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Questions and Answers
What is converted to the active form of vitamin D in the kidneys?
What is converted to the active form of vitamin D in the kidneys?
Which consequence is primarily associated with vitamin D deficiency in children?
Which consequence is primarily associated with vitamin D deficiency in children?
What mechanism does calcitonin use to decrease plasma calcium levels?
What mechanism does calcitonin use to decrease plasma calcium levels?
What clinical condition can result from hypercalcemia?
What clinical condition can result from hypercalcemia?
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Which hormone is primarily secreted in response to elevated calcium levels?
Which hormone is primarily secreted in response to elevated calcium levels?
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What does hypocalcemia primarily lead to in terms of neuromuscular symptoms?
What does hypocalcemia primarily lead to in terms of neuromuscular symptoms?
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What can cause abnormal calcium levels in patients?
What can cause abnormal calcium levels in patients?
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Which of the following is NOT a consequence of vitamin D deficiency?
Which of the following is NOT a consequence of vitamin D deficiency?
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Which of the following statements about the forms of calcium in plasma is correct?
Which of the following statements about the forms of calcium in plasma is correct?
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What role does parathyroid hormone (PTH) play in calcium homeostasis?
What role does parathyroid hormone (PTH) play in calcium homeostasis?
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Which statement accurately describes the process of bone remodeling?
Which statement accurately describes the process of bone remodeling?
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What is the primary physiological function of calcium in the human body?
What is the primary physiological function of calcium in the human body?
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How do dietary phosphorus levels affect its absorption in the human body?
How do dietary phosphorus levels affect its absorption in the human body?
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Which of the following hormones primarily decreases blood calcium levels?
Which of the following hormones primarily decreases blood calcium levels?
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Which location in the kidneys is responsible for the majority of calcium reabsorption?
Which location in the kidneys is responsible for the majority of calcium reabsorption?
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What is the role of 1,25 dihydroxycholecalciferol in the body?
What is the role of 1,25 dihydroxycholecalciferol in the body?
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What triggers the secretion of parathormone (PTH)?
What triggers the secretion of parathormone (PTH)?
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What is one primary consequence of primary hyperparathyroidism?
What is one primary consequence of primary hyperparathyroidism?
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Which effect does parathormone have on the kidneys?
Which effect does parathormone have on the kidneys?
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What condition results from the hyper-excitability of excitable tissues due to decreased calcium levels after parathyroidectomy?
What condition results from the hyper-excitability of excitable tissues due to decreased calcium levels after parathyroidectomy?
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What is the role of vitamin D in calcium absorption?
What is the role of vitamin D in calcium absorption?
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What process is primarily affected by chronic renal failure in relation to phosphate levels?
What process is primarily affected by chronic renal failure in relation to phosphate levels?
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Which vitamin D form is synthesized in the skin upon exposure to sunlight?
Which vitamin D form is synthesized in the skin upon exposure to sunlight?
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Which statement regarding secondary hyperparathyroidism is correct?
Which statement regarding secondary hyperparathyroidism is correct?
