[Functional changes in embryonal heart]

Intracardiac pressure in the developing chick was measured throughout the embryonic and fetal period. Amplitude and heart rate were used to estimate cardiac output and power of the heart. In addition, the cardiac performance per milligram dry weight was calculated. Initially, the cardiac tube displays a peristaltic movement which results in a continuous pressure within the vascular lumen. Switch to rhythmic atrial and ventricular contractions occurs between the 6th and the 9th developmental day. This change is directly correlated with differentiation of the conduction system. The simultaneous formation of the cardiac septa and valves is of only minor importance for this change in function. Between the 4.5th and 5.5th day, functional development is found to halt temporarily. This could be due to the enormous growth of the extra-embryonic vascular network. During this period, the risk of damage to the embryo by endogenous or exogenous factors is highest. In the human embryo, this developmental stage corresponds to the period between the 28th and 34th day. After this change in function the heart is able to build up a reserve of power, since it has less work to perform per unit weight.