Essentials of Anatomy and Physiology

Third stage—delivery of the placenta (afterbirth).
Continued contractions of the uterus expel the pla-
centa and membranes, usually within 10 minutes
after delivery of the infant. There is some bleeding
at this time, but the uterus rapidly decreases in size,
and the contractions compress the endometrium to
close the ruptured blood vessels at the former site
of the placenta. This is important to prevent severe
maternal hemorrhage.

THE INFANT AT BIRTH

Immediately after delivery, the umbilical cord is
clamped and cut, and the infant’s nose and mouth are
aspirated to remove any fluid that might interfere with
breathing (see Box 21–6: Apgar Score). Now the
infant is independent of the mother, and the most
rapid changes occur in the respiratory and circulatory
systems.
As the level of CO 2 in the baby’s blood increases,
the respiratory center in the medulla is stimulated and
brings about inhalation to expand and inflate the
lungs. Full expansion of the lungs may take up to 7
days following birth, and the infant’s respiratory rate
may be very rapid at this time, as high as 40 respira-
tions per minute.

Breathing promotes greater pulmonary circula-

tion, and the increased amount of blood returning

to the left atrium closes the flap of the foramen ovale.

The ductus arteriosusbegins to constrict, apparently

in response to the higher blood oxygen level. Full

closure of the ductus arteriosus may take up to 3

months.

The ductus venosusno longer receives blood

from the umbilical vein and begins to constrict within

a few minutes after birth. Within a few weeks the duc-

tus venosus becomes a non-functional ligament.

The infant’s liver is not fully mature at birth and

may be unable to excrete bilirubin efficiently. This

may result in jaundice, which may occur in as many as

half of all newborns. Such jaundice is not considered

serious unless there is another possible cause, such as

Rh incompatibility (see Chapter 11).

GENETICS

Geneticsis the study of inheritance. Most, if not vir-

tually all, human characteristics are regulated at least

partially by genes. We will first look at what genes are,

then describe some patterns of inheritance.

486 Human Development and Genetics

BOX21–6 APGAR SCORE

The highest possible score is 10. Infants who score

less than 5 require immediate medical attention.

The Apgar score is an overall assessment of an

infant and is usually made 1 minute after birth (may

be repeated at 5 minutes if the first score is low).

Characteristic Description Score

Heartbeat

Respiration

Muscle tone

Reflex response to stimulation of sole

of the foot

Color

2 1 0 2 1 0 2 1 0 2 1 0 2 1 0

• Over 100 bpm


• Below 100 bpm


• No heartbeat


• Strong, vigorous cry


• Weak cry


• No respiratory effort


• Spontaneous, active motion


• Some motion


• No muscle tone


• A cry in response


• A grimace in response


• No response


• Healthy coloration


• Cyanotic extremities


• Cyanosis of trunk and extremities