686
UNIT 4
Maintenance of the Body
18
Homeostatic Imbalance
18.10
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
con-
genital
heart defects
, 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-
tion occur.
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
the heart.
Examples of the first type of defect are
septal defects
(Figure 18.25a)
and
patent ductus arteriosus
, in which the
connection between the aorta and pulmonary trunk remains
open.
Coarctation of the aorta
(Figure 18.25b) is an example of
the second type of problem.
Tetralogy of Fallot
(te-tral
9
o-je ov
fal-o
9
), 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.
3.
Ventricle.
Te strongest pumping chamber of the early
heart, the ventricle gives rise to the
le±
ventricle.
4.
Bulbus cordis.
Tis chamber plus its cranial extension, the
truncus arteriosus
(labeled 4a in Figure 18.24b), give rise to
the pulmonary trunk, the first part of the aorta, and most
of the
right
ventricle.
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
foramen ovale
(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
ductus arteriosus
, exists
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
ligamentum arteriosum
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:
Endothelial
tubes begin
to fuse.
(b) Day 22:
Heart starts
pumping.
(c) Day 24:
Heart
continues to
elongate and
starts to bend.
(d) Day 28:
Bending
continues as ventricle
moves caudally and
atrium moves cranially.
(e) Day 35:
Bending is
complete.
Tubular
heart
Ventricle
Ventricle
Atrium
Venous end
Venous end
Arterial end
Arterial end
4a
4
3
2
1
Aorta
Superior
vena cava
Inferior
vena cava
Ductus
arteriosus
Pulmonary
trunk
Foramen
ovale
Ventricle
Figure 18.24
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.
(b)
1 is the sinus venosus; 2, the atrium; 3, the ventricle; 4, the bulbus cordis; and 4a, the
truncus arteriosus.
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