Newborn Physiology

Cardiovascular System

To understand the changes occurring in the cardiovascular physiology of the newborn, one must first understand intrauterine fetal circulation and the shunts involved, which include the ductus venosus, foramen ovale, and ductus arteriosus. 

Oxygenated blood from the mother becomes oxygenated in the placenta and enters the fetus through the umbilical vein. The oxygenated blood in the umbilical vein bypasses the hepatic circulation and is delivered to the inferior vena cava (IVC) via the ductus venosus.

From the IVC, the blood flows into the right atrium. Oxygenated blood is then shunted from the right atrium to the left atrium through the foramen ovale. Blood travels to the left ventricle and into the coronary arteries and aorta from the left atrium. A small amount of blood from the right atrium does not travel through the foramen ovale and instead flows to the right ventricle and into the pulmonary artery, then the lungs. However, the majority of blood is shunted from the pulmonary artery directly to the aorta, bypassing the lungs via the ductus arteriosus. From the aorta, oxygenated blood is then delivered to the systemic circulation.[2]

• Changes in Fetal Circulation at Birth
• • Decreased Pulmonary Vascular Resistance and Increased Systemic Vascular Resistance

    • With the cutting of the umbilical cord comes the removal of the low-resistance circuit, allowing for an increase in systemic circulation. Upon taking its first breath, the fluid in the newborn lungs is replaced with air, and oxygen diffuses into the blood vessels surrounding the alveoli. Relaxation of the pulmonary arteries occurs, which allows the pulmonary resistance to fall and blood to flow into the lungs.
  • Closure of the Ductus Arteriosus

    • In utero, patency of the ductus arteriosus is maintained by high levels of prostaglandins, carbon monoxide, nitric oxide, and low oxygen tension. Once the newborn begins breathing, functional closure of ductus arteriosus begins and can last several days. Due to the decrease in pulmonary arterial resistance and increase in oxygen, there is a decrease in prostaglandins which aids in the closure of the ductus arteriosus. With the placenta now separated, there is also a decrease in prostaglandin synthesis, further contributing to the closure of the ductus arteriosus.[3] 
  • Closure of the Foramen Ovale

    • Decreased pulmonary vasculature resistance upon the newborn's first breath causes the left atrial pressure to increase. With the left atrial pressure now higher than the right atrial pressure, the foramen ovale begins to close.
  • Closure of the Ductus Venosus (DV)
• • • Permanent closure of the DV takes approximately one to three months, with the remnant consisting of connective tissue forming the ligamentum venosum of the liver.[4] 
  • Maternal-fetal circulation is no longer needed after birth, so the umbilical vein's remnant forms the liver's round ligament.[5]

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