CHAPTER TWENTY
Content Review
1. Hormones are released into the bloodstream. Only cells with specific receptors for the
hormone (enabling the hormone to bind to the cell) respond to that hormone. These
cells are called target cells, and the organs that contain them are called target organs.
In contrast, organs, tissues, or cells that do not have the specific receptor for a
hormone do not bind or attach the hormone and do not respond to its stimulating
effects.
2. Peptide hormones are formed from chains of amino acids; an example is growth
hormone. Steroid hormones are a type of lipid derived from cholesterol; an example
is testosterone. Biogenic amines are small molecules produced by altering the
structure of an amino acid; an example is thyroid hormone.
3. The hypothalamus has special neurosecretory cells that secrete hormones to influence
the secretory activity of the anterior pituitary gland. These hormones are called
regulatory hormones because they are secreted into the blood to regulate the secretion
of most anterior pituitary hormones. Regulatory hormones fall into one of two
groups: Releasing hormones (RH) stimulate the production and secretion of specific
anterior pituitary hormones, and inhibiting hormones (IH) deter the production and
secretion of specific anterior pituitary hormones. Additionally, the hypothalamus
produces two hormones that are transported through axons to the posterior pituitary
and stored there until needed. These two hormones are oxytocin and antidiuretic
hormone (ADH).
4. The anterior pituitary gland is controlled by regulatory hormones secreted by the
hypothalamus. These regulatory hormones reach the anterior pituitary by traveling
through a blood vessel network called the hypothalamo-hypophyseal portal system,
which is essentially a “shunt” that takes venous blood (carrying regulatory hormones)
from the hypothalamus directly to the anterior pituitary first, before the blood returns
to the heart. Thus, it provides a pathway for hypothalamic hormones to immediately
reach the anterior pituitary. In addition, the veins that drain this portal system provide
a pathway by which the anterior pituitary hormones may be released into the general
bloodstream.
5. The anterior pituitary secretes (1) thyroid- stimulating hormone (TSH) to stimulate
the thyroid gland to release thyroid hormone; (2) prolactin (PRL) to regulate
mammary gland growth and breast milk production; (3) adrenocorticotropic hormone
(ACTH) to stimulate the adrenal cortex to produce and secrete some corticosteroid
hormones; (4) growth hormone (GH) to affect cellular activities in most body cells,
especially growth in the skeletal and muscular systems, and to cause the liver to
produce somatomedin, which stimulates growth at the epiphyseal plate of long bones;
(5, 6) follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to regulate
reproductive system activities and the production and maturation of gametes; and (7)
melanocyte-stimulating hormone (MSH), which stimulates both the rate of melanin
synthesis by melanocytes in the integument and their distribution through the skin.
6. Parafollicular cells secrete the hormone calcitonin in response to elevated blood
calcium levels. Calcitonin reduces blood calcium levels by stimulating osteoblast
activity and inhibiting osteoclast activity. New bone matrix is formed, with
simultaneous deposition of calcium salts onto this matrix, and resorption of bone
matrix decreases.
7. PTH secretion occurs when the calcium ion concentration in the body decreases.
Calcium ions are needed for many body functions, such as activity at nerve synapses
and muscle contraction. Without adequate levels of calcium in the blood, the body
cannot function properly. Symptoms of insufficient PTH are mostly neuromuscular,
ranging from mild tingling in the fingers and limbs to marked muscle cramps and
contractions (tetany); in severe cases, convulsions may occur.
8. The adrenal cortex is partitioned into three separate regions, each of which
synthesizes and secretes a different functional category of steroid hormones: The
zona glomerulosa produces mineralocorticoids; the zona fasciculata produces
glucocorticoids; and the zona reticularis produces androgens.
9. Four endocrine cell types form the islets: Alpha cells secrete glucagon to raise blood
glucose levels; beta cells produce insulin to cause blood glucose levels to decline;
delta cells produce somatostatin to slow down or inhibit activities in the digestive
tract; and F cells secrete pancreatic polypeptide to suppress and regulate somatostatin
secretion from delta cells.
10. The kidneys, heart, some GI tract organs, and gonads contain endocrine cells that
secrete their own hormones. These hormones help regulate blood electrolyte levels
and erythrocyte production; blood volume and blood pressure; digestion; and sexual
maturation and activity, respectively.