In humans, the circulatory system is different before and after birth. The fetal circulation is composed of the placenta, umbilical blood vessels encapsulated by the umbilical cord, heart and systemic blood vessels. A major difference between the fetal circulation and postnatal circulation is that the lungs are not used during the fetal stage resulting in the presence of shunts to move oxygenated blood and nutrients from the placenta to the fetal tissue. At birth, the start of breathing and the severance of the umbilical cord prompt various changes that quickly transform fetal circulation into postnatal circulation. Oxygenation, nutrient, and waste exchange Placenta The placenta functions as the exchange site of nutrients and wastes between the maternal and fetal circulation. Water, glucose, amino acids, vitamins, and inorganic salts freely diffuse across the placenta along with oxygen. Two umbilical arteries carry deoxygenated blood and waste from the fetus to the placenta where waste is exchanged for oxygen and nutrients. The oxygenated blood will then return to the fetus from the placenta via the umbilical vein. Oxygenation transport in fetus Hemoglobin is a structure found within the red blood cells that binds to and carries oxygen. Fetal hemoglobin enhances the fetus' ability to draw oxygen from the placenta. This is facilitated by the hemoglobin molecule that made up of two alpha and two gamma chains (2α2γ). Its oxygen-hemoglobin dissociation curve is shifted to the left, meaning that it is able to absorb oxygen at lower concentrations than adult hemoglobin. This enables fetal hemoglobin to absorb oxygen from adult hemoglobin in the placenta, where the oxygen pressure is lower than at the lungs. Around 6 months of age after birth, the gamma chains will gradually be replaced by beta chains. This new hemoglobin structure is known as hemoglobin A, composed of two alpha and two beta chains (2α2β). Hemoglobin A is the predominant form found in adults. Before birth Oxygenated blood from the placenta is carried to the fetus by the umbilical vein, which will drain into the inferior vena cava (IVC) through the ductus venosus or the liver. When oxygenated blood enters the IVC, it moves in parallel with deoxygenated blood from the fetal systemic veins, establishing a bilaminar blood flow as it enters the right atrium. The fetal heart contains two upper atria and two lower ventricles. It also contains two additional structures, the foramen ovale and the ductus arteriosus, that function