THE CIRCULATION OF THE BLOOD 



lift a unit of weight, i.e., i gram or i kilogram, through a unit of height; i.e., 

 i centimeter or i meter, the work required being i gramcentimeter for small 

 units, and i kilogrammeter for large units, respectively. The average work 

 done by the heart at each contraction can be readily computed by multi- 

 plying the weight of blood expelled by the ventricles by the height through 

 which it would have to be lifted to overcome the resistance to its discharge 

 from the cavities into the arteries. 



The quantity of blood expelled and the pressure of the arteries can only 

 be estimated for man. But the computations from indirect observations 

 on other mammals indicate that the quantity of blood discharged from each 

 ventricle at a single contraction is from 80 to 100 c.c. The pressure of the 

 aorta, see page 208, is an average of, say, 150 mm. of mercury, or 200 cm. of 

 blood. The pressure in the pulmonary artery is much less, say, 30 mm. (20 

 to 40), of mercury or 40 cm. of blood. Collecting these facts, we have the 

 following computation: 



Total . 



90 c.c. 



240 cm. 



21,600 



This computation shows that each heart-beat expends 21,600 gramcenti- 

 meters (21.6 grammeters) of work. The amount of energy developed in the 

 contractions of the auricles may be ignored in this calculation, which is at 

 best only of relative value. Calculations based on the determinations of 

 Vierordt, also other earlier determinations, give much higher figures than 

 are presented here. 



The Properties of the Heart Muscle. It is evident that if we are 

 to arrive at any adequate explanation of the action of the heart, one of the 

 first questions that must be considered is, what are the fundamental 

 properties of heart muscle, as such? 



It has already been shown, page 62, that the muscular fibers of the 

 heart differ in structure from skeletal muscle fibers on the one hand, and 

 from unstriped muscle on the other, occupying an intermediate position 

 between the two varieties. The heart muscle, however, possesses a property 

 which is not possessed by skeletal muscle, or by unstriped muscle to such a 

 degree, namely, the property of contracting rhythmically. 



