 0.45 meter (one Ib. 3.23 feet). Work is 

 measured by the amount of energy needed to raise a definite 

 weight a given distance against gravity at the earth's surface, 

 the unit, called a kilogr ammeter, being either that necessary 

 to raise one kilogram one meter, or, called a foot-pound, that 

 necessary to raise one pound one foot. Expressed thus the 

 work of the left ventricle in one minute, when the heart's 

 rate is seventy strokes in that time, is 0.45 X 70 31 50 kilo- 

 grammeters (3.23 x 70 = 226.1 foot-pounds) ; in one hour it 

 is 31.50 X 60 = 1890 kilogrammeters (226.1 X 60 = 13,566 

 foot-pounds); and in twenty-four hours 1890 X 24 = 45,360 



