KeywordsPeristaltic pumpValveless embryo heartCardiac jellyLiebau pump principleDynamic suction pumpContraction waveBlood flow reversalEjection fraction
Rhythmical contractions of the embryonic heart have traditionally been assumed to be the mechanical cause for impelling the blood around the circuit. The proposed nature of these contractions is a peristaltic wave which originates in sinus venosus and follows the direction of blood toward the outlet of the tubular heart. The time-sequenced nature of the propulsive myocardial peristalsis in the chick embryo has been subject of investigation ever since the myogenic contraction of the heart was first described in the nineteenth century (for review see ref. [1, 2]).
In fact, the term “peristaltoid” rather than peristaltic was specifically chosen by Patten and Kramer because of the absence of definite longitudinal and circular muscle characteristic of hollow organs such as the gut or the ureters which have a differential dilating effect on the wall [3, 4]. With the help of lumen silhouette tracings from motion pictures of the early chick heart, Patten and Kramer documented passive dilation of the lumen and “heaping up” of plasma ahead of contraction waves. They further commented that, “Although there is unmistakably fluid in the heart for a considerable time previous to the beginning of circulation we were not able to satisfy ourselves that this passive dilation was appreciable until just about the time that the blood began to be propelled through the heart. Its significance, of course, lies in the well-known heightened responsiveness of stretched muscle” .
No doubt Patten and Kramer here described the movement of plasma from the yolk sac to the heart before the function of the sinus node pacemaker. It is possible that during this stage, the heart does contract in response to being stretched just like other hollow organs. To that extent, the term “peristaltic” is certainly justified. Despite the fact that the peristaltic nature of the valveless tube heart clearly differs from its mature counterpart, the actions of the two were considered to be analogous in the sense that they both affect fluid propulsion.
On the basis of their high-resolution imaging data analysis of the early zebrafish heart, Forouhar et al. showed in their landmark study that the valveless zebrafish tube heart does not function like a peristaltic pump for the following reasons :