498 



ORGAN SYSTEMS OF MAN 



force to flow into the ventricle, hence the 

 muscular walls of the auricles are thin. 

 The ventricle fills completely, then sud- 

 denly contracts with sufficient force to 

 exceed the pressure in the pulmonary 

 artery, thus opening the semilunar valves 

 which allow the blood to pass to the 

 lung capillaries. The blood then returns 

 to the left auricle by way of the four pul- 

 monary veins. The path taken by blood 

 passing through the left heart is similar to 

 that of the right, although the force with 

 which it leaves is greater because the blood 

 must go throughout the body as a result of 

 the impetus received from the muscle of the 

 left ventricle. It passes over the semilunar 

 valves of the dorsal aorta and out to the 

 capillaries of the body, eventually return- 

 ing to the right auricle again through the 

 vena cavas. This complete circuit is some- 

 times called tlie cardiac cycle. 



How the heart is nourished 



It is obvious that such an active muscle 

 as the heart must receive an ample supply 

 of oxygen and food continuously. Even 

 though tons of blood flow through the heart 

 chambers, none of it reaches the heart mus- 

 cle because there are no direct connections 

 to the muscle from the chambers. The heart 

 has a system of blood vessels of its own, 

 however, called the coronary circulation. 

 This rather strange name was given to it 

 because the vessels reminded early anato- 

 mists of a crown, since they encircle the top 

 part of the heart. The two coronary arteries 

 leave the dorsal aorta just above the semi- 

 lunar valves (Fig. 19-8) and pass through- 

 out the heart muscle, ultimately becoming 

 capillaries. Blood returns through a system 

 of veins (coronary veins) which coalesce 

 and eventually empty into the right auricle 

 via the coronary sinus. Approximately one- 

 fourth of the total blood pumped out by 

 the left ventricle passes through the coro- 

 nary circulation. This is an extremely im- 

 portant system because the slightest impair- 

 ment, such as a tiny clot of blood lodging in 



it, profoundly affects heart action. This is 

 one form of heart attack. Sudden death re- 

 sults when the stoppage of blood is suf- 

 ficient to hinder seriously the contraction 

 of the muscle. 



Listening to the heart during health and 

 illness led more alert physicians many cen- 

 turies ago to recognize that certain sounds 

 could be associated with heart malfunction- 

 ing. The characteristic "lubb-dup," as heard 

 with the stethoscope, is caused by the clos- 

 ing of the various heart valves to prevent 

 the blood from rushing back into the auri- 

 cles from the ventricles and back into the 

 ventricles from the great arteries. The low 

 "lubb" sound is due to the closing of the 

 mitral and tricuspid valves during the ini- 

 tial stages of ventricular systole, whereas 

 the sharp "dup" which follows the first 

 sound very closely is caused by the sudden 

 closing of the semilunar valves in the ar- 

 teries. There is a brief pause during which 

 time the heart rests and the auricles fill. 

 Any abnormalities in the closing of these 

 valves can be detected by various "murmur- 

 ing" sounds that are easily recognized by 

 the trained ear of the physician. During 

 some kinds of infectious diseases, and per- 

 haps allergies, the edges of the valves be- 

 come inflamed and injured; upon healing 

 they frequently do not fit as well as they 

 once did, resulting in the so-called "heart 

 murmur." Such an injured heart can, how- 

 ever, increase its size and output so that 

 even with faulty valves it may still be ade- 

 quate. Such a heart is often spoken of as a 

 "compensated heart" and within limits may 

 function normally for many years. 



The work of the heart is almost unbeliev- 

 able when it is compared to that accom- 

 plished by other muscles of the body. Start- 

 ing long before birth, indeed, when the 

 embryo is no more than 25 days old, it con- 

 tinues its ceaseless contractions until old 

 age; during all this time it does not falter, 

 and its only rest comes between systoles. 

 Beating at 70 times per minute, over 100,- 

 000 per day, and nearly 40,000,000 per year, 



