Embryo Heart is Not a Peristaltic Pump




(1)
Professor of Anesthesiology, Albany Medical College, Albany, NY, USA

 



Keywords

Peristaltic 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” [3].


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.


Depending on the range of pressure within which they operate, the biological peristaltic pumps, such as hollow organs and even hearts, are considered positive displacement, low or high impedance pumps [5]. They function by enclosing fluid in a chamber (organ lumen) and expel it by reducing the chamber volume in the process of muscular contraction. The mechanical equivalent of such a pump is a roller pump of the type used during cardiopulmonary bypass or in hemodialysis machines. It is a characteristic of these pumps that the fluid bolus displaced is equal to the amount of wall deformation produced by external compression (roller) on the tubing which generates a flow. The volume flow is a function of the pressure head generated by the pump and is proportional to the frequency of compressions (Fig. 6.1).

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Fig. 6.1

Schematic diagram of a roller pump (a) and pump function diagram (b). At constant roller speed, the pump operates at a given (control) pressure and output (red). At increased roller speed, the pump operates at higher working pressure and gives higher output (blue). When the pump works at increased pressure (load) at control speed, the output remains at control levels (green). (Adapted from ref. [6], used with permission of Springer Nature)


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 [7]:


(1) The maximal RBC velocity recorded in the lumen of the early tube heart exceeds the contraction wave velocity along the heart tube wall; in other words, the blood travels faster through the heart than the contractile wave which is supposed to propel it. This contradicts the peristaltic model in which, as mentioned, the peak blood velocity matches the traveling wave velocity. (2) An increase in embryo’s heart rate (caused by an increase of temperature of the embryo mount) did not increase the velocity of blood flow. This violates the above criteria for the peristaltic pump where a linear relationship exists between the flow volume and frequency of compression. (3) A synchronous expansion of cardiac jelly was noted at the side of atrial contraction, which rather than propelling, would inhibit the flow of blood during the systole (contraction), and finally, (4) in addition to the contraction wave traveling in the direction of the moving blood, they also recorded a phase-shift wave, reflected from the opposite direction, creating a possible suction effect (Fig. 6.2).

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May 1, 2020 | Posted by in CARDIOLOGY | Comments Off on Embryo Heart is Not a Peristaltic Pump
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