Chapter 16
The Endocrine System
621
16
Adipose Tissue
Adipose cells release
leptin
, which serves to
tell your body how much stored energy (as fat) you have. Te
more fat you have, the more leptin there will be in your blood.
As we describe in Chapter 24, leptin binds to CNS neurons con-
cerned with appetite control, producing a sensation of satiety. It
also appears to stimulate increased energy expenditure.
±wo other hormones released by adipose cells both affect the
sensitivity of cells to insulin.
Resistin
is an insulin antagonist,
while
adiponectin
enhances sensitivity to insulin.
Gastrointestinal
Tract
Enteroendocrine
cells
are
hormone-
secreting cells sprinkled in the mucosa of the gastrointestinal (GI)
tract. Tese scattered cells release several peptide hormones that
help regulate a wide variety of digestive functions, some of which
are summarized in ±able 16.5. Enteroendocrine cells also release
amines such as serotonin, which act as paracrines, diffusing to and
influencing nearby target cells without first entering the bloodstream.
Enteroendocrine cells have been referred to as
paraneurons
because
they are similar in certain ways to neurons and many of their hor-
mones and paracrines are chemically identical to neurotransmitters.
Heart
Te atria contain specialized cardiac muscle cells that
secrete
atrial natriuretic peptide (ANP).
As noted on p. 614,
ANP decreases the amount of sodium in the extracellular fluid,
thereby reducing blood volume and blood pressure.
Kidneys
Interstitial cells in the kidneys secrete
erythropoietin
(ĕ-rith
0
ro-poi
9
ĕ-tin; “red-maker”), a glycoprotein hormone that
signals the bone marrow to increase production of red blood
cells. Te kidneys also release
renin
, which acts as an enzyme to
Te
placenta
is a temporary endocrine organ. Besides sus-
taining the fetus during pregnancy, it secretes several steroid
and protein hormones that influence the course of pregnancy.
Placental hormones include estrogens and progesterone (hor-
mones more oFen associated with the ovary), and human cho-
rionic gonadotropin (hCG).
We will discuss the roles of the gonadal, placental, and go-
nadotropic hormones in Chapters 27 and 28, where we consider
the reproductive system and pregnancy.
Check Your Understanding
16.
Which of the two chemical classes of hormones introduced
at the beginning of this chapter do the gonadal hormones
belong to? Which major endocrine gland secretes hormones
of this same chemical class?
For answers, see Appendix H.
Hormone Secretion by Other Organs
Name a hormone produced by the heart.
State the location of enteroendocrine cells.
Briefly explain the hormonal functions of the kidney, skin,
adipose tissue, bone, and thymus.
Other hormone-producing cells occur in various organs includ-
ing the heart, gastrointestinal tract, kidneys, skin, adipose tis-
sue, skeleton, and thymus
(Table 16.5)
.
Table 16.4
Symptoms of Insulin Deficit (Diabetes Mellitus)
ORGANS/TISSUES INVOLVED
ORGAN/TISSUE
RESPONSES TO
INSULIN DEFICIENCY
RESULTING CONDITIONS
IN BLOOD
IN URINE
SIGNS AND
SYMPTOMS
Adipose
Muscle
tissue
Decreased glucose
uptake and utilization
Hyperglycemia
 
 
Glycosuria
Osmotic diuresis
 
 
Polyuria (and
dehydration, soft eyeballs)
Polydipsia (and fatigue,
weight loss)
Polyphagia
 
 
Liver
Glycogenolysis
 
Protein catabolism
and gluconeogenesis
 
Lipolysis and
ketogenesis
Lipidemia and
ketoacidosis
Ketonuria
Loss of Na
1
,
K
1
; electrolyte
and acid-base
imbalances
Acetone breath
Hyperpnea
Nausea, vomiting,
abdominal pain
Cardiac irregularities
Central nervous system
depression; coma
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