The fact that the forces for propulsion of the circulating blood do in fact originate in the periphery, specifically in the microcirculation, is further substantiated by specific flow patterns, originating in the peripheral circulation, which continue relatively undisturbed as they course through the heart. Invariant flow patterns present strong evidence against the heart as a suction or pressure propulsion pump.
The ease of reproducibility and transparency of the zebrafish and chick embryos offer a unique model for the study of such patterns. A number of studies exist, which were performed with the aim of elucidating the effect of blood flow on early embryo’s heart development [1–3]. The most illustrative work is perhaps that of Hogers et al., who reproduced detailed circulation maps for chick embryo HH stages 12–17 (Figs. 4.1 and 4.2). Their flow diagrams reveal vitelline and intracardiac flow patterns which are consistent with the developmental changes of the yolk sac circulation. The first heart beat in the chick embryo occurs at around stage 9; however, before stage 12, the blood flow is still reported to be irregular with occasional backflow. From stage 12 onwards, a steady laminar flow can be observed by means of injected intravital dye, such as India ink, into the vitelline veins. Soon after the onset of the circulation, the blood courses along the marginal sinus of the vitelline membrane and flows towards the heart via the left and right anterior vitelline veins, symmetrically from each half of the embryonic disc, with respect to the embryo’s longitudinal axis. Despite heart pulsations, the blood can be observed to course in parallel streams through the heart to supply specific regions in the embryo proper. From stage 12 on, with the accelerated growth of the embryo in cranio-caudal direction and increased vascularization of the vitelline membrane, the original left-right symmetry of blood flow shifts to anterior-posterior, giving way to six “watershed” regions of the vitelline membrane which supply the embryo . Oxygen tensions in the specific parts of the intra- and extra-embryonic circulations have been reported, and it is possible that different embryonic organs are supplied with blood containing specific respiratory gas and nutrient compositions .