218 



SPEEDS OF SOME PROCESSES IN BIOLOGICAL SYSTEMS 



0.5 1.0 1.5 



Tube radius (mm) ■»" 



Figure 8-13. Fluidity of Whole Blood 

 vs Glass Tube Radius. 



vessels, makes the whole study very complicated, easily subject to gross mis- 

 interpretation, and certainly needing more careful experimental definition. 



Circulation of Blood 



Description of the circulatory system is not our objective here. This was 

 done in 1628 by, at the time, the radical physician, Sir William Harvey, 

 whose description of experiments proving the continuous circulation of the 

 blood — from the heart through the systemic arterial and venous systems, 

 back to the heart, thence through the pulmonary arterial and venous sys- 

 tems, and again back to the heart — is still one of the classics of clarity in 

 medical literature. 



The pressure difference between aorta and vena cava across the pump, the 

 heart, is about 100 mm Hg, or 0.13 atm. Along the large arteries and veins 

 and in the main arterial and venous branches, the pressure gradient is small; 

 but because these vessels are of large radius, the flow rate is rapid. The 

 pressure gradient is at its peak along the capillaries and the arterioles; be- 

 cause they have very small radii, the flow there is slowest — just where it 

 should be the slowest— so that plenty of time exists for exchange to occur by 

 diffusion through the walls of arterioles and capillaries. Figure 8-14 illus- 



° O 



vena 

 aorta cava capillaries 



aorta 1 vena 



R cava 



, 1 » 



capillaries 



Figure 8-14. Relative Areas and Pressure Drops in Different Parts of the Human 



Circulatory System. 



