Page 233 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Fetal and Neonatal Circulation The arteries of the head and upper body are
supplied with partly arterialized blood from
Placenta. The maternal placenta acts as the the left ventricle (! A). This is important since
“gut” (absorption of nutrients), “kidneys” (re- brain tissue is susceptible to hypoxia. The re-
moval of waste products) and “lungs” of the maining blood leaves the aorta and mixes with
fetus (uptake of O 2 and elimination of CO 2). Al- venous blood from the ductus arteriosus. As a
though the fetal O 2-hemoglobin dissociation result, the blood supplied to the lower half of
curve is shifted to the left compared to that of the body has a relatively low O 2 concentration
adults (! p. 129 C), only 60% (0.6) of placental (O 2 saturation = 0.3; ! A). The majority of this
hemoglobin is saturated with O 2 (! A). blood returns via the umbilical arteries to the
Fetal blood is distributed according to need. placenta, where it is oxygenated again.
Inactive or hardly active organs receive little Circulation during birth. The exchange of O 2,
blood. The fetal cardiac output (from both ven- nutrients, and waste materials through the
Cardiovascular System 65 min –1 (week 5) to 130–160 min –1 in later mosensors (! p. 132) that induce a strong
placenta stops abruptly during birth. This
tricles) is about 0.2 L/min per kg body weight.
leads to a rise in blood PCO 2, triggering che-
The fetal heart rate rises from an initial
weeks. Approx. 50% of the blood ejected from
breathing reflex. The resultant inspiratory
the heart flows through the placenta, the other
movement causes negative pressure (suction)
half supplies the body (35%) and lungs (15%) of
in the thoracic cavity, which removes the
the fetus. This is supplied by the left and right
blood from the placenta and umbilical vein
The unfolding of the lungs and the rise in alve-
The serial connection of the systemic circula-
8 heart, which function essentially in parallel. (placental transfusion) and expands the lungs.
olar PO 2 reduces the resistance in the pulmo-
tion to the pulmonary circuit (as in adults) is
not fully necessary in the fetus. nary circulation, and the blood flow increases
Fetal circulation. The blood flows through while the pressure decreases (! B1, 2). Mean-
the fetal body as follows (! A): After being while, the resistance in the systemic circula-
arterialized in the placenta, the blood passes tion increases due to occlusion or clamping of
into the fetus via the umbilical vein and part of the umbilical cord. This changes the direction
it travels through the ductus venosus (Arantii), of blood flow in the ductus arteriosus, result-
thereby bypassing the liver. When entering the ing in a left-to-right shunt. The pulmonary
inferior vena cava, the blood mixes with venous circulation therefore receives aortic blood for a
blood from the lower half of the body. Guided few days after birth. The right atrial filling
by special folds in the vena cava, the mixed volume decreases due to the lack of placental
blood passes directly from right atrium to the blood, while that of the left atrium increases
left atrium through an opening in the atrial due to the increased pulmonary blood flow.
septum (foramen ovale). From the left atrium, Due to the resultant pressure gradient from
it then proceeds to the left ventricle. While in the left to right atrium and to a decrease in va-
the right atrium, the blood mingles with sodilatory prostaglandins, the foramen ovale
venous blood from the superior vena cava usually closes within about 2 weeks after
(only slight mixing), which is received by the birth. The ductus arteriosus and ductus veno-
right ventricle. Only about one-third of this sus also close, and the systemic and pulmo-
blood reaches the lungs (due to high flow re- nary circulation now form serial circuits.
sistance since the lungs are not yet expanded, Shunts occur when the foramen ovale or
and due to hypoxic vasoconstriction, ! C and ductus arteriosus remains open, placing a
p. 122). The other two-thirds of the blood strain on the heart. In patent foramen ovale
travels through the ductus arteriosus (Botalli) (atrial septum defect), the blood flows from
to the aorta (right-to-left shunt). Due to the low left atrium ! right atrium (left-to-right shunt)
peripheral resistance (placenta), the blood ! right ventricle (volume overload) ! lungs
pressure in the aorta is relatively low—only ! left atrium. In patent ductus arteriosus, the
about 65 mmHg towards the end of pregnancy. blood flows from aorta ! pulmonary artery
220 (= left-to-right shunt) ! lungs (pressure over-
load) ! aorta.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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