Pacific Science (1982), vol. 36, no. 3 © 1983 by the University of Hawaii Press. All rights reserved The Biom~chanics of the Arteries of Nautilus, Nototodarus, and Sepia! JOHN M. GOSLINE2 and ROBERT E. SHADWICK2 ABSTRACT: The mechanical properties of the dorsal aorta of three cepha­ lopod mollusks, Nautilus pompilius, Nototodarus sloani, and Sepia latimanus, were investigated by in vitro inflations ofisolated arterial segments. As expected, all three arteries exhibit nonlinear, J -shaped stress-extension curves, and all are highly extensible in the circumferential direction. Differences in longitudinal extensibility appear to be correlated to specific features ofthe tissue architecture. The squid, Nototodarus, and to a lesser extent the cuttlefish, Sepia, arteries are reinforced longitudinally with a dense layer of longitudinally oriented elastic fibers. Analysis of the form of the incremental wall stiffness data for Nautilus and Nototodarus suggests that the in vivo blood pressures for these animals fall in the ranges 20-60 cm H 2 0 and 100-200 cm H 2 0, respectively. Nautilus has a thin­ walled, low-pressure arterial system that is in keeping with its relatively limited locomotory capabilities. Nototodarus has a high-pressure, thick-walled circu­ lation that is required to support the high-speed, aerobic locomotion generally common in squid. Analysis ofpressure wave velocities for these arteries indicates that the Nautilus circulatory system contains a true Windkessel whereas it appears possible that wave propogation effects may make a relatively minor contribution to the hemodynamics of Nototodarus. THE ROLE OF ELASTIC ARTERIES in the circu­ is constructed must be quite low. However, lation of vertebrates has been known for the cylindrical geometry ofthe artery imposes many years. At its simplest, the rubberlike some very important constraints on the me­ elasticity ofthe arteries provides a mechanism chanical properties ofthe artery wall material. in which some of the energy released in the If the vessel is to inflate in a uniform, stable pulsatile contractions of the heart is stored. manner along its entire length and not "bal­ This stored energy can then be used in main­ loon out" in forming aneurisms, then the stiff­ taining the flow of blood to the tissues dur­ ness of the artery wall must increase sharply ing the refilling phase of the cardiac cycle with radial expansion over the physiological (McDonald 1974). In order for a system range ofextensions (Gosline 1980, Wainwright of elastic arteries to function efficiently, the et al. 1976). Thus, arteries always show non­ arteries must be relatively easy to inflate, so linear, J -shaped stress-extension curves, and the heart will not have to work against a large in higher vertebrates the incremental stiffness systemic pressure. Therefore, the modulus of or incremental modulus of elasticity (Einc) elasticity ofthe material from which the artery will increase with roughly the fifth power of radius in the physiological range (Bergel 1961). This dramatic shift in elastic properties I This research was conducted as part of the Alpha Helix Cephalopod Expedition to the Republic of the is achieved by the parallel arrangement of a Philippines, supported by National Science Foundation low-modulus, rubbery material (elastin) with grant PCM 77-16269 to J. Arnold. This study was also a very stifffiber (collagen) (Roach and Burton supported in part by Natural Sciences and Engineering 1957). At low extensions (i.e., at low pres­ Research Council of Canada grant 67-6934 to J. M. Gosline. sures) the collagen fibers are somewhat coiled 2 University of British Columbia, Department of Zo­ and do not act to resist the internal pressure. ology, Vancouver, British Columbia, Canada V6T 2A9. The elastic properties of the composite are 283 ,.~ <~, "," ~,..-H " " ... " ',w V~JI-'''''' """.'r~'"'' ,_'."..,.. 'v,," ..' ,T", ,......,,-,.., .,-, '" '.'" ...~ • ,,'" "' _,,, 'O'~" • "~ 284 PACIFIC SCIENCE, Volume 36, July 1982 dominated by the elastin and the modulus is MATERIALS AND METHODS low [of the order of 10 5 N m-z (Newtons per square meter)]. As pressure increases and the Experimental animals were purchased alive artery is inflated, the collagen fibers become and in good condition from fishermen on straightened and begin to resist the internal the east coast of the island of Negros in the pressure. At the upper end of the physiolog­ southern Philippines. Arterial tissues sampled ical range of pressures the stiffness has in­ for histology were dissected from the dorsal creased by roughly an order of magnitude to aorta just anterior to the systemic heart and about 106 N m-z, and this dramatic increase were placed directly in Bouin's fixative. The in stiffness occurs over a range of radial ex­ tissues were dehydrated, embedded in wax, pansion ofthe order of25%. Clearly, the pre­ and sectioned according to standard histol­ cise design of the arterial wall material is an ogical techniques. The sections were stained essential component in the vertebrate circu­ using the Gomori aldehyde-fuchsin method latory system. (Cameron and Steele 1959). This staining Until very recently, virtually nothing was procedure has been shown to preferentially known about the mechanics ofthe circulation stain the elastic fibers in the octopus aorta, in any invertebrate. Cephalopod mollusks leaving the muscle cells and the collagen fibers have fairly complex, high-pressure, closed cir­ essentially unstained. culatory systems, and it is likely that elastic Arterial tissue isolated for mechanical tests arteries may function in the cephalopods was quickly dissected from freshly killed in much the same manner as they do in the animals and placed in seawater at ambient vertebrates. The available blood pressure temperatures (appox. 