Bone Physiology: Calcium & Phosphate Metabolism
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Questions and Answers

What is the general formula for hydroxyapatites in bone?

  • Ca10(PO4)6(OH)2 (correct)
  • Ca7(PO4)4(OH)3
  • Ca5(PO4)3(OH)
  • Ca8(PO4)5(OH)

    • What percentage of bone in the body is made up of trabecular bone?

  • 20% (correct)
  • 30%
  • 10%
  • 40%

    • What is the function of canaliculi in compact bone?

  • To receive nutrients (correct)
  • To facilitate bone resorption
  • To provide structural support
  • To allow for bone growth

    • What is the total blood flow to bone in adult humans?

    <p>200–400 mL/min</p> Signup and view all the answers

    What is the purpose of bone remodeling?

    <p>To respond to stresses and strains</p> Signup and view all the answers

    What is the function of compact bone?

    <p>To provide structural support</p> Signup and view all the answers

    What percentage of bone mass is being remodeled by bone-remodeling units in the human skeleton at any one time?

    <p>5%</p> Signup and view all the answers

    What is the primary function of free, ionized calcium in the body?

    <p>Cell signaling, nerve function, muscle contraction, and blood coagulation</p> Signup and view all the answers

    Where is the majority of calcium stored in the body?

    <p>Bones</p> Signup and view all the answers

    What is the role of calcitonin in calcium regulation?

    <p>Lowering calcium levels in the blood</p> Signup and view all the answers

    What is the rate of renewal for compact bone?

    <p>4% per year</p> Signup and view all the answers

    Which of the following is NOT a way in which phosphate levels are regulated?

    <p>Hormonal regulation of vitamin D levels</p> Signup and view all the answers

    What is the primary mechanism of nutrient supply in compact bone?

    <p>Haversian canals containing blood vessels</p> Signup and view all the answers

    What is the composition of collagen in bone matrix?

    <p>Over 90% type I collagen</p> Signup and view all the answers

    How many collagen genes have been identified so far?

    <p>Over 40</p> Signup and view all the answers

    What is the process called when cartilage is transformed into bone?

    <p>Enchondral bone formation</p> Signup and view all the answers

    Which bones are formed directly from mesenchymal cells?

    <p>Clavicles, mandibles, and certain skull bones</p> Signup and view all the answers

    What is the arrangement of collagen around each haversian canal?

    <p>Concentric layers forming cylinders</p> Signup and view all the answers

    What is the primary action of parathyroid hormone (PTH) on calcium levels?

    <p>Mobilize calcium from bone and increase urinary phosphate excretion</p> Signup and view all the answers

    What is the role of 1,25-Dihydroxycholecalciferol in calcium regulation?

    <p>Increasing calcium absorption in the intestine</p> Signup and view all the answers

    What is the function of the kidneys in calcium homeostasis?

    <p>To excrete excess calcium</p> Signup and view all the answers

    What is the significance of calcium in the body?

    <p>It is an essential intracellular signaling molecule</p> Signup and view all the answers

    What is the main target of parathyroid hormone (PTH) in the body?

    <p>The bones</p> Signup and view all the answers

    What is the primary function of the endocrine system in calcium regulation?

    <p>To regulate calcium levels in the blood</p> Signup and view all the answers

    What percentage of total plasma calcium is ionized?

    <p>10%</p> Signup and view all the answers

    What is the result of a decrease in extracellular Ca2+?

    <p>Hypocalcemic tetany</p> Signup and view all the answers

    What is the effect of hyperventilation on the symptoms of tetany?

    <p>It increases the symptoms of tetany</p> Signup and view all the answers

    What is the importance of knowing the plasma protein level when evaluating the total plasma calcium?

    <p>It affects the binding of Ca2+ by plasma proteins</p> Signup and view all the answers

    What is the role of Ca2+ in blood clotting?

    <p>It is essential for blood clotting</p> Signup and view all the answers

    What is the effect of laryngospasm on the airway?

    <p>It obstructs the airway</p> Signup and view all the answers

    What happens to plasma proteins when the pH is high?

    <p>They become more ionized, providing more protein anions to bind with Ca2+</p> Signup and view all the answers

    What is the approximate amount of Ca2+ moved into and out of the readily exchangeable pool in the bone per day?

    <p>500 mmol</p> Signup and view all the answers

    What regulates the overall transportation process of Ca2+ across the brush border of intestinal epithelial cells to the bloodstream?

    <p>1,25-dihydroxycholecalciferol</p> Signup and view all the answers

    What percentage of filtered Ca2+ is reabsorbed in the kidneys?

    <p>98-99%</p> Signup and view all the answers

    Where does about 60% of Ca2+ reabsorption occur?

