What are the effects of hypercalcemia?
-increases in Ca above normal progressive depression of the nervous system
+symptoms appear when the bloof Ca level rises above 12mg/dL
+reflex activities of the nervous system are sluggish
+there is constipation and lack of appetite
Decreases in calcium concentration causes the nervous system to become more __________.
Excited
What is the distribution of calcium stored in the body?
- 0.1% in extracellular fluid
- 1% in the cells and organelles
- rest is stored in the bones
How is phosphate storied in the body?
- 85% in bones
- 14-15% cells
- less than 1% in extracellular fluid
- extracellular phosphate conc is not nearly as well regulated as that of Ca
What are the two forms of phosphate in the plasma?
HPO4- and H2PO2-
What can hypocalcemia do in the body?
- causes nervous system excitement and tetany
- nervous system is more excitable due to increased neuronal permeability to Na+
-hypocalcemia may also causes seizures
+first signs of tetany typically occurs in the hand, resulting in capopedal spasm
When does tetany typically occur?
- when blood conc of Ca falls below normal level of 9.5mg/dL to 6mg/dL (35% below normal Ca conc)
- usually lethal at 4mg/dL
How much calcium is typically taken into the body? How is it absorbed?
- 1000mg/day
- usually divalent ions are poorly absorbed but vitamin D promotes Ca absorption by the intestines, so 35% if ingested Ca is absorbed
Calcium excretion
-Ca not absorbed is excreted in the feces
+250mg/day enter intestines via secreted GI juices and sloughed mucosal cells
+90% of daily intake is excreted
Renal excretion of calcium and phosphate
- about 100mg/day (10%) of the ingested Ca is excreted in the urine
- 41% of plasma Ca bound to plasma proteins is not filtered by glomerular capillaries
- the rest is combined with anions or ionized and filtered through the glomeruli
- renal tubules reabsorb 99% Ca in filtrate
What can increase phosphate excretion?
-PTH
Hydroxyapatite crystals fail to precipitate in normal tissues except in what? What prevents this precipitation?
- Bones
- pyrophosphate
What are the initial calcium salts deposited in the bone?
CaHPO4- * 2H2O + Ca3(PO4)
What happens when soluble calcium is injected intravenously?
- Ca+2 conc may increase immediately to high levels
- within 30-60mins, the Ca+2 conc returns to normal
How can calcium concs be maintained so well in the body?
- bone contains exchangeable calcium that is always in equilibrium with the Ca+2 in the extracellular fluids
- small amount of exchangeable Ca is found in tissue cells, especially in highly permeable types of cells such as those of the liver and GI tract
- exchangeable Ca provides a rapid buffering system
What is the active form of vitamin D?
1,25-dihydroxycholecalciferol
What is the importance of vitamin D?
-causes kidneys, intestines, and bones to increases absorption of Ca adn P into the extracellular fluid and contribute to feedback regulation of these substances
Where are vitamin D receptors found? What do they do?
-present in most cells in the body and are located mainly in the nuclei of target cells
+hormone and DNA binding domains
+can suppress transcription
How does vitamin D promote intestinal absorption of Ca?
-by increasing, over a period of 2 days, formation of calbindin (Ca binding protein) in epithelial intestine cells
+functions in brush border to transport Ca into the cell cytoplasm
- Ca moves through the basolateral membrane of the cell by facilitated diffusion
- calbindin remains in the cell for weeks after the vitamin D has been removed
Vitamin D also promotes Ca absorption through the formation of what?
- a Ca stimulated ATPase in the brush border of the epithelial cells
- an alkaline phosphate in the epithelial cells
Vitamin D also increases Ca and P reabsorption in the renal tubules
Weak effect
What happens when there is an excess of vitamin D?
- causes absorption of bone
- in the absence of vitamin D, the effect of PTH in causing bone formation Iis greatly reduced or prevented
How can vitamin D in smaller quantities promote bone calcification?
-increasing Ca and P absorption from the intestines
Excess activity of the parathyroid gland causes rapid absorption of ________ from the bones.
- Ca salts -> results in hypercalcemia
- hypo function of parathyroid causes hypocalcemia
