BLOOD AND CIRCULATION 553 



ward by the closure of a system of valves. One with three cusps, known 

 as the tricuspid valve, lies between the right atrium and ventricle; one 

 with two cusps, the bicuspid valve, between the left chambers. These 

 valves operate automatically as pressures change, opening when atrial 

 pressure is greater than ventricular, closing when ventricular pressure 

 is greater. Tendinous cords extend from the free margins of the cusps to 

 the ventricular wall, and prevent them from turning into the atria 

 during the powerful ventricular contractions. When the ventricles relax, 

 blood in the pulmonary artery and aorta, which is under pressure, tends 

 to back up into them. This closes the pocket-shaped semilunar valves at 

 the base of these vessels which prevent blood from returning to the 

 ventricles. Abnormalities in the structure of the valves produced con- 

 genitally or by disease organisms may prevent their closing properly. 

 Blood then leaks back during diastole; the leaking blood is heard as a 

 heart "murmur." 



Cardiac muscle has an inherent capacity for beating, and the hearts 

 of vertebrates, if properly cultured, will continue to beat rhythmically 

 when excised from the body. Each contraction is initiated in the sino- 

 atrial node, or "pacemaker"-a node of specialized cardiac muscle 

 (Purkinje fibers) located in that part of the wall of the right atrium into 

 which the primitive sinus venosus is incorporated. 1 he impulse spreads 

 through a network of Purkinje fibers to all parts of the atria, and to an 

 atrioventricular node from which the impulse continues to all parts of 

 the ventricles. The factors that stimulate the sinoatrial node and cause 

 it to send out impulses to other parts of the heart are not completely 

 understood. Apparently the leakage of positively charged sodium ions, 

 which are abundant outside the cells, through the plasma membrane 

 and into the cells of the node, and a temporary reversal of the electrical 

 polarity of their membranes, is involved. A similar phenomenon occurs, 

 as we shall see later, in the initiation and transmission of the nerve 

 impulse. The sinoatrial node has a shorter refractory period than other 

 cardiac muscle; thus it recovers more rapidly after each beat and is 

 ready to act again before the rest of the heart has recovered. 



Though the heart has an inherent rhythm, its rate of contraction 

 and the volume of blood pumped per stroke can be regulated by a 

 number of extrinsic factors so as to adjust the heart output to body 

 requirements. Nervous pathways are present for many cardiac reflexes. 

 Motor nerves that increase or decrease the heart rate go to the heart 

 from centers in the brain, and sensory impulses from many parts of the 

 body reach these centers. For example, sensory fibers in the right atrium 

 are stimulated by the increase in the pressure of the venous blood re- 

 turning to the heart which occurs during exercise. They initiate a reflex 

 that increases the heart rate. If the arterial pressure becomes too high, 

 sensory fibers from the arch of the aorta reflexly reduce the heart rate. 

 The increased pressure and more rapid return of venous blood 

 during exercise stretches the heart musculature. This causes it to contract 

 with greater force, and to send out the greater volume of blood received 

 during each period of atrial diastole. Within physiologic limits, the 

 greater the tension on cardiac (or any other) muscle, the more powerful 



