Inferior Vena Cava

Twenty-first through thirty-eighth week

Body gains weight, subcutaneous fat deposited; eyebrows and lashes appear; eyelids reopen; testes descend Size: 21 inches (53 centimeters), weight: 6 to 10 pounds (2.7 to 4.5 kilograms)

Placenta

Uterine wall

Placenta

Placental Blood Supply Uterine Artery

Umbilical arteries Umbilical vein

Maternal blood in lacuna

Blood flow from fetus, branch of umbilical artery

Chorionic villus

Blood flow to fetus, branch of umbilical vein

Figure 23.21

Oxygen and nutrients diffuse into the fetal blood from the maternal blood. Waste diffuses into the maternal blood from the fetal blood.

Uterine wall

Umbilical arteries Umbilical vein

Maternal blood in lacuna

Diffusion

Oxygen and nutrients into fetal blood

Diffusion

Waste substance into maternal blo

Blood flow from fetus, branch of umbilical artery

Chorionic villus

Blood flow to fetus, branch of umbilical vein

Figure 23.21

Oxygen and nutrients diffuse into the fetal blood from the maternal blood. Waste diffuses into the maternal blood from the fetal blood.

Blake Schultz made medical history when he underwent major surgery seven weeks before birth. Ultrasound had revealed that his stomach, spleen, and intestines protruded through a hole in his diaphragm, the muscle sheet that separates the abdomen from the chest. This defect would have suffocated him shortly after birth were it not for pioneering surgery that exposed Blake's left side, gently tucked his organs in place, and patched the hole with a synthetic material.

Some prenatal medical problems can be treated by administering drugs to the pregnant woman or by altering her diet. An undersized fetus can receive a nutritional boost by putting the pregnant woman on a high-protein diet. It is also possible to treat prenatal medical problems directly: A tube inserted into the uterus can drain the dangerously swollen bladder of a fetus with a blocked urinary tract, providing relief until the problem can be surgically corrected at birth. A similar procedure can remove excess fluid from the brain of a fetus with hydrocephaly (a neural tube defect, also called "water on the brain").

U What major changes occur during the fetal stage of development?

^9 When can the sex of a fetus be determined?

^9 How is a fetus usually positioned within the uterus at the end of pregnancy?

Fetal Blood and Circulation

Throughout fetal development, the maternal blood supplies oxygen and nutrients and carries away wastes. These substances diffuse between the maternal and fetal blood through the placental membrane, and the umbilical blood vessels carry them to and from the fetus (fig. 23.21). Consequently, the fetal blood and vascular system are adapted to intrauterine existence. For example, the concentration of oxygen-carrying hemoglobin in the fetal blood is about 50% greater than in the maternal blood. Also, fetal hemoglobin has a greater affinity for oxygen than does an adult's hemoglobin. Thus, at the oxygen partial pressure of the placental capillaries, fetal hemoglobin can carry 20%-30% more oxygen than maternal hemoglobin.

In the fetal cardiovascular system, the umbilical vein transports blood rich in oxygen and nutrients from the placenta to the fetal body. This vein enters the body through the umbilical ring and travels along the anterior abdominal wall to the liver. About half the blood it carries passes into the liver, and the rest enters a vessel called the ductus venosus, which bypasses the liver.

The ductus venosus extends a short distance and joins the inferior vena cava. There, oxygenated blood from the placenta mixes with deoxygenated blood from the lower parts of the fetal body. This mixture continues through the vena cava to the right atrium.

In an adult heart, the blood from the right atrium enters the right ventricle and is pumped through the pulmonary trunk and pulmonary arteries to the lungs. In the fetus, however, the lungs are nonfunctional, and the blood largely bypasses them. As blood from the inferior vena cava enters the fetal right atrium, much of it is shunted directly into the left atrium through an opening in the atrial septum. This opening is called the foramen ovale, and the blood passes through it because the blood pressure in the right atrium is somewhat greater than that in the left atrium. Furthermore, a small valve (septum primum) located on the left side of the atrial septum overlies the foramen ovale and helps prevent blood from moving in the reverse direction.

The rest of the fetal blood entering the right atrium, including a large proportion of the deoxy-genated blood entering from the superior vena cava, passes into the right ventricle and out through the pulmonary trunk. Only a small volume of blood enters the pulmonary circuit because the lungs are collapsed and their blood vessels have a high resistance to blood flow. However, enough blood reaches the lung tissues to sustain them.