    <p>Proximal tubules</p> Signup and view all the answers

    What regulates distal tubular reabsorption of Ca2+?

    <p>PTH</p> Signup and view all the answers

    What is the approximate plasma level of 1,25-dihydroxycholecalciferol?

    <p>0.03 ng/mL</p> Signup and view all the answers

    What is the effect of high plasma Ca2+ levels on the production of 1,25-dihydroxycholecalciferol?

    <p>It stimulates the production of 24,25-dihydroxycholecalciferol instead</p> Signup and view all the answers

    What is facilitated by 1,25-dihydroxycholecalciferol in the kidneys?

    <p>Ca2+ reabsorption</p> Signup and view all the answers

    What is the consequence of 1,25-dihydroxycholecalciferol stimulating osteoblasts?

    <p>An increase in osteoclast activity</p> Signup and view all the answers

    What is the role of renal 1α-hydroxylase in the regulation of calcium metabolism?

    <p>It catalyzes the formation of 1,25-dihydroxycholecalciferol</p> Signup and view all the answers

    What is the primary function of 1,25-dihydroxycholecalciferol in the intestine?

    <p>It increases Ca2+ absorption</p> Signup and view all the answers

    What percentage of total body phosphorus is found in the skeleton?

    <p>85-90%</p> Signup and view all the answers

    What is the approximate amount of phosphorus that normally enters bone per kilogram of body weight per day?

    <p>3 mg (97 μmol)</p> Signup and view all the answers

    What percentage of filtered Pi is reabsorbed in the proximal tubule?

    <p>85-90%</p> Signup and view all the answers

    What is the effect of PTH on NaPi-Iia?

    <p>It inhibits NaPi-Iia</p> Signup and view all the answers

    What is the form of phosphorus found in plasma?

    <p>Two-thirds organic compounds and one-third inorganic phosphorus</p> Signup and view all the answers

    What stimulates phosphate absorption in the duodenum and small intestine?

    <p>1,25-dihydroxycholecalciferol and other stimuli that increase Ca2+ absorption</p> Signup and view all the answers

    What stimulates calcitonin secretion in the thyroid gland?

    <p>Plasma calcium level above 9.5 mg/dL</p> Signup and view all the answers

    Which hormone does gastrin stimulate the secretion of?

    <p>Calcitonin</p> Signup and view all the answers

    What is the half-life of calcitonin in humans?

    <p>Less than 10 minutes</p> Signup and view all the answers

    In which organs are receptors for calcitonin found?

    <p>Bones and kidneys</p> Signup and view all the answers

    What is the effect of β-adrenergic agonists on calcitonin secretion?

    <p>Stimulation</p> Signup and view all the answers

    What is the consequence of elevated plasma calcium levels on calcitonin secretion?

    <p>Stimulation</p> Signup and view all the answers

    What is the primary function of bone in the body?

    <p>To protect vital organs and support locomotion</p> Signup and view all the answers

    What type of bone has a high surface-to-volume ratio and many cells sitting on the surface of the plates?

    <p>Trabecular bone</p> Signup and view all the answers

    What is the approximate percentage of bone in the body that is compact bone?

    <p>80%</p> Signup and view all the answers

    What is the total blood flow to bone in adult humans?

    <p>200-400 mL/min</p> Signup and view all the answers

    What is the formula for hydroxyapatites in bone?

    <p>Ca10(PO4)6(OH)2</p> Signup and view all the answers

    What is the process of bone resorption and formation permitting?

    <p>Bone remodeling</p> Signup and view all the answers

    What is the primary effect of high plasma Ca2+ levels on the production of 1,25-dihydroxycholecalciferol?

    <p>Decreased production of 1,25-dihydroxycholecalciferol</p> Signup and view all the answers

    What is the consequence of 1,25-dihydroxycholecalciferol facilitating Ca2+ reabsorption in the kidneys?

    <p>Decreased excretion of Ca2+ in the urine</p> Signup and view all the answers

    What is the role of 24,25-dihydroxycholecalciferol in calcium regulation?

    <p>It is the less active metabolite of vitamin D</p> Signup and view all the answers

    What is the effect of 1,25-dihydroxycholecalciferol on osteoblasts?

    <p>It stimulates the activity of osteoblasts</p> Signup and view all the answers

    What is the normal plasma level of 25-hydroxycholecalciferol?

    <p>30 ng/mL</p> Signup and view all the answers

    What is the role of renal 1α-hydroxylase in the regulation of calcium metabolism?

    <p>It converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol</p> Signup and view all the answers

    What is the consequence of having a high pH on plasma proteins?

