Maintenance of the Body
Building a perfect heart is diﬃcult. Each year about 30,000 in-
fants are born in the U.S. with one or more of 30 diﬀerent
, making them the most common of all
birth defects. Some congenital heart problems are traceable to
environmental inﬂuences, such as maternal infection or drug
intake during month 2 when the major events of heart forma-
Te most prevalent abnormalities produce two basic kinds
of disorders in the newborn. Tey either (1) lead to mixing of
oxygen-poor blood with oxygenated blood (so that inadequately
oxygenated blood reaches the body tissues) or (2) involve nar-
rowed valves or vessels that greatly increase the workload on
Examples of the ﬁrst type of defect are
patent ductus arteriosus
, in which the
connection between the aorta and pulmonary trunk remains
Coarctation of the aorta
(Figure 18.25b) is an example of
the second type of problem.
Tetralogy of Fallot
), a serious condition in which the baby becomes cyanotic
within minutes of birth, encompasses both types of disorders
(Figure 18.25c). Modern surgical techniques can usually cor-
rect these congenital defects.
Heart Function Throughout Life
In the absence of congenital heart problems, the heart functions
admirably throughout a long lifetime for most people. Homeo-
static mechanisms are normally so eﬃcient that people rarely
notice when the heart is working harder.
Te strongest pumping chamber of the early
heart, the ventricle gives rise to the
Tis chamber plus its cranial extension, the
(labeled 4a in Figure 18.24b), give rise to
the pulmonary trunk, the ﬁrst part of the aorta, and most
During the next three weeks, the heart “tube” exhibits dra-
matic contortions as it undergoes rightward looping, and major
structural changes convert it into a four-chambered organ ca-
pable of acting as a double pump—all without missing a beat!
Te ventricle moves caudally and the atrium cranially, assuming
their adult positions. Te heart divides into its four deﬁnitive
chambers (via a number of stages), the midline septum forms,
and the bulbus cordis splits into the pulmonary trunk and as-
cending aorta. A±er the second month, few changes other than
growth occur until birth.
Te interatrial septum of the fetal heart is incomplete. Te
(literally, “oval door”) connects the two atria
and allows blood entering the right heart to bypass the pulmo-
nary circuit and the collapsed, nonfunctional fetal lungs (Fig-
ure 18.24e). Another lung bypass, the
between the pulmonary trunk and the aorta. At or shortly a±er
birth, these shunts close, completing the separation between the
right and le± sides of the heart.
In the adult heart, the fossa ovalis reveals the position of the
foramen ovale, and the
is the ﬁbrous
remnant of the ductus arteriosus (see Figure 18.5b). We give a
more complete description of the fetal and newborn circulation
in Chapter 28 (see Figure 28.14).
(a) Day 20:
(b) Day 22:
(c) Day 24:
starts to bend.
(d) Day 28:
continues as ventricle
moves caudally and
atrium moves cranially.
(e) Day 35:
Development of the human heart.
Ventral views, with the cranial direction
toward the top of the ﬁgures. Arrows show the direction of blood ﬂow. Days are approximate.
1 is the sinus venosus; 2, the atrium; 3, the ventricle; 4, the bulbus cordis; and 4a, the