158 Mechanical and Electrical Character of the Heartbeat /9 : 2 



blood contains antitoxins and phagocytic cells which help protect the 

 organism from external invaders. 



The vertebrate circulatory system, then, is a major internal trans- 

 portation line for chemical substances. The vessels into which the heart 

 pumps blood are named arteries. These branch into smaller and 

 smaller arteries; the smallest are called arterioles. The arterioles empty 

 into the capillaries. Here, most of the exchanges occur between the blood 

 and the surrounding tissues. The capillaries join to form venules, which 

 in turn join to form larger and larger veins leading back to the heart. 

 The circulatory system is not completely closed, however. Some fluid 

 leaves the capillaries, passing into the tissue spaces; it is then called 

 lymph. The lymph filters back slowly through several nodes, finally 

 entering the venous portion of the circulatory system. 



2. Blood Pressures and Velocities 



Before the action of the heart is examined, the flow of the blood through 

 the arteries and veins will be discussed briefly. The flow of the blood 

 can be described in terms of its linear velocity v and its pressure p. The 

 velocity v is, in general, a function both of time and of the point in space 

 at which it is measured. The pressure p is the force per unit area of the 

 fluid. It is a scalar quantity; that is, p is independent of the orientation 

 of the areas used to define it. The zero point for pressure is somewhat 

 arbitrary. So-called "gauge pressure" is the difference between the 

 absolute pressure and the atmospheric pressure. Absolute pressure is 

 measured relative to a zero of no net external forces on the system. 



Pressure is a stress and has the dimensions of force per unit area. In 

 the mks system it is measured in newtons/m 2 . Instead of absolute units, 

 pressure is often measured in terms of the height of a column of liquid 

 which it will support. Thus, it may be measured in terms of meters of 

 mercury or meters of water. Some convenient reference numbers to 

 remember are: 



1 atmosphere = 1.0 x 10 5 newtons/m 2 

 1 meter of H 2 = 9.8 x 10 3 newtons/m 2 

 1 meter of Hg = 1.33 x 10 5 newtons/m 2 



Any convenient height units may be used. The most frequent ones in 

 describing the circulatory system are mm of Hg. 



Besides pressure and velocity, another fundamental property of a 

 fluid is its density p. For all purposes in this chapter, the blood may 

 be considered as incompressible. Its density is approximately that of 

 water. 