Most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arte-riosus, which connects the pulmonary trunk to the descending portion of the aortic arch. As a result of this connection, the blood with a relatively low oxygen concentration, which is returning to the heart through the superior vena cava, bypasses the lungs and does not enter the portion of the aorta that branches to the heart and brain.

The more highly oxygenated blood that enters the left atrium through the foramen ovale mixes with a small amount of deoxygenated blood returning from the pulmonary veins. This mixture moves into the left ventricle and is pumped into the aorta. Some of it reaches the myocardium through the coronary arteries, and some reaches the brain tissues through the carotid arteries.

The blood the descending aorta carries is partially oxygenated and partially deoxygenated. Some of it is carried into the branches of the aorta that lead to the lower regions of the body. The rest passes into the umbilical arteries, which branch from the internal iliac arteries and

Function

Adaptation

Function

Fetal blood Umbilical vein Ductus venosus

Foramen ovale

Ductus arteriosus Umbilical arteries

Has greater oxygen-carrying capacity than adult blood Carries oxygenated blood from the placenta to the fetus Conducts about half the blood from the umbilical vein directly to the inferior vena cava, thus bypassing the liver

Conveys a large proportion of the blood entering the right atrium from the inferior vena cava, through the atrial septum, and into the left atrium, thus bypassing the lungs Conducts some blood from the pulmonary trunk to the aorta, thus bypassing the lungs Carry the blood from the internal iliac arteries to the placenta lead to the placenta. There the blood is reoxygenated (fig. 23.22).

The umbilical cord usually contains two arteries and one vein. Rarely, newborns have only one umbilical artery. This condition is often associated with other cardiovascular, urogenital, or gastrointestinal disorders. Because of the possibility of these conditions, the vessels within the severed cord are routinely counted following a birth.

Table 23.2 summarizes the major features of fetal circulation. At the time of birth, important adjustments must occur in the cardiovascular system when the placenta ceases to function and the newborn begins to breathe. Clinical Application 23.3 describes a case in which fetal ultrasound revealed two hearts and bloodstreams, yet a single body.

H How does the pattern of fetal circulation differ from that of an adult?

Which umbilical vessel carries oxygen-rich blood to the fetus?

What is the function of the ductus venosus?

How does fetal circulation allow blood to bypass the lungs?

What characteristic of the fetal lungs tends to shunt blood away from them?

Postnatal Period

The postnatal period of development lasts from birth until death. It can be divided into the neonatal period, infancy, childhood, adolescence, adulthood, and senescence.

Arch of the aorta

Superior vena cava

Foramen ovale Right atrium

Inferior vena cava Right ventricle

Ductus venosus

Hepatic portal vein

Ductus arteriosus

Pulmonary trunk Pulmonary artery Pulmonary veins Left atrium

Left ventricle

Umbilicus

Umbilical vein

Umbilical arteries

Arch of the aorta

Superior vena cava

Foramen ovale Right atrium

Inferior vena cava Right ventricle

Ductus venosus

Hepatic portal vein

Umbilicus

Umbilical vein

Umbilical arteries

Fetal Interrupted Inferior Vena Cava

Decreasing oxygen concentration

Decreasing oxygen concentration

Figure 23.22

The general pattern of fetal circulation is shown anatomically (a) (highlighted labels indicate structures unique to fetal circulation) and schematically (b).

Neonatal Period

The neonatal period (ne"o-na'tal pe're-od), which extends from birth to the end of the first four weeks, begins very abruptly at birth (fig. 23.23). At that moment, physiological adjustments must occur quickly because the newborn must suddenly do for itself what the mother's body had been doing for it. Thus, the newborn (neonate) must respire, obtain and digest nutrients, excrete wastes, and regulate body temperature. However, a newborn's most immediate need is to obtain oxygen and excrete carbon dioxide, so its first breath is critical.

The first breath must be particularly forceful because the newborn's lungs are collapsed and the airways are small, offering considerable resistance to air movement. Also, surface tension tends to hold the moist membranes of the lungs together. However, the lungs of a full-term fetus continuously secrete surfactant (see chapter 19, p. 795), which reduces surface tension. After the

Figure

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Pregnancy And Childbirth

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