Podcast
Questions and Answers
What effect does hypocalcaemia have on parathyroid hormone (PTH) secretion?
What effect does hypocalcaemia have on parathyroid hormone (PTH) secretion?
Which condition is most commonly associated with hypercalcaemia?
Which condition is most commonly associated with hypercalcaemia?
What is a common consequence of hypophosphataemia?
What is a common consequence of hypophosphataemia?
In cases of hyperphosphataemia, which physiological process is typically inhibited?
In cases of hyperphosphataemia, which physiological process is typically inhibited?
Signup and view all the answers
Which disorder results in bone and abdominal pain when severe?
Which disorder results in bone and abdominal pain when severe?
Signup and view all the answers
How does hypocalcaemia affect muscle activity?
How does hypocalcaemia affect muscle activity?
Signup and view all the answers
What is an effect of calcitriol in hypophosphataemia?
What is an effect of calcitriol in hypophosphataemia?
Signup and view all the answers
Which of these is a less common cause of hypercalcaemia?
Which of these is a less common cause of hypercalcaemia?
Signup and view all the answers
Which of the following statements about calcium in the body is true?
Which of the following statements about calcium in the body is true?
Signup and view all the answers
What is the primary form of calcium present in plasma?
What is the primary form of calcium present in plasma?
Signup and view all the answers
How does alkalosis affect calcium levels in the plasma?
How does alkalosis affect calcium levels in the plasma?
Signup and view all the answers
Which mechanism is essential for bone formation?
Which mechanism is essential for bone formation?
Signup and view all the answers
What happens to ionized calcium levels in acidosis?
What happens to ionized calcium levels in acidosis?
Signup and view all the answers
Which anticoagulant should not be used for measuring ionized calcium?
Which anticoagulant should not be used for measuring ionized calcium?
Signup and view all the answers
What vital role does alkaline phosphatase serve during childhood?
What vital role does alkaline phosphatase serve during childhood?
Signup and view all the answers
What is the impact of changes in plasma albumin concentration on calcium measurement?
What is the impact of changes in plasma albumin concentration on calcium measurement?
Signup and view all the answers
What is the corrected calcium formula when plasma albumin is less than 4 g/dL?
What is the corrected calcium formula when plasma albumin is less than 4 g/dL?
Signup and view all the answers
Which hormone primarily increases plasma calcium concentration in response to low ionized calcium levels?
Which hormone primarily increases plasma calcium concentration in response to low ionized calcium levels?
Signup and view all the answers
What is the reference range for total calcium in mg/dL?
What is the reference range for total calcium in mg/dL?
Signup and view all the answers
Which action does parathyroid hormone (PTH) NOT perform?
Which action does parathyroid hormone (PTH) NOT perform?
Signup and view all the answers
What is the primary function of calcitriol in the body?
What is the primary function of calcitriol in the body?
Signup and view all the answers
What is the role of calcitonin in calcium homeostasis?
What is the role of calcitonin in calcium homeostasis?
Signup and view all the answers
What effect does mild hypomagnesaemia have on parathyroid hormone (PTH) secretion?
What effect does mild hypomagnesaemia have on parathyroid hormone (PTH) secretion?
Signup and view all the answers
How is calcitriol formed in the body?
How is calcitriol formed in the body?
Signup and view all the answers
Study Notes
Calcium
- Most abundant mineral in the human body
- Plays structural role in bones and teeth
- Essential for muscle contraction
- Affects excitability of nerves
- Second messenger, involved in the action of hormones
- Required for blood coagulation
Bone
- Composed of collagenous organic matrix (osteoid)
- Contains inorganic hydrated calcium salts (hydroxyapatites)
- Continuously remodeled through bone resorption (osteoclasts) and bone formation (osteoblasts)
- Bone formation requires calcium and phosphate
- Alkaline phosphatase, secreted by osteoblasts, is essential for phosphate release from pyrophosphate
- Alkaline phosphatase levels are higher in children than in adults
Plasma Calcium
- Exists in three forms:
- Protein bound (45%): mainly bound to albumin, not diffusible
- Complexed with citrate and phosphate (7%): diffusible
- Free ions (ionized) (47%): physiologically active, maintained by homeostatic mechanisms
- Ionized calcium is affected by alkalosis and acidosis
- Alkalosis: increases calcium binding to albumin, leading to decreased ionized calcium, possibly causing hypocalcemia despite normal total calcium levels
- Acidosis: has the reverse effect, increasing ionized calcium
- Ionized calcium is measured using an ion-selective electrode, requiring exclusion of air from the sample and tight capping of the container
- Citrate, oxalate, and EDTA should not be used as anticoagulants for measuring ionized calcium, as they can bind to calcium and decrease its concentration
- Heparin is used as an anticoagulant
- Low heparin syringes contain 2 u/ml of heparin
- Calcium titrated heparin syringes are also used
- Changes in plasma albumin concentration affect total calcium concentration, leading to misinterpretation of results in hypoproteinemic and hyperproteinemic states
- Corrected calcium concentration is used in these states:
- If plasma albumin is < 4 g/dL, corrected Ca = Ca + 0.8 (4 – serum alb.)
