CHAPTER 23Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone 375phosphate by increasing the latter’s renal excretion. Calcitonin
lowers both calcium and phosphate levels.
■ Deficiencies of 1,25-dihydroxycholecalciferol or mutations in
its receptor, lead to decreases in circulating calcium, defective
calcification of the bones, and bone weakness. Disease states also
result from either deficiencies or overproduction of parathyroid
hormone, with reciprocal effects on calcium and phosphate.
■ Bone is a highly structured mass with outer cortical and inner
trabecular layers. The larger cortical layer has a high surface to
volume layer with haversian canals that provide nutrients and
gaps (lacunae) inhabited by bone cells that are connected by a
canaliculi network. The smaller trabecular layer has a much
higher surface to volume layer that relies on diffusion for nutri-
ents supply.
■ Regulated bone growth through puberty occurs through epiphy-
sial plates. These plates are located near the end of the bone shaft
and fuse with the shaft of the bone to cease linear bone growth.
■ Bone is constantly remodeled by osteoclasts, which erode and
absorb bone, and osteoblasts, which lay down new bone.MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.
- A patient with parathyroid deficiency 10 days after inadvertent
damage to the parathyroid glands during thyroid surgery would
probably have
A) low plasma phosphate and Ca2+ levels and tetany.
B) low plasma phosphate and Ca2+ levels and tetanus.
C) a low plasma Ca2+ level, increased muscular excitability, and
a characteristic spasm of the muscles of the upper extremity
(Trousseau sign).
D) high plasma phosphate and Ca2+ levels and bone demineral-
ization.
E) increased muscular excitability, a high plasma Ca2+ level,
and bone demineralization. - A high plasma Ca2+ level causes
A) bone demineralization.
B) increased formation of 1,25-dihydroxycholecalciferol.
C) decreased secretion of calcitonin.
D) decreased blood coagulability.
E) increased formation of 24,25-dihydroxycholecalciferol. - Which of the following is not involved in regulating plasma Ca2+
levels?
A) kidneys
B) skin
C) liver
D) lungs
E) intestine
4. 1,25-dihydroxycholecalciferol affects intestinal Ca2+ absorption
through a mechanism that
A) includes alterations in the activity of genes.
B) activates adenylyl cyclase.
C) decreases cell turnover.
D) changes gastric acid secretion.
E) is comparable to the action of polypeptide hormones.
5. Which of the following would you expect to find in a patient
whose diet has been low in calcium for 2 mo?
A) increased formation of 24,25-dihydroxycholecalciferol
B) decreased amounts of calcium-binding protein in intestinal
epithelial cells
C) increased parathyroid hormone secretion
D) a high plasma calcitonin concentration
E) increased plasma phosphates
6. In osteopetrosis, which of the following is defective?
A) phosphate deposition in trabecular bone
B) structure of parathyroid hormone related protein (PTHrP)
C) osteoblasts
D) osteoclasts
E) bone collagen
7. At epiphysial closure
A) cortical bone and trabecular bone merge.
B) osteoclasts and osteoblasts undergo differentiation.
C) there is an extended amount of proliferating cartilage that
contributes to bone elongation.
D) lacunae meet the trabecular bone.
E) ephyses unite with the shaft to end normal linear bone
growth.
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