Ganong's Review of Medical Physiology, 23rd Edition

386
SECTION IV
Endocrine & Reproductive Physiology

occurs appears to be an important consideration. For exam-
ple, once the pubertal growth spurt has commenced, consid-
erable linear growth continues even if caloric intake is
reduced. Injury and disease likewise stunt growth because
they increase protein catabolism.

GROWTH PERIODS

Patterns of growth vary somewhat from species to species.
Rats continue to grow, although at a declining rate, through-
out life. In humans, two periods of rapid growth occur (Figure
24–8): the first in infancy and the second in late puberty just
before growth stops. The first period of accelerated growth is
partly a continuation of the fetal growth period. The second
growth spurt, at the time of puberty, is due to growth hor-
mone, androgens, and estrogens, and the subsequent cessa-
tion of growth is due in large part to closure of the epiphyses
in the long bones by estrogens (see Chapter 25). After this
time, further increases in height are not possible. Because girls
mature earlier than boys, this growth spurt appears earlier in
girls. Of course, in both sexes the rate of growth of individual
tissues varies (Figure 24–9).
It is interesting that at least during infancy, growth is not a con-
tinuous process but is episodic or saltatory. Increases in length of
human infants of 0.5 to 2.5 cm in a few days are separated by
periods of 2 to 63 d during which no measurable growth can be
detected. The cause of the episodic growth is unknown.

HORMONAL EFFECTS

The contributions of hormones to growth after birth are

shown diagrammatically in Figure 24–10. Plasma growth hor-

mone is elevated in newborns. Subsequently, average resting

levels fall but the spikes of growth hormone secretion are larg-

er, especially during puberty, so the mean plasma level over

24 h is increased; it is 2 to 4 ng/mL in normal adults, but 5 to

8 ng/mL in children. One of the factors stimulating IGF-I se-

cretion is growth hormone, and plasma IGF-I levels rise during

childhood, reaching a peak at 13 to 17 years of age. In contrast,

IGF-II levels are constant throughout postnatal growth.

The growth spurt that occurs at the time of puberty (Figure

24–8) is due in part to the protein anabolic effect of andro-

gens, and the secretion of adrenal androgens increases at this

time in both sexes; however, it is also due to an interaction

among sex steroids, growth hormone, and IGF-I. Treatment

FIGURE 24–8
Rate of growth in boys and girls from birth to
age 20.

201816141210

Girls

Height gain (cm/yr)

Boys

Age in years

8642

0

5.1

10.2

15.2

20.3

25.4

FIGURE 24–9

Growth of different tissues at various ages as a

percentage of size at age 20.

The curves are composites that include

data for both boys and girls.

FIGURE 24–10

Relative importance of hormones in human

growth at various ages.

(Courtesy of DA Fisher.)

101214 201816

Brain

and

head

Percent of size at age 20

Body and

most visceral

organs

Age in years

86420

20

40

60

80

100

120

140

160

180

200

Lymphoid

tissue

Reproductive

organs

Birth 2 4 6 8 10 12 14 16 18 20

Thyroid

hormones

Growth

hormone

Androgens

and

estrogens

Age (years)