Bone Growth and Calcium Homeostasis
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Questions and Answers

What is the approximate percentage of the body's calcium stored in bones?

  • 99% (correct)
  • 10%
  • 50%
  • 1%

    • Which of these is NOT a function of bone?

  • Attachment site for muscles and nerve endings
  • Organ protection
  • Structural support
  • Hormone production (correct)

    • What type of bone is found in the ends of long bones?

  • Compact bone
  • Spongy bone
  • Cortical bone
  • Trabecular bone (correct)

    • Which of the following factors affects bone strength?

    <p>All of the above (D)</p> Signup and view all the answers

    What is the main predictor of fracture risk?

    <p>Bone mineral density (BMD) (C)</p> Signup and view all the answers

    When does bone loss typically begin in both men and women?

    <p>30s and 40s (A)</p> Signup and view all the answers

    What is the primary goal of bone remodeling?

    <p>Maintaining calcium levels and bone strength (A)</p> Signup and view all the answers

    What is considered the reference range for blood calcium levels?

    <p>8.5-10.5 milligrams per deciliter (C)</p> Signup and view all the answers

    Which of the following is NOT a risk factor for osteoporosis?

    <p>High calcium intake (D)</p> Signup and view all the answers

    What is the corrected calcium formula used for?

    <p>Adjusting for variations in serum albumin levels (D)</p> Signup and view all the answers

    Which of the following medications is NOT known to decrease bone mineral density?

    <p>Metformin (A)</p> Signup and view all the answers

    Which of these is NOT a symptom of hypercalcemia?

    <p>Tetany (B)</p> Signup and view all the answers

    What is the main reason estrogen deficiency leads to bone loss in post-menopausal women?

    <p>Increased osteoclast activity (D)</p> Signup and view all the answers

    Which type of osteoporosis is associated with specific lifestyle factors or medical conditions?

    <p>Secondary (D)</p> Signup and view all the answers

    What is the single best predictor of hip fracture in individuals with osteoporosis?

    <p>Bone mineral density of the hip (A)</p> Signup and view all the answers

    Which of these is NOT a recommendation for preventing osteoporosis?

    <p>Regular use of bisphosphonates (A)</p> Signup and view all the answers

    What is the recommended daily calcium intake for adults?

    <p>1000 mg (D)</p> Signup and view all the answers

    Which of the following is NOT a common cause of hypocalcemia?

    <p>Hyperthyroidism (C)</p> Signup and view all the answers

    What is the primary role of osteoclasts in bone remodeling?

    <p>To break down existing bone matrix (B)</p> Signup and view all the answers

    Which of the following is NOT a direct effect of parathyroid hormone on calcium levels?

    <p>Inhibits calcium absorption in the small intestine (C)</p> Signup and view all the answers

    Which cell type is responsible for initiating a new bone remodeling cycle?

    <p>Lining cells (B)</p> Signup and view all the answers

    What is the main function of osteoprotegerin (OPG)?

    <p>To act as a decoy for RANKL, preventing osteoclast formation (A)</p> Signup and view all the answers

    Which of the following hormones directly decreases circulating calcium levels?

    <p>Calcitonin (D)</p> Signup and view all the answers

    What is the primary mechanism by which vitamin D increases plasma calcium levels?

    <p>By promoting calcium absorption in the gastrointestinal tract (C)</p> Signup and view all the answers

    Which of the following is NOT a trigger for bone remodeling?

    <p>Low levels of vitamin D (B)</p> Signup and view all the answers

    When bone resorption exceeds bone formation, which of the following occurs?

    <p>Decreased bone mass (C)</p> Signup and view all the answers

    What is the role of RANKL in osteoclast formation?

    <p>It binds to RANK on osteoclast precursor cells, promoting their differentiation and survival (A)</p> Signup and view all the answers

    What is the primary source of vitamin D3 (cholecalciferol)?

    <p>Animal sources like fatty fish (A)</p> Signup and view all the answers

    What is the role of the paracellular pathway in calcium absorption?

    <p>It occurs between cells and is a passive process, particularly significant when calcium intake is high (D)</p> Signup and view all the answers

    Which of the following is NOT a mechanism by which parathyroid hormone increases calcium levels?

