612
UNIT 3
Regulation and Integration of the Body
16
Zona fasciculata
(fah-sik
0
u-la
9
tah). Te cells of this middle
layer, arranged in more or less linear cords, mainly produce
the metabolic hormones called glucocorticoids.
Zona reticularis
(rĕ-tik
0
u-lar
9
is). Te cells of this innermost
layer, abutting the adrenal medulla, have a netlike arrange-
ment. Tey mainly produce small amounts of adrenal sex
hormones, or gonadocorticoids.
Note, however, that the two innermost layers of the adrenal cor-
tex share production of glucocorticoids and gonadocorticoids,
although each layer predominantly produces one type.
Mineralocorticoids
Te essential function of
mineralocorticoids
is to regulate the
electrolyte (mineral salt) concentrations in extracellular fluids,
particularly of Na
1
and K
1
. Te single most abundant cation in
extracellular fluid is Na
1
, and the amount of Na
1
in the body
largely determines the volume of the extracellular fluid—where
Na
1
goes, water follows. Changes in Na
1
concentration lead to
changes in blood volume and blood pressure. Moreover, the reg-
ulation of Na
1
is coupled to the regulation of many other ions,
including K
1
, H
1
, HCO
3
(bicarbonate), and Cl
(chloride).
Te extracellular concentration of K
1
is also critical—it sets
the resting membrane potential of all cells and determines how
easily action potentials are generated in nerve and muscle. Not
surprisingly, Na
1
and K
1
regulation are crucial to overall body
homeostasis. Teir regulation is the primary job of
aldosterone
and
Figure 16.14
). Tey are also called the
suprarenal glands
(
supra
5
above).
Each adrenal gland is structurally and functionally two en-
docrine glands. Te inner
adrenal
medulla
, more like a knot of
nervous tissue than a gland, is part of the sympathetic nervous
system. Te outer
adrenal
cortex
, encapsulating the medulla
and forming the bulk of the gland, is glandular tissue derived
from embryonic mesoderm. Each region produces its own set
of hormones summarized in
Table 16.3
(p. 616), but all adrenal
hormones help us cope with stressful situations.
The Adrenal Cortex
Te adrenal cortex synthesizes well over two dozen steroid hor-
mones, collectively called
corticosteroids
. Te multistep steroid
synthesis pathway begins with cholesterol, and involves varying
intermediates depending on the hormone being formed. Un-
like the amino acid–based hormones, steroid hormones are not
stored in cells. Consequently, their rate of release depends on
their rate of synthesis.
Te large, lipid-laden cortical cells are arranged in three lay-
ers or zones (Figure 16.14). From the outside in, they are:
Zona glomerulosa
(zo
9
nah glo-mer
0
u-lo
9
sah). Te cell clus-
ters forming this superficial layer produce mineralocorti-
coids, hormones that help control the balance of minerals
and water in the blood.
Cortex
Kidney
• Medulla
Adrenal gland
C
apsule
Aldost
erone
Epinephr
ine
and
nor
epinephr
ine
Cor
tisol
and
androgens
Zona
glomerulosa
Zona
f
asciculata
Cortex
Medulla
Zona
r
eticularis
Adr
enal
medulla
(a) Drawing of the histology of the
adrenal cortex and a portion of
the adrenal medulla
(b) Photomicrograph (115
m
)
Hormones
secreted
Figure 16.14
Microscopic structure of the adrenal gland.
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