9 : 3/ Mechanical and Electrical Character of the Heartbeat 161 



this pressure is very small, it is conveniently measured in mm of water. 

 In a normal adult human, the venous gauge pressure at the heart is 

 about —40 mm of H 2 0. 



The arteries and veins have similar flow rates but verv different 

 pressures. Accordingly, both have about the same diameter (0.5-12.5 

 mm i.d.), but the arterial walls are thick and elastic, whereas the venous 

 walls are very thin. The larger pressures in the arteries make reverse 

 flow unlikely; valves limit reverse flow in the veins. 



The capillaries are the location of most exchanges of gases, meta- 

 bolites, and metabolic products. They are thin walled and small in 

 diameter. A red blood cell, 8 /* in diameter, distorts the shape of the 

 capillary as it passes through. At the capillary walls, the excess gauge 

 pressure, osmotic forces, and active transport all combine to promote 

 exchanges between the blood stream and the surrounding tissues. 



3. The Vertebrate Heart 



In warm-blooded vertebrates, the heart keeps pumping for the entire life 

 of the organism. If the heart stops even for a short time, the animal 

 dies. This continuous activity is regulated by both the nervous and the 

 endocrine systems. However, even without these regulatory influences, 

 the heart maintains its rhythmic beat. In cold-blooded animals, the 

 temperature also influences the heart rate. At close to freezing tempera- 

 tures, their heart rate slows almost to zero. 



The heart of the cold-blooded vertebrates is simpler than the mam- 

 malian heart. Most fishes and amphibians have a heart made up of a 

 series of chambers as shown in Figure 3. The first, which receives the 

 blood from the veins, is called the sinus venosus. It is the pacemaker and 

 originates the heartbeat. 



The reptilian heart, also shown in Figure 3, is more specialized. 

 Instead of one auricle, there are two. One receives blood from the lungs 

 only and the other from the remainder of the body. This system is more 

 efficient in aerating the blood than is that of the amphibians and fishes. 

 The sinus venosus does not exist as a separate chamber, but its homolog 

 persists as a sino-auricular (s-a) node on the wall of the auricle serving 

 the body proper. 



The mammalian heart is illustrated in diagrammatic form in Figure 4. 

 It consists of four chambers : two auricles and two ventricles. The blood 

 from all the body except the lungs enters the right auricle. It is forced 

 from there into the right ventricle, then into the lungs and back to the 

 left auricle. From there it is forced into the left ventricle and finally 

 through the aorta to all arteries of the body except those going to the 



