fetal circulation

Oxygenated, nutrient-rich blood from the placenta travels via the umbilical vein to the fetus. Most of this blood bypasses the liver through the ductus venosus, draining into the IVC and mixing with deoxygenated blood returning from the lower body. This blood enters the right atrium, where the majority is shunted through the foramen ovale directly into the left atrium (bypassing the right ventricle and pulmonary circulation), then to the left ventricle and out the aorta to supply the brain and upper body with the most oxygenated blood available (the 'preferential streaming' phenomenon). Deoxygenated blood from the superior vena cava (returning from the fetal head/upper body) preferentially flows into the right ventricle and pulmonary artery. Since fetal lungs are fluid-filled and non-functional for gas exchange, pulmonary vascular resistance is high, so most of this blood bypasses the lungs via the ductus arteriosus, shunting from the pulmonary artery to the descending aorta. This lower-oxygen blood then supplies the lower body and returns to the placenta via the two umbilical arteries for reoxygenation. Key pressure relationships: right atrial pressure > left atrial pressure in utero, keeping the foramen ovale open; ductus arteriosus patency is maintained by low fetal PaO2 and high circulating prostaglandins (PGE2) produced by the placenta. At birth, several changes occur: (1) the umbilical cord is clamped, removing the low-resistance placental circuit and increasing systemic vascular resistance; (2) the first breaths inflate the lungs, dramatically decreasing pulmonary vascular resistance and increasing pulmonary blood flow; (3) increased pulmonary venous return raises left atrial pressure above right atrial pressure, functionally closing the foramen ovale (anatomic closure occurs later, forming the fossa ovalis); (4) increased PaO2 and decreased prostaglandins cause functional closure of the ductus arteriosus within hours to days (becomes the ligamentum arteriosum); indomethacin can pharmacologically close a patent ductus arteriosus (PDA) by inhibiting prostaglandin synthesis, while PGE1 (alprostadil) keeps it open—clinically useful in ductal-dependent congenital heart lesions (e.g., transposition of the great arteries, coarctation, hypoplastic left heart). The ductus venosus becomes the ligamentum venosum, and the umbilical vein becomes the ligamentum teres. Understanding fetal circulation is essential for grasping congenital heart defects: failure of foramen ovale closure results in patent foramen ovale (PFO); failure of ductus arteriosus closure results in PDA, associated with congenital rubella and maternal conditions causing chronic fetal hypoxia; persistent pulmonary hypertension of the newborn can cause right-to-left shunting through fetal channels postnatally, causing differential cyanosis.