Ұрықтың қан айналым жүйесі

Circulatory system of fetuses
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.

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.

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.

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 core concept behind fetal circulation is that fetal hemoglobin (HbF) has a higher affinity for oxygen than does adult hemoglobin, which, together with the difference in partial pressure of oxygen, allows a diffusion of oxygen from the mother's circulatory system to the fetus.

It is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive its blood through the fetal circulation. Intracardiac pressure remains identical between the right and left ventricles of the human fetus. The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to ca 45 mmHg at 40 weeks of gestation. The fetal pulse pressure is ca 20 mmHg at 20 weeks of gestation, increasing to ca 30 mmHg at 40 weeks of gestation.

The blood flow through the umbilical cord is approximately 35 mL/min at 20 weeks, and 240 mL/min at 40 weeks of gestation. Adapted to the weight of the fetus, this corresponds to 115 mL/min/kg at 20 weeks and 64 mL/min/kg at 40 weeks. It corresponds to 17% of the combined cardiac output of the fetus at 10 weeks, and 33% at 20 weeks of gestation. Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation. On the other hand, there is no neural vascular regulation, and catecholamines have only little effect.