7.2 The human heart

7.2.4 Heart development

In the human embryo, the heart is the first functional organ to develop, and the pericardium the first structure to form. There are 5 stages to heart development:

1. The development of a heart tube, and early stages of the five regions of the adult heart structure.
   
   
2. The heart chambers begin to take form and the heart tube loops into an asymmetrical structure.
   
   
3. The formation of the two septa, the interatrial septum and the intraventricular septum (critical for correct positioning and functioning of heart valves). There are endocardial cushions (cells collected together as swellings), which fuse to form the septa of the four heart chambers.
   
   
4. The aorticopulmonary septum forms to separate the aorta and pulmonary arteries; later this will fuse with the intraventricular septum.
   
   
5. Heart completion with the formation of the heart valves.
   The embryonic heart begins beating at around 21 days after conception. It begins beating at a rate near to  the mother’s heart rate, (75–80 beats per minute or BPM). During the first month, the HER (embryonic heart rate) accelerates by about 100 BPM to peak at 165–185 BPM in early week seven. The acceleration is at a rate of 3.3 BPM per day, or about 10 BPM every three days. After week seven, it decelerates to about 152 BPM by week thirteen. After the week thirteen, deceleration slows to an average of about 145 BPM. Male and female heart rates do not differ before birth.