Calcium Balance Flashcards
Effects of Hypercalcemia
Excitable cells, such as neurons, are sensitive to changes in calcium ion concentrations Increases in calcium ion above normal cause progressive depression of the nervous system. •Symptoms begin to appear when the blood calcium level rises above 12 mg/dl. •Reflex activities of the nervous system are sluggish. •There is constipation and lack of appetite. •Decreases in calcium concentration cause the nervous system to become more excited
Storage of calcium in the body
•0.1 percent of the total body calcium is in the extracellular fluid •1 percent is in the cells and organelles •The rest is stored in the bones. even slight changes in extracellular calcium can cause extreme immediate physiological effects
Storage of Phosphate
•85 percent of the total body phosphate is stored in bones. •14 to 15 percent is in the cells. •Less than 1 percent is in the extracellular fluid. •Extracellular phosphate concentration is not nearly as well regulated as that of calcium. •Inorganic phosphate in the plasma is mainly in two forms: HPO-4and H2PO-4. Large changes in the level of phosphate in the extracellular fluid do not cause major immediate effects on the body.
Hypocalcemia
Causes nervous system excitement and tetany. •The nervous system becomes progressively more excitable: •Due to increased neuronal membrane permeability to sodium ions •Hypocalcemia may also cause seizures. •The first sign of tetany typically occurs in the hand, resulting in carpopedal spasm.
Tetany
Ordinarily occurs when the blood concentration of calcium falls from its normal level of 9.5 mg/dl to about 6 mg/dl (35% below normal calcium concentration). It is usually lethal at about 4 mg/dl.
Absorption and Excretion of Calcium and Phosphate
Usual rates of intake are about 1000 mg/day each for calcium and phosphorous. Normally, divalent ions are poorly absorbed, but vitamin D promotes calcium absorption by the intestines, so that about 35% of ingested calcium is absorbed. Calcium not absorbed is excreted in the feces. •About 250 mg/day of the absorbed calcium enters intestines via secreted GI juices and sloughed mucosal cells. •Thus, about 90 percent of the daily intake of calcium is excreted.
Renal Excretion of Calcium and Phosphate
•About 100 mg/day (10 percent) of the ingested calcium is excreted in the urine. •The 41 percent of the plasma calcium bound to plasma proteins is not filtered by glomerular capillaries. •The rest is combined with anions or ionized and filtered through the glomeruli. •The renal tubules reabsorb about 99 percent of the calcium in the filtrate. •Renal phosphate excretion is controlled by an over-flow mechanism. •PTH can greatly increase phosphate excretion.
In what tissue do hydroxyapatite crystals form?
Only Bone -This is an exception to normal tissues. inhibitors are present in other tissues to prevent the formation or precipitation. Eg- pyrophosphate
pyrophosphate
an inhibitor of hydroxyapatite crystal precipitation (bone formation) in bone, an inhibitor of pyrophosphate is used so crystals can accumulate
What happens in early bone production?
osteoblasts secrete collagen monomers which polymerize rapidly to form collagen fibers. the precipitation of calcium along these fibers eventually forms the hydroxyapatite crystals.
How long does it take for calcium salts in amorphous r noncrystalline form to be converted to hydroxyapatite crystals?
weeks or months. some never change –> can be absorbed rapidly when there is a need for calcium
Do calcium salts normally precipitate in other tissues than bone?
only under abnormal conditions. when inhibitor factors normally present in said tissue disappear and the result is deposition of calcium salts.
What happens with calcium injections? (IV drip)
Salt levels may initially rise dramatically, but return to normal levels within 30-60 min
What happens if large amounts of calcium ions are REMOVED from circulating body fluids?
the ion concentration should return to normal within 30ish min
Why do calcium levels regulate fairly quickly in circulating fluids?
Bone contains exchangeable calcium that is always in equilibrium with the calcium ions in extracellular fluids. a small portion is also found in all tissue cells, especially in highly permeable cells in liver or GI tract. The presence of rapidly exchangeable calcium presents a buffer system.
Compare effects of changing levels of phosphate to that of calcium.
Large changes in the level of phosphate in the extracellular fluid do not cause major immediate effects on the body. Conversely, even slight changes in extracellular calcium can cause extreme immediate physiological effects.
List 3 hormones that control hormone balance
parathyroid hormone calcitonin
where are most vit D receptors found?
most cells of body, mainly on nuclei of target cells have hormone binding and DNA binding domains forms a complex, sometimes suppresses transcription
Diagram the synthetic pathway of vit D and explain the involvement of parathyroid hormone.
parathyroid hormone is a negative feedback inhibitor
Describe the mechanism by which vit D promotes intestinal absorption of calcium.
By increasing, over a period of about 2 days, formation of calbindin, a calcium binding protein, in the intestinal epithelial cells. This protein functions in the brush border of the epithelial cells to transport calcium into the cell cytoplasm. •Then the calcium moves through the basolateral membrane of the cell by facilitated diffusion. Calbindin remains in the cell for weeks after the 1,25-dihydroxycholecalciferol has been removed from the body. OR Formation of: •A calcium-stimulated ATPase in the brush border of the epithelial cells •An alkaline phosphatase in the epithelial cells. OR Vitamin D also increases calcium and phosphate reabsorption by the epithelial cells of the renal tubules. •This is a weak effect
Describe and explain the effects of administering small quantities of vit D versus extreme quantities of vit D.
Vitamin D in smaller quantities promotes bone calcification by increasing calcium and phosphate absorption from the intestines. The administration of extreme quantities of vitamin D causes absorption of bone. •In the absence of vitamin D, the effect of PTH in causing bone absorption is greatly reduced or even prevented. •The mechanism of this action of vitamin D is believed to result from the effect of 1,25-dihydroxycholecalciferol to increase calcium transport through cellular membranes
Rickets
Rickets is the softening and weakening of bones in children, usually because of an extreme and prolonged vitamin D deficiency. Vitamin D promotes the absorption of calcium and phosphorus from the gastrointestinal tract.
pain or tenderness in the bones of the arms, legs, pelvis, or spine.
stunted growth and short stature.
bone fractures.
muscle cramps.
teeth deformities, such as: delayed tooth formation. holes in the enamel. …
skeletal deformities, including: an oddly shaped skull. bowlegs, or legs that bow out.
Hypoparathyroidism
Occurs when parathyroid glands do not secrete sufficient PTH. •Osteoclasts become almost totally inactive. •Calcium reabsorption from the bones is so depressed that blood calcium levels decrease. •When the parathyroid glands are removed suddenly, the calcium level in the blood falls from the normal 9.4 mg/dl to 6 to 7 mg/dl within 2 to 3 days and the blood phosphate concentration may double. •Signs of tetany begin to develop at this level. •The laryngeal muscles are especially sensitive to tetanic spasms, which is the usual cause of death
List the effects of PTH on osteoclasts
The osteoclasts do not have membrane receptor proteins for PTH. •activated osteoblasts and osteocytes send secondary “signals” to the osteoclasts. •A major secondary signal is osteoprotegerin ligand, which activates receptors on preosteoclast cells and transforms them into mature osteoclasts that remove bone over a period of weeks or months. Activation of the osteoclastic system occurs in two stages: 1.Immediate activation of the osteoclasts that are already formed and 2.Formation of new osteoclasts