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Study Notes
Calcium Homeostasis & Balance
- Calcium is 1.5% of adult human body weight
- 99% of calcium is in bones
- 98-99% of filtered calcium in the kidney is reabsorbed, with 60% occurring in the proximal tubule
- Plasma calcium is approximately 9-10.5 mg/dL and exists in three forms:
- 50% ionized calcium (Ca2+), freely diffusible
- 10% non-ionized or complexed with organic ions (citrate, phosphate)
- 40% protein-bound calcium (primarily to albumin), non-diffusible through capillary membranes
Calcium Functions
- Muscle contraction and nerve excitability
- Neurotransmitter and hormonal release
- Enzyme function
- Blood coagulation
- Maintaining healthy bones and strong teeth
- Maintaining membrane integrity and permeability
Phosphorus
- About 90% of phosphorus is found in the skeleton
- Plasma phosphates are approximately 2.48-4.34 mg/dL
- Phosphorus absorption occurs in the duodenum and small intestine via active transport and passive diffusion
- Absorption is directly correlated with dietary intake
- Vitamin D increases phosphorus absorption
- 85-90% of filtered phosphorus is reabsorbed in the proximal tubules
- Parathyroid hormone (PTH) inhibits phosphorus absorption
Bone Structure and Physiology
- Bone is a connective tissue composed of microscopic calcium phosphate crystals in a collagen matrix
- Osteoblasts and osteoclasts, derived from bone marrow, are primarily involved in bone formation and resorption
- Bone growth occurs at the ends of long bones (epiphyses), separated from the shaft by epiphyseal plates
- Bone remodeling is a local process where bone is resorbed and replaced by osteoclasts and osteoblasts, respectively, in concentric circles (lamellae)
- Remodeling adjusts bone shape and strength in response to stress
Calcium Homeostasis Regulation
- Blood calcium is regulated by:
- Principle organ systems: intestines, bone, kidney
- Hormones: parathyroid hormone (PTH), vitamin D, calcitonin
Parathyroid Hormone (PTH)
- There are four parathyroid glands on the dorsal side of the thyroid
- Blood supply to the glands comes from thyroid arteries
- Chief cells secrete PTH
- Oxyphil cells' function is unknown and they are possibly degenerated chief cells
- PTH is a peptide hormone derived from preproparathyroid hormone
- PTH secretion is stimulated by low blood calcium levels and inhibited by high blood calcium levels
PTH Mechanism of Action
- Bone: Stimulates osteoclast activity, increasing bone resorption and calcium release
- Kidney: Increases calcium reabsorption in renal tubules, promotes phosphate excretion, and stimulates Vitamin D conversion to calcitriol
- Intestines: Enhances intestinal calcium absorption indirectly via calcitriol
Vitamin D
- Fat-soluble vitamin that functions as a hormone
- Two forms: vitamin D2 (ergocalciferol), vitamin D3 (cholecalciferol)
- Vitamin D2 comes from dietary sources and supplements
- Vitamin D3 is synthesized in the skin upon exposure to sunlight
Vitamin D Synthesis and Activation
- Synthesis: 7-dehydrocholesterol converts to cholecalciferol via UVB radiation in the skin
- Activation:
- Liver: Converts vitamin D to 25-hydroxy cholecalciferol (calcidiol).
- Kidney: Converts calcidiol to 1,25-dihydroxy cholecalciferol (calcitriol).
Vitamin D Actions
- Bone: Promotes bone resorption and increases calcium and phosphate release
- Kidney: Increases calcium and phosphate reabsorption
- Intestine: Increases calcium and phosphate absorption
Calcitonin
- Protein hormone produced by parafollicular cells (C cells) of the thyroid gland
- Secreted in response to hypercalcemia
- Inhibitory feedback by low calcium levels
- Calcitonin secretion increases with increasing calcium levels
- Calcitonin receptor mainly in bones and kidney
Calcitonin Mechanism of Action
- Direct effect: Inhibits osteoclast activity inhibiting bone resorption
- Indirect effect: Reduces the formation of new osteoclasts
- Increases calcium and phosphate excretion in urine
Clinical Significance
- Deficiency: Inadequate sunlight exposure or poor dietary intake, leading to rickets in children and osteomalacia in adults
- Excess: Can result in hypercalcemia and complications
- Hypocalcemia: A decrease in plasma calcium, leading to muscle tetany and laryngeal spasm
- Hypercalcemia: An increase in plasma calcium associated with nervous system depression, ECG changes, and increased risk for kidney stones
Bone Diseases
- Rickets and osteomalacia: Caused by vitamin D deficiency
- Osteoporosis: Loss of bone mass and strength, with increased fracture risk; may be related to age, menopause, lack of activity, malnutrition, Vitamin C deficiency, or excess glucocorticoids
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Description
Test your knowledge on calcium homeostasis and its vital functions in the human body. This quiz covers calcium balance, its presence in bones, and various physiological roles including muscle contraction and blood coagulation. Additionally, you'll explore the role of phosphorus in the body.