    <p>They become more ionized</p> Signup and view all the answers

    What is the approximate amount of Ca2+ moved into and out of the readily exchangeable pool in the bone per day?

    <p>500 mmol</p> Signup and view all the answers

    What percentage of filtered Ca2+ is reabsorbed in the kidneys?

    <p>98-99%</p> Signup and view all the answers

    Which system regulates the overall transportation process of Ca2+ across the brush border of intestinal epithelial cells to the bloodstream?

    <p>1,25-dihydroxycholecalciferol system</p> Signup and view all the answers

    What is the primary location of Ca2+ reabsorption in the kidneys?

    <p>Proximal tubules and distal tubules</p> Signup and view all the answers

    What regulates distal tubular reabsorption of Ca2+?

    <p>Parathyroid hormone (PTH)</p> Signup and view all the answers

    What is the primary factor that determines the width of the epiphysial plate?

    <p>Pituitary growth hormone and insulin-like growth factor-I</p> Signup and view all the answers

    What is the outcome when the epiphyses unite with the shaft of the bone?

    <p>Linear bone growth ceases</p> Signup and view all the answers

    What is the purpose of radiographing the skeleton in young individuals?

    <p>To determine bone age</p> Signup and view all the answers

    What is the function of the periosteum in bones?

    <p>To cover the surface of bones</p> Signup and view all the answers

    What is the sequence of epiphysial closure?

    <p>Orderly temporal</p> Signup and view all the answers

    What is the consequence of cartilage cells becoming hypertrophic?

    <p>Secretion of vascular endothelial growth factor</p> Signup and view all the answers

    Study Notes

    Bone Structure and Composition

    • Bone is a special form of connective tissue with a collagen framework impregnated with Ca2+ and PO4(3–) salts, particularly hydroxyapatites.
    • Collagen, which is 90% type I, provides strength and structure to bone, and is made up of a triple helix of three polypeptides bound tightly together.
    • Two types of bone: compact or cortical bone (80% of bone in the body) and trabecular or spongy bone (20% of bone in the body).

    Bone Physiology

    • Bone is a living tissue that is well vascularized and has a total blood flow of 200–400 mL/min in adult humans.
    • Old bone is constantly being resorbed and new bone formed, permitting remodeling that allows it to respond to the stresses and strains that are put upon it.
    • Bone is involved in overall Ca2+ and PO4(3–) homeostasis, and protects vital organs, and provides support for locomotion and gravity.

    Calcium Metabolism

    • The free, ionized calcium (Ca2+) in the body fluids is a vital second messenger and is necessary for blood coagulation, muscle contraction, and nerve function.
    • Decrease in extracellular Ca2+ exerts a net excitatory effect on nerve and muscle cells, leading to hypocalcemic tetany.
    • Total plasma calcium is composed of diffusible and nondiffusible components, with the ionized calcium being the most important.
    • Calcium is regulated by the kidneys, which filter and reabsorb Ca2+, and the intestine, which absorbs Ca2+.

    Parathyroid Hormone (PTH)

    • PTH is secreted by the parathyroid glands and mobilizes calcium from bone and increases urinary phosphate excretion.
    • PTH regulates the reabsorption of Ca2+ in the proximal tubules and the distal tubule.

    Vitamin D

    • Vitamin D is a steroid hormone that is formed from vitamin D by successive hydroxylations in the liver and kidneys.
    • 1,25-Dihydroxycholecalciferol is the active form of vitamin D, which facilitates Ca2+ absorption from the intestine, reabsorption in the kidneys, and bone formation.
    • The formation of 1,25-dihydroxycholecalciferol is regulated by plasma Ca2+ and PO43+, and is increased by PTH.

    Calcitonin

    • Calcitonin is a Ca2+-lowering hormone produced by the parafollicular cells of the thyroid gland.
    • Calcitonin secretion is increased by high plasma calcium levels, β-adrenergic agonists, dopamine, and estrogens.
    • Calcitonin receptors are found in bones and the kidneys, and its actions are short-lived.

    Bone Remodeling

    • Bone remodeling is a continuous process of bone resorption and deposition, which allows the bone to respond to the stresses and strains that are put upon it.
    • Two independent but interacting homeostatic systems affect the calcium in bone: one that regulates plasma Ca2+ and the other that involves bone remodeling.

    Bone Growth

    • Bone growth occurs at the epiphyses, which are separated from the shaft of the bone by a plate of actively proliferating cartilage.
    • The width of the epiphysial plate is proportional to the rate of growth, and is affected by several hormones, including pituitary growth hormone and IGF-I.
    • Linear bone growth ceases after the epiphyses unite with the shaft (epiphysial closure).

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