Circulation of Body Fluids 553 



perature; the rate of increase is not a linear but rather a logarithmic func- 

 tion of temperature, and temperature characteristics have been obtained for 

 the hearts of many species. ^^ There is no uniformity in temperature charac- 

 teristics of hearts, but several insects and crustaceans give Arrhenius /x values 

 of 12,200 calories, whereas several molluscs give values of 11,200.^'^^ 



Heart rates, then, are directly related to rates of metabolism, and any al- 

 teration in metabolism is reflected in an alteration in heart rate. In addition, 

 the ability to increase or decrease heart rate, and with it the cardiac output, 

 is necessary for the efficient functioning of any cardiovascular system. 



Electrocardiograms. The conduction of the excitation from the pace- 

 maker to and within the muscle of the heart is manifest by an action poten- 

 tial wave. Because of the synchronization of many discharging units the 

 potentials which can be recorded are large, even at some distance from the 

 heart. At the same time the geometrical relations in chambered hearts are 

 such that a complex wave is recorded. The electrocardiogram (ECG) is dif- 

 ferent in hearts with muscular conduction from the ECG in hearts with 

 nervous conduction. 



The electrocardiogram is similar in vertebrates whether ventricular con- 

 duction is in Purkinje tissue (mammals and birds) or in muscle (poikilo- 

 therms). Typically the ECG consists of a series of slow waves, upward (nega- 

 tive) deflections called P, R, and T, and downward (positive) deflections Q 

 and S (Fig. 199). The complex wave form of the ECG cannot be inter- 



1 it III 



•UJ 



^4*" 



Fig. 199. Electrocardiograms from man, leads 1, U and III. From Katz."* 



preted simply as propagation along a polarized membrane, as in nerve or 

 skeletal muscle conduction. Rather, the ECG appears to represent a wave 

 front spreading over the heart. An isolated sinus from the eel may show 

 rhythmic simple diphasic waves (Fig.. 200, B).^'' In A-V block or isolated 

 auricle and sinus the auricle gives the P wave, often followed by a re- 

 covery wave sometimes called X.^^ There is no doubt that the P wave cor- 

 responds to conduction in the auricles. The PQ interval is delay at the au- 

 riculoventricular junction, and the QRS complex corresponds to conduction 

 in the ventricles. Early theories postulated a spread of excitation in Pur- 

 kinje tissue down the ventricular septum and then in muscle fibers from 

 endocardium out to epicardium, the different waves representing different 

 times of arrival in the various parts of the ventricles. However, the com- 

 plex is similar in frogs and fish, which lack a septum. Records from the endo- 

 cardium and epicardium, and from different electrical leads (across the heart, 

 on one side, etc.) under conditions of local changes in temperature and ex- 



