THE OSMOTIC FORCE 35 



the earth at sea level (980 dynes/g). Our goal now will be to show what 

 problem is introduced by the simple facts that man's head is 6 ft away from 

 his feet and he walks upright. 



Two fluids circulate independently through the body: blood and lymph. 

 Both move via a canal system. The former is a closed system driven by a 

 pump; the latter is driven by muscle movement along the canals. 



Because a column of air 6 ft high, of 1 in. 2 cross-section, has negligible 

 weight, there is no difference in the weight force of air at the head and feet. 

 However, the weight of a column of water (or blood) of the same dimen- 

 sions is 2.8 lb, quite an appreciable fraction (12 per cent) of 14.8 psi of at- 

 mospheric pressure. In terms of the mercury manometer (1 atm, 14.8 psi, 

 supports a column of mercury 30 in., or 760 mm high, remember?) this 

 extra pressure at the feet due to the weight of the blood is 120 mm over 760. 

 Hence the pump must force blood along against a 120-mm back-pressure. 

 Add to this a small resistance to flow, mostly in the large arteries and veins 

 in which the total area of flow is relatively small and the flow rate high. 



The heart is a pulse pump. It distends, collecting a volume of blood 

 freshly oxygenated in the lungs, closes its inlet valves, and contracts, forcing 

 the blood out through the aorta. The aorta, like the rest of the circulating 

 system, has elastic walls, which, in turn, distend under the hydraulic force 

 impressed by the contracting heart muscles. The pressure-rise in the aorta, 

 for a rather typical stroke-volume of 30 cc, may vary from 30 to 150 mm Hg 

 pressure depending upon the reaction of the walls of the arterial system to 

 the pulse and the physical position of the person. In the highly elastic walls 

 of the young and healthy the value will be small; as the tissues become 

 harder with age, or disease, it will rise. 



The maximum value is called the systolic pressure, and is due directly to 

 the factors outlined. It is usually of the order of 120 mm. The minimum 

 value — reached after the walls of the aorta, distended by the stroke from the 

 heart, have relaxed to the original diameter, having forced the blood along 

 the artery-capillary system — is called the diastolic pressure. Typically in a 

 healthy, adult male it is ~80 mm Hg. The mean value is about 100. The 

 pulse period is about 1 sec. Because the veins in the legs are more easily 

 distended than the arteries, most of the venous blood is stored there and re- 

 called when needed. The center of gravity is thus lowered, and storage re- 

 quires less work. 



THE OSMOTIC FORCE 



What Is It? 



One of the most important forces at work in the living system is the os- 

 motic (literally, Greek: "impulse") one. It is the force which drives the dif- 

 fusion of water, nothing more, and is a property of a solution just as are 