- If albumin is > 4.5 g/dL, corrected Ca = Ca – 0.8 (serum alb.– 4.5)
- 1 g albumin in 100 ml serum binds 0.8 mg of Ca/dL
Reference Ranges
- Albumin: 3.5 – 5 g/dL = 35 – 50 g/L
- Total Ca: 8.5 – 10.2 mg/dL = 2-2.5 mmol/L
- Ionized Ca+: 4.6 – 5.3 mg/dL = 1.2 – 1.32 mmol/L
- Total phosphate (adult): 2.5 – 4.5 mg/dL = 0.8 – 1.5 m mol/L
- Phosphate (children): 4-7 mg/dl
Calcium Homeostasis
- Maintained by two hormones: parathyroid hormone (PTH) and calcitriol (1.25 – dihydroxycholecalciferol)
- Calcitonin has a minor role
Parathyroid Hormone (PTH)
- Secreted by the parathyroid glands in response to a fall in plasma (ionized) calcium
- Inhibited by hypercalcemia
- Acts on bone and kidneys, increasing plasma calcium concentration and reducing phosphate
- Actions of PTH:
- Rapid release of Ca from bone: plasma Ca+
- osteoclastic resorption: plasma Ca+
- calcium reabsorption: plasma Ca+
- phosphate reabsorption: plasma Pi
- 1-hydroxylation of 25-hydroxycholcalciferol: Ca and PI absorption from gut
- bicarbonate reabsorption: → acidosis
- Calcitriol inhibits PTH synthesis
- PTH stimulates the formation of calcitriol
- Changes in phosphate concentration do not directly affect PTH secretion
- Mild hypomagnesaemia stimulates PTH secretion, but severe hypomagnesaemia reduces it (PTH secretion is magnesium dependent)
- PTH is metabolized in the liver and kidneys
Calcitriol
- Derived from vitamin D after successive hydroxylation in the liver (25-hydroxylation) and kidney (1 hydroxylation)
- Actions of calcitriol:
- Stimulates absorption of dietary calcium and phosphate in the gut
- Promotes bone mineralization through the maintenance of ECF calcium and phosphate concentrations (at normal concentrations)
- At high concentrations, stimulates osteoclastic bone resorption, releasing Ca and Pi into the ECF
- Inhibits its own synthesis in the kidneys
Calcitonin
- Polypeptide hormone secreted by C-cells of the thyroid gland
- Secreted when plasma calcium concentration rises, and also in response to certain gut hormones
- Its physiological role is uncertain; subjects with thyroidectomy do not develop a clinical syndrome due to calcitonin deficiency
- Calcium homeostasis is normal in patients with medullary carcinoma of the thyroid (tumor secretes calcitonin)
Calcium and Phosphate Homeostasis
Hypocalcaemia
- Stimulates the secretion of PTH and increases the production of calcitriol
- Increases the uptake of both calcium and phosphate from the gut, and their release from bone
- Phosphaturia (by PTH) and calcium reabsorption by the kidney, leading to serum calcium (becomes normal)
Hypophosphataemia
- Increased secretion of calcitriol, but not PTH secretion
- Any tendency of plasma calcium by calcitriol should inhibit PTH secretion
- Calcium and phosphate absorption from the gut is stimulated
- Calcitriol has a much smaller effect on renal calcium reabsorption than PTH, so in the absence of PTH, excess Ca absorbed from the gut is excreted in the urine
- The net outcome is restoration of phosphate concentration to normal, independently of that of calcium
Disorders of Calcium, Phosphate, and Magnesium Metabolism
Hypercalcaemia
- Common causes:
- Primary hyperparathyroidism due to parathyroid adenoma or hyperplasia
- Malignant disease, with or without metastasis to bone, including myeloma (due to secretion of calcium-mobilizing substances by tumor cells)
- Less common causes:
- Sarcoidosis
- Overdosage with vitamin D or its derivatives
- Mild hypercalcemia is often asymptomatic, but when more severe, clinical features may include:
- Bone and abdominal pain
- Renal calculi
- Polyuria
- Thirst
- Behavioral disturbances
Hypocalcaemia
- Causes increased excitability of nerve and muscle, leading to muscle spasm (tetany) and in severe cases, convulsions
- Causes:
- Vitamin D deficiency
- Hypoparathyroidism
- Vitamin D deficiency may be:
- Dietary in origin, often increased by poor exposure to sunlight
- Due to malabsorption
Hyperphosphataemia
- Associated with renal failure
- Inhibits vitamin D metabolism (inhibits 1 hydroxylation to 25-hydroxycholcalcifrol in the kidney), which can cause hypocalcemia
Hypophosphataemia
- Occurs with inadequate phosphate provision during intravenous feedings
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
Test your knowledge on the vital roles of calcium in the human body and its impact on bone health. This quiz covers the different forms of plasma calcium, the structural composition of bones, and the essential functions of calcium in physiological processes. Challenge yourself and learn more about this abundant mineral!
