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Questions and Answers
What percentage of adult body weight is comprised of calcium?
What percentage of adult body weight is comprised of calcium?
Which hormone is primarily responsible for increasing phosphate absorption in the intestines?
Which hormone is primarily responsible for increasing phosphate absorption in the intestines?
What is the primary protein to which calcium is bound in plasma?
What is the primary protein to which calcium is bound in plasma?
Where does the majority of calcium reabsorption occur in the kidneys?
Where does the majority of calcium reabsorption occur in the kidneys?
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What is the main function of osteoclasts in bone physiology?
What is the main function of osteoclasts in bone physiology?
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Which of the following physiological actions is NOT a function of calcium?
Which of the following physiological actions is NOT a function of calcium?
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What percentage of ionized calcium (Ca2+) is considered free and diffusible in plasma?
What percentage of ionized calcium (Ca2+) is considered free and diffusible in plasma?
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What is the role of the epiphyseal plate in bone growth?
What is the role of the epiphyseal plate in bone growth?
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What triggers the secretion of parathormone (PTH)?
What triggers the secretion of parathormone (PTH)?
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What effect does parathormone have on the kidneys?
What effect does parathormone have on the kidneys?
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Which condition is characterized by hypercalcemia and increased PTH levels due to a parathyroid tumor?
Which condition is characterized by hypercalcemia and increased PTH levels due to a parathyroid tumor?
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What occurs as a result of decreased glomerular filtration rate in chronic renal failure?
What occurs as a result of decreased glomerular filtration rate in chronic renal failure?
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What is the primary action of parathormone in bone tissue?
What is the primary action of parathormone in bone tissue?
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What is a potential consequence of hypoparathyroidism?
What is a potential consequence of hypoparathyroidism?
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Which form of Vitamin D is synthesized in the skin upon exposure to sunlight?
Which form of Vitamin D is synthesized in the skin upon exposure to sunlight?
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How does parathormone affect intestinal calcium absorption?
How does parathormone affect intestinal calcium absorption?
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What is the primary substance formed in the liver during the vitamin D activation process?
What is the primary substance formed in the liver during the vitamin D activation process?
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What condition can result from a deficiency of vitamin D in children?
What condition can result from a deficiency of vitamin D in children?
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What is the primary function of the hormone calcitonin?
What is the primary function of the hormone calcitonin?
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What effect does hypercalcemia have on nervous system activity?
What effect does hypercalcemia have on nervous system activity?
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In the context of calcitonin, what is hypercalcitoninemia primarily associated with?
In the context of calcitonin, what is hypercalcitoninemia primarily associated with?
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What effect can hypocalcemia have on the electrocardiogram (ECG)?
What effect can hypocalcemia have on the electrocardiogram (ECG)?
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Which of these factors can contribute to rickets and osteomalacia?
Which of these factors can contribute to rickets and osteomalacia?
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What is a characteristic effect of calcitonin on calcium and phosphate levels?
What is a characteristic effect of calcitonin on calcium and phosphate levels?
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Study Notes
Calcium Homeostasis & Balance
- Calcium (Ca) makes up 1.5% of the adult human body's weight
- 99% of Ca is found in bones
- 98-99% of filtered Ca++ in kidneys is reabsorbed, with 60% occurring in proximal tubules; less in ascending loop of Henle and distal tubules
- Plasma Ca++ is about 9-10.5 mg/dL
- Ca++ exists in three forms:
- 50% ionized calcium (Ca2+), freely diffusible
- 10% non-ionized/complexed with organic ions (citrate, phosphate)
- 40% protein-bound (mainly albumin), non-diffusible through capillary membranes
Functions and Health Benefits of Calcium
- Muscle contraction and nerve excitability
- Neurotransmitter and hormonal release
- Enzyme function
- Blood coagulation
- Maintenance of healthy bones and strong teeth
- Membrane integrity and permeability
Phosphorus
