Flashcards in MEH - Calcium Metabolism Deck (29)
Where is most calcium stored in the body?
In the skeleton
How much calcium is stored in the average adult human?
Around 1000g - 99% of this is in the bones in hydroxyapatite crystals
How does the skeleton help with calcium metabolism?
It helps to buffer serum calcium levels and releases calcium phosphate into the interstitium
How does parathyroid hormone affect calcium levels? (It is active when serum calcium is low)
- stimulates bone resorption and release of calcium into circulation
- stimulates calcium resorption in kidney
How does dietary vitamin D affect calcium levels?
It increases intestinal absorption of dietary calcium and renal reabsorption of calcium, and increases bone resorption
What is the role of calcitonin?
It counteracts the effects of PTH
What do chief cells in the parathyroid gland do?
Produce parathyroid hormone
How long is the half life of parathyroid hormone?
What is the structure of PTH?
It is a straight chain polypeptide hormone
Why must PTH be continually synthesised?
Chief cells degrade the hormone as well as synthesising it, so it is not stored
What are the actions of PTH on bone?
- induces osteoblastic cells to synthesise and secrete cytokines on the cell surface
- cytokines stimulate differentiation and activity in osteoclasts and protect from apoptosis
- PTH decreases osteoblast activity
- resorption of mineralised bone and release of calcium and phosphate into extracellular fluid
What is the typical dietary calcium intake per day?
What is vitamin D?
Lipid soluble vitamin which comes in two forms - D3 (made in skin and from dairy), and D2 (from yeast and fungi)
Where does vitamin D come from?
Can be derived from plants or the action of sunlight on cholesterol in the skin
What does vitamin D3 bind to in the circulation?
What is the half life of vitamin D3?
What is the main action of calcitriol?
Decreases the urinary loss of calcium by stimulating reabsorption in the kidney
Where is calcitonin secreted?
Parafollicular cells of the thyroid gland
How is calcitonin used clinically?
To lower serum calcium in cases of hypercalcaemia
True or false - calcium is not involved in blood clotting?
False - it is factor IV in the clotting cascade
Give some causes of hypercalcaemia in a hospital setting
Malignant osteocytic bone metastases, multiple myeloma
Give some common cancers which can metastasise to bone causing hypercalcaemia
Breast, lung, renal, thyroid
What causes primary hyperparathyroidism?
One of the four parathyroid glands develops an adenoma and secretes excessive parathyroid hormone, causing serum calcium to rise
What are the symptoms of primary hyperparathyroidism?
Bones, (kidney) stones, abdominal groans and 'psychic moans'
What are the effects of hypercalcemia on neurones?
Lethargy, confusion, coma due to suppression of neuronal activity
What are the effects of hypocalcemia on nerves?
Leads to 'excitable' nerves because it lowers the threshold for depolarisation
Why is it important to keep patients who have severe hypercalcemia hydrated?
The polyuria can lead to dehydration which exacerbates the hypercalcemia
What is osteomalacia?
The ratio of mineral to matrix is decreased, so there is not enough mineral content. It leads to soft bones which are prone to bending.