544 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION 



considered unresolv^ed in intact animals where record- 

 ings are not sufficiently precise for this kind of distinc- 

 tion. This is not to say that occurrence of changes in 

 mvocardial distensibllity has been effectively excluded 

 in the intact animal or man. For example, the increase 

 in ventricular dimensions with little or no sign of 

 increased filling pressure on reclining certainly sug- 

 gests that distensibllity has changed (see fig. 4). 

 Measurements of diastolic filling pressure in human 

 subjects generally fail to include the extracardiac 

 distending pressure because it is rarely convenient 

 to record intrapleural pressure. Holmgren (18) 

 reviewed the literature and reported his own data to 

 support the conclusion that both right and left atrial 

 pressure are essentially unchanged during the transi- 

 tion from rest to exercise. Sjostrand (44) reported that 

 the intrathoracic blood content neither increases nor 

 decreases during work. Thus, shifts in blood from 

 various portions of the venous system toward the 

 heart need not be postulated. Marshall et al. (25) 

 recently demonstrated that computed values for 

 pulmonary blood volume would err consistently 

 toward excessive values if the indicator dilution 

 sample were withdrawn too slowly, it being possible 

 to obtain values up to 100 per cent too large. In their 

 experiments this factor was carefully controlled, and 

 their values for pulmonary blood volume were either 

 unchanged or increased less than 20 per cent under 

 conditions of severe exertion. Kjellberg el al. (21) 

 questioned the existence anywhere in the systemic 

 circulation of blood "depots" in the usual meaning 

 of this term. When Guntheroth (16) continuously 

 recorded various dimensions of the liver and spleen 

 in dogs, he found no consistent changes in the size 

 of these organs during exercise. If there is no signifi- 

 cant shift of blood from one part of the circulation 

 to another, then the i)lood flow must accelerate so 

 smootlily and uniformly throughout the entire circula- 

 tion that little or no dislocation of blood occurs. 



Ventricular Dimensions During Exertion 



The available evidence clearly indicates that 

 diastolic dimensions of the heart are not consistently 

 increased and arc often decreased during exercise. 

 Directly recorded ventricular dimensions in dogs 

 generally change little during treadmill exercise. 

 Additional evidence to support this conclusion has 

 stemmed from studies on both dogs (33, 42, 51 ) and 

 men (21, 45). Indeed there is now little reason to 

 doubt that diastolic size of the heart commonly 

 remains unchanged or actually diminished during 



exercise. Systolic ejection may increase as a result of 

 increased myocardial contractility owing to sympa- 

 thetic discharge. Contrary to previous concepts, in- 

 creased stroke volume is not a prominent factor in 

 achieving increased cardiac output during a wide 

 range of muscular exercise in most healthy dogs or 

 men. 



Constancy of Stroke \olume During Exercise 



For many years the concept has prevailed that 

 augmented stroke volume and accelerated heart rate 

 contribute about equally to an increase in cardiac 

 output during exercise, so that oxygen extraction in 

 the tissues, indicated by the arteriovenous oxygen 

 difference, is only slightly increased. The basic idea 

 stemmed from extensive studies in which indirect 

 Fick procedures were employed and the subjects were 

 often trained athletes. These indirect methods, which 

 invohed use of carbon dio.xide or foreign gases 

 (acetylene, nitrous oxide, or ethyl iodide) to estimate 

 the composition of mixed venous blood, have been 

 largely abandoned in favor of the more accurate 

 direct Fick method (catheterization) and indicator 

 dilution techniques. As these newer methods have 

 been applied to many normal but untrained persons 

 it has become clear that, in contrast to the older 

 view, increased oxvgen delivery to the tissues during 

 exertion is accomplished primarily by tachycardia 

 and greater extraction of oxvgen from the blood (fig. 



7)- ^ '~ ' 



The severity of exertion is generally expressed in 

 terms of the total oxygen consumption by the body. 

 This practice is reasonable since the oxygen delivery 

 to the tissues depends upon the combined effects of 

 stroke volume and heart rate (cardiac output) and 

 the arteriovenous oxygen difference (oxygen extrac- 

 tion from blood). Part D of figure 7 schematically 

 summarizes data obtained in ten studies in which 

 various investigators applied the direct Fick method 

 or indicator dilution techniques to normal persons 

 (33). These data include control values at rest and the 

 changes during various levels of exercise in both ei-ect 

 and recumbent positions. Over a very wide range of 

 exercise the stroke volume did not increase progres- 

 sively (fig. 7). Sjostrand (46) reached the same con- 

 clusion from his studies. However, stroke volume 

 did increase during the transition from standing 

 quietly to mild or moderate exercise (9), and an 

 additional increase in stroke volume occurred during 

 exercise so severe that the oxygen consumption had 

 reached a plateau (i. e., at 6 to 9 mph on a treadmill 



