THE NUTRITION OF THE HEART 



falls towards zero, owing to the increased intrathoracic pressure- 

 obstructing the filling of the heart, while the vena cava pressure rises 

 a few millimetres of Hg (Fig. 66). The extent of the effect depends 

 greatly on the rigidity of the thorax and resisting-power of the 

 animal. In man, compression of the chest produces the same result; 

 loss of consciousness may be induced and convulsive spasms owing 

 to the production of acute cerebral anaemia. 



In the fatal crushes of panic-stricken crowds, death is produced 

 by compression of the thorax and circulatory failure. The women 

 and children with collapsible chests are the first to die, while the 

 men with most rigid chests escape. When a person is strangled, or 

 the larynx is blocked by some hard, impacted 

 mass of food too hastily swallowed, death is 

 hastened by the violent expiratory spasms, 

 which drive the blood out of the abdominal 

 veins into the heart and engorge the face 

 with venous blood. 



To sum up, the heart is filled in diastole 



by- 



1. The contraction of the heart produc- 

 ing a positive pressure in the veins (the 

 vis a tergo). 



2. The action of the respiratory pump 

 creating a negative pressure in the thorax 

 and a positive pressure in the abdomen. 



3. The contraction of the muscles of the 

 body generally impelling the blood onward 

 toward the heart. 



4. Change of posture and the action of 

 gravity. 



FIG. 66. COMPRESSION OF 

 THE THORAX (A-B). (Hill 

 and Barnard.) 



Compensvtory effect of 

 powerful inspirations, 

 alternating with forced 

 expiratory efforts (glot- 

 tis closed). Aorta mm. 

 Hg, vena cava mm. 

 water. 



The Systolic Output and the Work of the 

 Heart. To estimate the work of the heart, 

 it is necessary to know the mean pressure 

 (H), the velocity, of blood in the aorta (V), 

 and the volume of systolic output (Q). 



The velocity of blood in the aorta may 

 be obtained by one of the methods given (From Schafer's "Physiology.") 

 later (p. 203). 



Having obtained the velocity (V), the output can be reckoned if 

 the sectional area of the aorta (A) and the time of the cardiac cycle 

 (T) be known. To calculate half the diameter of the aorta, thus 



22 

 getting the radius (r). The sectional area (A) = irr 2 (ir=- ); 60 



divided by the number of heart-beats per minute gives T in seconds. 

 ThenQ = AVT. 



The output of the heart may be determined by means of the heart- 

 lung preparation (Fig. 67). Under an anaesthetic, and after injection 

 of hirudin to prevent the clotting of the blood, artificial respiration is 

 established, and the common carotid artery, the descending aorta and 



