Early development
		The developing circulation

Key Points

1. Heart cells first form a tube, which then loops around to develop the chambers of the heart.
   
   
2. By the time a developing baby in the womb is just three months old, all of the chambers of the heart have formed.
   
   
3. The heart of the developing baby gets oxygen-rich blood directly from the mother rather than using its own lungs.
   
   
4. The hearts of newborn babies change after birth to allow the lungs to get oxygen through the process of breathing.

Early development

While an embryo is forming, the circulatory system is the first working system to develop, and the heart is the first functioning organ.¹ Formation of the heart begins soon after conception. By the time the embryo is just three weeks old, these cells have formed the heart tube that begins to beat even before the chambers, valves, and vessels have formed.¹ Learn more about the adult heart in How the Heart Works and The Vessels. By the time the embryo is five weeks old, the heart has formed into a two-chambered tube with one atrium and one ventricle. As it grows, the tube loops over to form left and right sides.

By the 60th day in the embryo's life, the heart's major vessels – including the aorta – have formed. By the embryo's third month, all four of the heart's chambers – two atria and two ventricles – have developed.

The developing circulation

The circulation of the developing baby differs from a newborn baby’s in important ways. The developing baby gets its oxygen and nutrients directly from the mother’s placenta because the baby’s lungs and intestines are not yet functioning. Also, two openings within the developing heart allow the circulation to bypass the lungs. This makes it possible for oxygen and nutrients to be passed from the mother to the developing baby.

But what happens when you are born? When a newborn baby takes in its first breath, the two openings in the heart close off and separate the body’s circulation from the circulation in the lungs. This allows the newborn to breathe in enough oxygen to live on its own and keep oxygen-rich blood flowing through its body.

Source

1. Gilbert, Scott. Developmental Biology, Seventh Edition, Sinauer Associates, Inc., 2003, pp. 491-500.