- About 90% of phosphorus (Pi) is found in the skeleton
- Plasma phosphates are about 2.48-4.34 mg/dL
- Pi absorption occurs in the duodenum and small intestine via active and passive diffusion
- Dihydroxycholecalciferol increases Pi absorption
- 85-90% of filtered Pi is reabsorbed in proximal tubules; active transport is inhibited by PTH
Bone Structure and Physiology
- Bone is a special form of connective tissue built from microscopic crystals of phosphates and calcium within a collagen matrix
- Osteoblasts and osteoclasts (both derived from bone marrow) are primarily involved in bone formation and resorption
- Bone growth happens at the ends of long bones (epiphysis) separated by the epiphyseal plate (actively proliferating cartilage)
- Bone remodeling is a local process, done in small areas by bone-remodeling units. Osteoclasts resorb bone, creating a tunnel; osteoblasts then lay down new bone in the same area in concentric layers (lamellae)
- Bone remodeling adjusts bone shape and strength in response to stress
Calcium Homeostasis
- Blood calcium regulation relies on:
- Principle organ systems: intestines, bone, kidneys
- Hormones: parathyroid hormone (PTH), vitamin D, calcitonin
Parathyroid Hormone (PTH)
- Four parathyroid glands located on the dorsal side of the thyroid gland
- Blood supply comes from thyroid arteries
- Chief cells secrete PTH
- Oxyphil cells (function unknown, possibly degenerated chief cells)
- PTH is a peptide hormone made from preproparathyroid hormone within the parathyroid glands
- PTH secretion is stimulated by low blood calcium levels; inhibited by high blood calcium levels (negative feedback)
Mechanism of Action (PTH)
- Bone: Increases osteoclast activity; increases bone resorption and calcium release
- Kidneys: Increases calcium reabsorption in renal tubules, promotes phosphate excretion, stimulates vitamin D conversion to calcitriol
- Intestines: Enhances intestinal calcium absorption indirectly via calcitriol
Regulation (PTH)
- Low blood calcium causes PTH release
- PTH increases blood calcium levels by:
- Ca++ efflux from bone
- Decreased loss of calcium in urine
- Enhanced calcium absorption from the intestine
Hyperparathyroidism
- Primary: Increased PTH in a functioning parathyroid tumor; characterized by hypercalcemia, hypophosphatemia, bone demineralization, hypercalciuria (excess Ca in urine), and kidney stones
- Secondary: In kidney disease or rickets; low Ca exerts feedback stimulation on PTH
Hypoparathyroidism
- PTH is essential. After parathyroidectomy, steady decline in plasma Ca can lead to hyper-excitability and hypocalcemia tetany. This can occur post-thyroid surgery.
Vitamin D
- Fat-soluble vitamin that functions as a hormone, exists in two forms:
- Vitamin D2 (ergocalciferol): From dietary sources/supplements
- Vitamin D3 (cholecalciferol): Synthesized in skin upon sunlight exposure
- Synthesis: Skin converts 7-dehydrocholesterol to cholecalciferol via UVB radiation
-
Activation:
- Liver converts vitamin D to 25-hydroxy cholecalciferol (calcidiol)
- Kidney converts calcidiol to 1,25-dihydroxy cholecalciferol (calcitriol, active form)
-
Clinical Significance:
- Deficiency: Rickets (children), osteomalacia (adults), increased fracture risk (inadequate sunlight/poor diet/malabsorption).
- Excess: Hypercalcemia and complications
Vitamin D Actions
- Bone: Increases bone resorption and Ca2+, phosphate
- Kidney: Increases Ca2+ and phosphate reabsorption
- Intestine: Increases Ca2+ and phosphate absorption; synthesis of calbindin D-28K.
Calcitonin
- Protein hormone produced by parafollicular cells (C cells) of the thyroid gland
- Secreted in response to hypercalcemia
- Inhibited by low calcium levels (negative feedback)
- Increased Ca++ levels increase calcitonin secretion; proportionate
-
Mechanism of Action:
- Direct (immediate): Inhibits osteoclast activity, inhibits bone resorption
- Indirect (prolonged): Reduces osteoclast formation
- Increased Ca++ and phosphate excretion in urine
Clinical Significance (Calcitonin)
- Hypercalcitoninemia: Can occur in thyroid carcinomas
- Monitoring: Levels assessed in contexts like thyroid tumors.
- Therapy: Used in osteoporosis to manage bone density
Hypocalcemia
- Decreased plasma calcium (causes muscle tetany, carpopedal/laryngeal spasm, ECG changes (ST-segment and prolonged QT interval)
Hypercalcemia
- Increased plasma calcium (causes depressed nervous system activity, sluggish reflexes, shortened QT in ECG, enhanced myocardial contractility, constipation, reduced appetite; predisposes to renal stone formation.)
Clinical Applications: Bone Diseases
- Rickets/Osteomalacia: Due to Ca deficiency, inadequate vitamin D, abnormal vitamin D receptors, inadequate sunlight exposure. Rickets in children, osteomalacia in adults
- Osteoporosis: Loss of bone mass and strength; matrix and minerals lost, incidences of fractures increases. Types include involutional (primary) osteoporosis (related to aging/menopause and estrogen), Lack of physical activity/malnutrition, Vit. C deficiency.
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Description
Test your understanding of calcium homeostasis and balance within the human body. This quiz covers essential facts about calcium's roles, its forms, and its health benefits, as well as information on phosphorus. Enhance your knowledge on how these minerals contribute to bodily functions and overall health.
