PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



80 I 



Further, most trunk vessels show a gradual taper 

 through their length. Exceptions to this, that come to 

 mind, are a region of the descending thoracic aorta 

 and one of the carotid artery, which appear to be 

 more nearly true cylinders. 



Too detailed a particularization of the various 

 factors which give rise to frictional resistance may be 

 nonessential. The total effect of them all should be 

 measurable by a decrease in mean pressure, over a 

 whole cardiac cycle, from the upper aorta to the 

 peripheral arteries. Several studies have shown that 

 in the aorta such a decrease is so small as to be within 

 the error of measurement (42, 68). In fact, there is no 

 clear loss in mean pressure in man until the brachial 

 or femoral arteries are reached. 



Despite this small frictional dissipation of energy 

 attending propagation, there is a very clear differ- 

 ence between the pressure energy developed in systole 

 and in diastole. Except under rare circumstances, the 

 mean systolic pressure is greater than the mean 

 diastolic pressure. This excess of pressure energy 

 could denote an inability of the stretch of the extensi- 

 ble wall to keep pace with the force applied by cardiac 

 ejection, so that energy is stored in potential form in 

 the visco-elastic walls, or it could indicate a different 

 pressure-flow relationship in the large vessels being 

 filled during systole, and that which marks "drainage" 

 through the peripheral arterioles. 



To go from generalities to the specifics, an analysis 

 of aortic function could be focused upon three large 

 questions: /) What are the essential features of the 

 tension-length curves shown by the walls and the 

 derived pressure-volume curves, and to what extent 

 are these curves subject to physiological and patho- 

 logical change? 2) How does wall distention affect the 

 conduit properties of the vessels? j) What factors 

 influence the capacity of the arteries to serve as a 

 blood reservoir? 



MEASUREMENT OF AORTIC DISTENSIBILITY 



General Characteristics of the Tension-Length Curve 



Until quite recently, measurements of the extensi- 

 bility ot blood vessels were made on isolated tissues, 

 using two procedures (11, 15, 22, 37, 51, 62, 65-67, 

 76, 107, 118). Usually a ring (for circular stretching 

 to produce an increase in circumference) or a cut 

 strip (for measuring longitudinal change) was sub- 

 jected to weight loads, the changes in length being 

 recorded. In a few cases, volume was injected into a 



tied-off vessel, recording pressure. Any change in the 

 other dimension, e.g., a longitudinal change during 

 circular stretching, was either inadequately measured 

 or ignored. Although the specific techniques for in- 

 creasing load have varied, the stretches were made 

 rather slowly so that the vessel could approach, if 

 not attain, a stable length value, i.e., a "static" value. 

 Whether the load was applied in a continuously in- 

 creasing manner or stepwise, the data were generally 

 presented as a single tension-length curve covering 

 the whole physiological range. All workers agree that 

 such a curve is not linear, but shows a relatively 

 great extensibility at low tension settings and a 

 progressive wall stiffening as the load increases (fig. 

 1). This curve is therefore different from that shown 

 by metals, even those that obey Hooke's law over the 

 greatest part of their extension, or by rubber, where 

 the length change becomes relatively greater at high 

 tension levels (46). A rubber tube wrapped with a 

 fibrous jacket, such as a garden hose, shows the same 

 type of curve as does the aorta (14). Rings taken from 

 the aorta or from arteries appear to differ only quan- 

 titatively. Further, the longitudinal stretch curves are 

 qualitatively similar to those obtained with a circular 

 stretch. 



The tension given in figure 1 A is the weight load 

 divided by the product of the length of the ring of the 

 thoracic aorta and the wall thickness. This can be 

 converted to internal pressure by dividing by the 

 radius. To express pressure in the usual physiological 

 terms of mm Hg, the obtained value is divided by 1 3.5. 

 We can calculate the volume per unit length of vessel 

 as tit 2 . Both pressure-diameter and pressure-volume 

 curves show two inflections, to give the curve a some- 

 what sigmoid appearance (figs. \B and C). The 

 pressure level at which these inflections are seen varies 

 with different regions of the aorta. Hence both inflec- 

 tions are set at a higher pressure in the upper aorta 

 than in the lower, and the lower inflection may not 

 be seen at all in the arteries (46). There is no simple 

 formula which will fit this sigmoid type of curve, or 

 even that portion of greatest physiological significance. 

 At outset, it is clear that any comparison of vessel dis- 

 tensibility from time to time, or between animals, will 

 require the use of the same arterial region and the 

 same pressure span. 



There are several inadequately explained properties 

 of the isolated specimens which seriously affect the 

 recorded extensibility curves. First, as the vessel is 

 dissected out of the body, there is an immediate 

 shortening of its length and a tensing of its walls. 

 This is true whether the animal has just been killed, 