    <p>Inhibits the conversion of vitamin D to its active form (A)</p> Signup and view all the answers

    What is the primary function of the parathyroid glands?

    <p>To regulate blood calcium levels (C)</p> Signup and view all the answers

    How does calcitonin directly contribute to lowering circulating calcium levels?

    <p>By inhibiting osteoclast activity and reducing bone resorption (A)</p> Signup and view all the answers

    What is the primary effect of cinacalcet, a calcimimetic drug, in the treatment of secondary hyperparathyroidism?

    <p>To inhibit parathyroid hormone secretion (D)</p> Signup and view all the answers

    What is the primary mechanism by which parathyroid hormone acts rapidly to increase calcium levels within minutes?

    <p>By stimulating osteoblasts to pump calcium ions out of the fluid surrounding the bone and activate osteoclasts (C)</p> Signup and view all the answers

    Flashcards

    Calcium Homeostasis

    Regulation of calcium ion concentration in extracellular fluid.

    Functions of Bone

    Support, protect organs, muscle attachment, and serve as a mineral reservoir.

    Cortical Bone

    Solid, dense bone comprising about 80% of bone mass, found in long bones.

    Trabecular Bone

    Spongy, metabolically active bone found in vertebrae and ends of long bones.

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    Bone Mineral Density (BMD)

    A predictor of fracture risk; more density means stronger bones.

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    Bone Remodeling

    The continuous process of bone resorption and formation throughout life.

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    Peak Bone Strength

    Occurs between ages 18 to 25, when bone mass is highest.

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    Factors Affecting Bone Strength

    Calcium, Vitamin D, exercise, lifestyle practices, and hormonal status.

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    Hypocalcemia

    Low calcium levels below 8.5 mg/dL.

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    Hypercalcemia

    High calcium levels above 10.5 mg/dL.

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    Osteogenesis imperfecta

    A genetic disease that causes brittle bones and easy fractures.

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    Paget's disease

    A chronic disorder leading to enlarged and weakened bones.

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    Rickets

    Softening of bones in children due to vitamin D deficiency.

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    Osteoporosis

    A skeletal disorder characterized by compromised bone strength, increasing fracture risk.

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    Calcium correction formula

    Corrected calcium = measured serum calcium + 0.8 * (4 - serum albumin).

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    Bone Mineral Density (BMD) Measurement

    DXA scan is used to measure BMD and predict fracture risk.

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    Risk factors for Osteoporosis

    Includes low bone density, female gender, advanced age, and others.

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    Pharmacist's role in bone health

    Counseling on calcium intake, exercise, and risk reduction.

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    Osteoclast

    A cell responsible for breaking down bone tissue during bone remodeling.

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    Osteoblast

    A cell that forms new bone tissue, promoting bone growth.

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    RANKL

    A cytokine emitted by osteoblasts that stimulates osteoclast activation.

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    OPG

    A cytokine that acts as a decoy to prevent bone resorption by binding to RANKL.

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    Vitamin D

    A vitamin that plays a crucial role in calcium absorption and bone health.

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    Parathyroid Hormone (PTH)

    A hormone that increases blood calcium levels by stimulating bone resorption and kidney reabsorption.

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    Calcitonin

    A hormone produced by the thyroid that decreases blood calcium levels by inhibiting osteoclasts.

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    Osteocyte

    A former osteoblast located within the bone matrix that helps communicate bone remodeling needs.

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    Bone Resorption

    The process of breaking down bone tissue to release minerals into the bloodstream.

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    Calcium Absorption

    The process by which the body takes in calcium from food, influenced by vitamin D.

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    Secondary Hyperparathyroidism

    A condition where high levels of parathyroid hormone occur due to low calcium absorption.

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    Cytokines

    Proteins that act as signaling molecules in bone remodeling, influencing osteoclast and osteoblast activity.

