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EndocrinE ` endocrine—Physiology EndocrinE ` endocrine—Physiology SEcTion iii 333
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Calcium homeostasis Plasma Ca exists in three forms: pH (less H ) albumin binds more
2+
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Ionized/free (~ 45%, active form) Ca ionized Ca (eg, cramps, pain,
Bound to albumin (∼ 40%) paresthesias, carpopedal spasm) PTH
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Bound to anions (∼ 15%) pH (more H ) albumin binds less Ca
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ionized Ca PTH
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Ionized/free Ca is 1° regulator of PTH;
changes in pH alter PTH secretion, whereas
changes in albumin concentration do not
Calcitonin
soUrce Parafollicular cells (C cells) of thyroid. Calcitonin opposes actions of PTH. Not
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important in normal Ca homeostasis
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FUnction bone resorption of Ca . Calcitonin tones down serum Ca levels and
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regUlAtion serum Ca calcitonin secretion.
keeps it in bones
Glucagon
soUrce Made by α cells of pancreas.
FUnction Promotes glycogenolysis, gluconeogenesis, lipolysis, ketogenesis. Elevates blood sugar levels to
maintain homeostasis when bloodstream glucose levels fall too low (ie, fasting state).
regUlAtion Secreted in response to hypoglycemia. Inhibited by insulin, hyperglycemia, somatostatin.
FAS1_2019_08-Endocrine.indd 333 11/7/19 4:30 PM
