The cardiovascular system is the first system to form and function in an embryo, and the heart is the first functional organ. In the mouse embryo, 50 founder cells make up the earliest heart precursor cells and are detectable as early as E6.5, on either side of the midline in the epiblast of early gastrula stage embryos. The human heart develops on day 18 or 19 following fertilization. In response to induction signals from the underlying endoderm, the mesoderm in the cardiogenic area forms the cardiogenic cords. A hollow center forms within the cords, giving rise to the endocardial tubes. With lateral folding of the embryo, the paired endocardial tubes approach each other and fuse into a single tube called the primitive heart tube (human: day 21 following fertilization; mouse: E8.0). On the day 22, the primitive heart tube develops into five distinct unpaired regions and begins to pump blood. Between days 23 and 28 (mouse E8.5-10.0), the primitive heart tube elongates unevenly, twisting and folding to form a U-shape and then an S-shape. As a result, the atria and ventricles of the future heart assume their final adult positions. Further heart development involves remodeling of the chambers and the formation of septa and valves to form a four-chambered heart.
The heart is part of the cardiovascular system. The heart is a muscular organ which undergoes repeated, rhythmic contractions (systole) and relaxations (diastole) periods, thereby pumping blood throughout the body. The right side of the heart pumps blood to the lungs, where the blood is then enriched by oxygen while carbon dioxide is removed from it. The left side of the heart pumps blood to the rest of the body. During diastole, the ventricles relax and fill with blood while the atria contract, forcing more blood into the ventricles. During systole, the ventricles contract and pump blood, while the atria relax and begin filling with blood again. The mitral valve regulates blood entry into the left ventricle, while the aortic valve regulates its exit. In the right ventricle, the Atrioventricular (tricuspid) valve serves as the inlet valve, and the semilunar (pulmonary) valve as the outlet valve.