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    Study Notes

    Bone Growth and Calcium Homeostasis

    • Calcium homeostasis is the regulation of calcium ion concentration in extracellular fluid.
    • Factors influencing calcium levels are absorption from diet, excretion in urine, and bone remodeling.
    • Bones store 99% of body's calcium, acting as a reservoir.
    • Calcium levels affect osteoporosis and fractures.

    Bone Physiology

    • Bone is dynamic and multifunctional tissue, supporting structure, protecting organs, and serving as a mineral reservoir (calcium and phosphorus).
    • 99% of calcium and 85% of phosphorus are in bones.
    • Calcium: 8.5-10.5 mg/dL; Phosphorus: 2.7-4.6 mg/dL (extracellular measurements).
    • Two types of bone:
      • Cortical bone: Dense, strong, stiff (80% bone mass); found in long bones.
      • Trabecular bone: Spongy, metabolically active; found in vertebrae and ends of long bones.
    • Bone composition: Collagen (flexibility, absorption) and minerals (stiffness, strength) are both crucial for preventing fractures.
    • Peak bone strength is 18-25 years old.
    • Modifiable factors affecting strength: calcium/vitamin D, exercise, lifestyle (smoking), hormonal status, diseases, medications.
    • Bone mineral density (BMD) predicts fracture risk; 10% bone mass decrease = 1.5-3x fracture risk.
    • Bone loss occurs when resorption exceeds formation, beginning in the 30s/40s, increasingly in older adults due to accelerated remodeling and reduced formation.
    • Bone strength is a better fracture predictor than BMD.

    Bone Remodeling

    • Ongoing throughout life, approximately 1-2 million tiny bone sections are remodeling simultaneously.
    • Goal: balance resorption and formation to maintain calcium levels and bone strength.
    • Triggers for remodeling: micro-damage repair, calcium homeostasis support, impact exercise.
    • Goals of remodeling: maintain serum calcium via bone calcium release, replace existing matrix.
    • Lower bone mass when resorption exceeds formation.

    Bone Remodeling Players

    • Hematopoietic stem cell: precursor to osteoclasts
    • Mesenchymal stem cell: precursor to osteoblasts
    • Osteoclast: resorbs bone
    • Osteoblast: forms bone
    • Osteocyte: communicates; former osteoblast within matrix; initiates remodeling
    • Lining cells: trigger new remodeling
    • RANKL: osteoblast/osteocyte cytokine; stimulates osteoclast activity and bone adherence
    • OPG: osteoblast cytokine; acts as a decoy to prevent resorption

    Steps of Bone Remodeling

    • Initiation: Lining cells/osteocytes signal (e.g., microfractures, calcium needs).
    • Osteoclast differentiation: Osteoblasts release cytokines; hematopoietic stem cells become mature osteoclasts.
    • Resorption: Activated osteoclasts break down bone.
    • Osteoblast differentiation: Osteoclasts' cytokines stimulate osteoblast differentiation from mesenchymal stem cells.
    • Bone formation: Mature osteoblasts inhibit osteoclasts; build/mineralize bone.
    • Quiescence: Formation stops; osteoblasts become lining cells/osteocytes, awaiting signals.

    RANK, RANKL, and OPG

    • RANK: Receptor on osteoclast surfaces
    • RANKL: Protein binding RANK; promotes osteoclast formation/function/survival
    • OPG: Decoy; binds RANKL; prevents RANKL-RANK binding; prevents osteoclast formation and bone resorption.

    Calcium Homeostasis and Hormones

    • Calcium homeostasis regulated by vitamin D, parathyroid hormone (PTH), and calcitonin.
    • Calcium absorbed in GI tract (30-35%; 10-15% with low vitamin D).
    • Vitamin D: Increases plasma calcium
    • PTH: Increases plasma calcium
    • Calcitonin: Decreases plasma calcium

    Vitamin D

    • Sources: UV light, plant/animal intake (Vitamin D2/D3).
    • PTH stimulates conversion to active form (1,25-dihydroxyvitamin D3 - Calcitriol).
    • Vitamin D levels measured via precursor (25-hydroxy vitamin D - Calcidiol).
    • Calcium absorption routes:
      • Paracellular: Passive between cells (high calcium intake)
      • Active: Through cells; influenced by calcitriol; uses calbindin transporter

    Kidney's Role in Calcium Homeostasis

    • Inadequate calcitriol leads to decreased absorption and hypocalcemia.
    • Hypocalcemia increases PTH, called secondary hyperparathyroidism.
    • Treatment: vitamin D/calcium supplements, calcimimetic drug (cinacalcet).
    • Kidney calcium reabsorption:
      • Proximal convoluted tubule: 60-70%
      • Loop of Henle: 20%
      • Distal tubules/collecting ducts: 15%
      • 98% filtered calcium reabsorbed.

