How Your Heart is Formed

Heart: The Engine of Life Educational Resources—The Franklin Institute

A human fetus forms in a very precise order.

While the heart is one of the first organs to begin development, it takes several weeks before it resembles the four-chambered structure that we all know.

During the fetal heart's developmental stages, the heart actually takes on several distinct appearances. These heart structures resemble other animal hearts.

When the human heart first begins to form, it looks like a simple tube, much like a fish’s heart. However, rapid growth soon causes the tube to bend and twist backward, beginning the formation of the familiar shape.

The second phase of heart development creates two chambers. At this time, the heart resembles a frog heart.

The third phase begins when the two atria (the top chambers of the heart) become completely separate and the ventricles (the bottom chambers) are just beginning to separate. During this three-chambered phase, the fetal heart may appear similar to a snake or a turtle heart.

Finally, the ventricles separate completely. The final four-chambered heart structure distinguishes the human heart from other living creatures.

Despite all the work is has to do, the average adult-size heart weighs only 8-12 ounces. Located in the middle of the chest behind the breastbone, between the lungs, the fully-developed heart rests in a moistened chamber called the pericardial cavity that is surrounded by the ribcage. The diaphragm, a tough layer of muscle, lies below. As a result, the heart is well protected.

How Fast Does it Go?

The heart's rate of pumping oxygen-rich blood is fastest in infancy, about 120 beats per minute. As a child grows, the heart rate slows down. A seven-year old child’s heart beats about 90 times per minute. By the age of 18, the heart rate stabilizes to about 60- 100 beats per minute.