30°C). Nautilus artery data (Bourne, Redmond, and Johansen 1978; samples were normally 4-6 cm long, and Johansen and Martin 1962) suggest that elas­ squid samples were usually 2-3 cm long. In tic arteries are present in the octopus and all cases, as much ofthe dorsal aorta as could in Nautilus. Shadwick (1978, 1980) and be easily isolated was used. Shadwick and Gosline (1981) have recently In inflation tests, the proximal end (toward shown that the dorsal aorta of Octopus dof­ the heart) ofthe sample was fitted over a piece leini is indeed elastic and that the animal ofpolyethylene tubing ofthe appropriate size makes use of the system's elasticity in main­ and tied in place with surgical silk. The distal taining blood flow during the entire cardiac end and any small branch arteries were tied cycle. Octopus arterial tissue exhibits a J­ off with fine surgical silk. The polyethylene shaped stress-extension curve that is matched tubing was attached to either a seawater-filled to the normal range of blood pressures in the or mercury-filled manometer, and the luminal animal. In addition, the arterial wall material pressure of the isolated arterial segment was is constructed from a protein rubber (not elas­ measured as the height of the fluid reservoir tin, but a previously undescribed material) above the specimen. The specimen was placed arranged in parallel with collagen fibers. untethered under seawater in a wax-filled petri Thus, it seems that the basic biophysical dish on the stage of a Wild M-3 stereomicro­ requirements ofa closed, pulsatile circulatory scope, and a Wild filar micrometer eyepiece system have led the cephalopods to evolve an (15 x ) was used to measure the longitudinal arterial tree that is functionally analogous to and radial expansion ofthe artery under pres­ the vertebrate system. This conclusion, how­ sure. Radial expansion was followed by ob­ ever, is based on observations ofa very seden­ serving the change in the outside diameter at tary animal, the octopus, and this animal may a single recognizable point along the artery. not correctly represent the more active cepha­ Longitudinal expansion was followed by ob­ lopods. This paper, therefore, presents a me­ serving the change in separation of two small chanical analysis of the major arteries of a (approx. 5-llm wide) metal slivers that were squid (Nototodarus sloani), a cuttlefish (Sepia inserted through the very outer layers of the latimanus), and Nautilus pompilius. artery wall. These markers were placed about "r.,...,~" .. ,,,,.., .-'.- ...',.... '.7<" -", ., ......... " .. " ,,.., '" .~ ,~ '" r,' • ,., ~ .,~ '" , Biomechanics of Cephalopod Arteries-GosLINE AND SHADWICK 285 1 mm on either side of the point at which the was possible to carry out tensile tests in the radial expansion measurements were taken. circumferential direction. This was done by The micrometer eyepiece gave excellent preci­ cutting ring samples of the artery (approx. sion and reproducibility, and we estimate that 1 cm long) and mounting the rings over stiff the expansion measurements are accurate to metal loops that were attached to the tensile better than about 2%. Wall thickness was de­ test apparatus. In the case of both longi­ termined by cutting a small ring sample ofthe tudinal and circumferential force-extension artery from the area where the radial and tests, the rate of extension was kept as low as longitudinal expansion measurements had possible (usually less than lO%/min) so that been taken. The ring sample was turned on its the measurements would at least approximate side, and 15-20 measurements of wall thick­ equilibrium values. ness were taken at various points around the In both inflation and force-extension mea­ ring. The average value was taken to be the surements, the extension of the sample is true wall thickness of the uninflated artery. expressed in terms of longitudinal and cir­ The wall thickness of the inflated artery was cumferential extension ratios, A.L = L/Lo and calculated from the radial and longitudinal A.c = R/Ro, respectively (where L is length, expansion values by assuming that the volume R is radius, and Lo and Ro are the initial of the arterial wall material remained con­ values of length and radius). Wall stress for stant. the inflation tests was determined as follows Before measurements were taken on an (Wainwright et al.