    Parathyroid Hormone (PTH)

    • Released by low calcium levels;
    • Elevates blood calcium:
      • Stimulates bone resorption (calcium release)
      • Decreases urinary calcium loss (renal reabsorption)
      • Stimulates vitamin D activation (indirect calcium absorption)
    • Located on thyroid's dorsal side.
    • Rapid effects (minutes):
      • Stimulates osteoblasts to release calcium
      • Stimulates bone resorption through osteoclasts
      • Stimulates osteoblasts to produce signaling molecule activating osteoclasts

    Calcitonin

    • Produced by thyroid gland (high blood calcium);
    • Receptors in bones and kidneys
    • Lowers calcium/phosphate; inhibits osteoclasts (decreases bone resorption); increases calcium excretion

    Disorders of Bone and Calcium Homeostasis

    • Hypocalcemia: Low calcium (below 8.5 mg/dL)
    • Hypercalcemia: High calcium (above 10.5 mg/dL)
    • Osteogenesis imperfecta: Genetic, easy fractures
    • Paget's disease: Enlarged, weakened bones
    • Hyperparathyroidism (Primary/Secondary): Calcium regulation issues
    • Renal osteodystrophy: Kidney disease skeletal manifestations
    • Rickets: Bone softening in children (vitamin D deficiency)

    Interpretation of Lab Results

    • 99% calcium in bones/teeth; <1% extracellular.
    • Extracellular calcium status:
      • 50% ionized
      • 40% bound (90% to albumin)
      • 10% bound to other anions
    • Corrected calcium = measured serum calcium + 0.8 * (4 - serum albumin). -Hypocalcemia causes: hypoparathyroidism, vitamin D deficiency, renal disease
    • Symptoms: convulsions, arrhythmias, tetany, stridor/spasms -Hypercalcemia causes: hyperparathyroidism, malignancy
    • Symptoms: painful bones, renal stones, abnormal groans, psychic moans, excessive thirst, frequent urination, nausea, vomiting, constipation, bone pain, muscle weakness, confusion, lethargy, fatigue

    Osteoporosis

    • Compromised bone strength, increasing fracture risk.
    • Risk factors: Low BMD, female, age, Asian heritage, fragility fractures, low BMI, premature menopause, chronic steroid use, tobacco, alcohol, low calcium/vitamin D, low physical activity, recent falls, impaired cognition/vision.
    • Post-menopausal women: estrogen deficiency increases osteoclast activity.
    • Age-related: accelerated turnover, reduced osteoblast formation.
    • Types of osteoporosis in men: Primary/Secondary (lifestyle/diseases/medications, e.g., endocrine/hormonal, GI, inflammatory, etc).
    • Medications decreasing bone density: diuretics, antiretrovirals, anti-convulsants, aromatase inhibitors, SGLT2 inhibitors, heparin, etc.
    • BMD measurement: DXA scan of hip and spine.
    • T-score interpretation:

      • -1: Normal

      • -1 to -2.4: Osteopenia
      • < -2.5: Osteoporosis

    Pharmacist's Role

    • Counseling on risk reduction, Calcium/Vitamin D intake, optimal absorption, Calcium citrate for elderly/PPI/antacid users, exercise guidelines, fall prevention, avoiding tobacco/alcohol.
    • Discussing bone health and diet.

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    Description

    Explore the intricate mechanisms of bone growth and calcium homeostasis in this quiz. Understand how calcium levels are regulated in the body and their impact on bone strength and health. The quiz covers the types of bone tissue, their functions, and the importance of calcium and phosphorus.

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