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666 Unit 3 Control and Regulation
Table 18–6 Representative Hormones Produced by Organs of Other Systems
Organ Hormone Primary Target Hormonal Effect
Intestines Many (secretin, gastrin, Other regions and organs of the Coordinate digestive activities
Kidneys cholecystokinin, etc.) digestive system
Erythropoietin (EPO) Red bone marrow Stimulates red blood cell production
Calcitriol Intestinal lining, bone, kidneys Stimulates calcium and phosphate absorption; stimulates
Ca2+ release from bone; inhibits PTH secretion
Heart Natriuretic peptides (ANP Kidneys, hypothalamus, adrenal Increase water and salt loss at kidneys; decrease thirst; suppress
and BNP) gland secretion of ADH and aldosterone
Thymus Thymosins (many) Lymphocytes and other cells of the Coordinate and regulate immune response
immune response
Gonads See Table 18–7
Adipose tissues Leptin Hypothalamus Suppression of appetite; permissive effects on GnRH and gonadotropin
synthesis
18 differentiation of osteogenic cells and osteoclasts, (2) stimulat- references consider renin to be a hormone. We will take a closer
ing bone resorption by osteoclasts, (3) stimulating Ca2+ reab- look at the renin-angiotensin-aldosterone system when we exam-
sorption by the kidneys, and (4) suppressing PTH production. ine the control of blood pressure and blood volume in Chapter 21.
Evidence indicates that calcitriol also affects lymphocytes and The Heart
keratinocytes in the skin, but these effects have nothing to do The endocrine cells in the heart are cardiac muscle cells in the
walls of the atria (chambers that receive blood from the veins)
with regulating calcium levels. and the ventricles (chambers that pump blood to the rest of
the body). If blood volume becomes too great, these cells are
Erythropoietin stretched excessively, to the point at which they begin to secrete
natriuretic peptides (na. -tre. -u. -RET-ik; natrium, sodium +
Erythropoietin (e-rith-ro. -POY-e-tin; erythros, red + poiesis, ouresis, urination). In general, the effects of natriuretic peptides
making), or EPO, is a peptide hormone released by the kid- oppose those of angiotensin II: Natriuretic peptides promote
neys in response to low oxygen levels in kidney tissues. EPO the loss of Na+ and water by the kidneys, and inhibit renin
stimulates the red bone marrow to produce red blood cells. release and the secretion of ADH and aldosterone. They also
The increase in the number of red blood cells elevates blood suppress thirst and prevent angiotensin II and norepinephrine
volume. Because these cells transport oxygen, this increase also from elevating blood pressure. The net result is a reduction in
improves oxygen delivery to peripheral tissues. We will con- both blood volume and blood pressure, thereby reducing the
sider EPO again in Chapter 19. stretching of the cardiac muscle cells in the heart walls. We dis-
cuss two natriuretic peptides—ANP (atrial natriuretic peptide)
Renin and BNP (brain natriuretic peptide)—when we consider the
control of blood pressure and volume in Chapters 21 and 26.
Specialized kidney cells called juxtaglomerular cells release renin
(RE. -nin) in response to (1) sympathetic stimulation or (2) a de- The Thymus
cline in renal blood flow. Once in the bloodstream, renin func-
tions as an enzyme that starts an enzymatic cascade known as The thymus is located in the mediastinum, generally just deep
the renin-angiotensin-aldosterone system (RAAS) (Figure 18–19b). to the sternum. This gland produces several hormones that are
RAAS is involved with blood pressure regulation and electrolyte important in developing and maintaining immune defenses.
metabolism. First, renin converts angiotensinogen, a plasma Thymosin (THI.-mo. -sin) is the name originally given to an
protein produced by the liver, to angiotensin I. In the capillaries extract from the thymus that promotes the development and
of the lungs, angiotensin-converting enzyme (ACE) modifies maturation of lymphocytes, the white blood cells responsible
angiotensin I to the hormone angiotensin II. This hormone, for immunity. The extract actually contains a blend of several
in turn, stimulates the secretion of aldosterone by the adrenal complementary hormones. The term thymosins is sometimes
cortex and ADH by the posterior lobe of the pituitary gland. used to refer to all thymic hormones. We consider the histo-
The combination of aldosterone and ADH retains salt and logical organization of the thymus and the functions of the
water by restricting their loss by the kidneys. Angiotensin II also thymosins in Chapter 22.
stimulates thirst and elevates blood pressure. For these reasons,
drugs known as ACE inhibitors are used to treat hypertension.
Because renin plays such a key role in the renin-angiotensin-
aldosterone system, many physiological and endocrinological
