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R10
Regulation of Calcium and Phosphate Balance
27
Physiology
Professional
02/24/2013

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Term
Where is calcium found in the body?
Definition

Bones

Extracellular fluid bound to proteins

Free ions in ECF (mainly heart and GI tract)

In intracellular organelles like the ER and mitochondria

Term
Where is most of the calcium in the body found? In what form?
Definition
In bone as calcium phosphate (Ca)3(PO4)2
Term
What is “fast time-scale” regulation of calcium concentrations in the plasma?
Definition
It is the shift of Ca2+ onto and off of the bone
Term
What is “slow time-scale” regulation of calcium concentrations in the plasma?
Definition
Altering the absorption and secretion from the GI tract and loss by renal processes
Term
What are the three forms of calcium in the blood?
Definition
1. freely ionized 2. reversibly bound to plasma proteins 3. complexed to plasma anions like citrate and phosphate
Term
Why is there very little calcium in the cytosol?
Definition
it would cause abnormal cell signalling
Term
What percentage of calcium is not absorbed by the body?
Definition
90%, the other 10% is mostly absorbed by the gut
Term
What regulates gut absorption of Ca2+?
Definition
The active form of Vitamin D3 = calcitriol
Term
What types of calcium is filterable in the kidney?
Definition
Only free ionized calcium
Term
How much of the filtered calcium is reabsorbed along the proximal tubule, thick ascending limb of Henle’s loop, distal tubule, and collecting duct?
Definition
97 to 99%
Term
What protein is involved in Ca2+ reabsorption?
Definition
Calbindin
Term
What is the short-term buffer for fast changes in plasma calcium concentrations?
Definition
bone
Term
How is calcium transported from the epithelial cells to the interstitium?
Definition
Calcium ATPase (primary active transporter)
Na/Ca exchanger (secondary active transporter)
Term
What three hormones participate in the regulation of calcium and phosphate levels in the plasma?
Definition
Calcitriol (activated Vitamin D3)
Parathormone (PTH)
Calcitonin
Term
Where is parathormone produced?
Definition
Parathyroid glands
Term
Where is calcitonin produced?
Definition
Thyroid glands
Term
Where is Vit D3 activated to calcitriol?
Definition
Proximal tubule cells in the nephron
Term
How does calcitriol react to hypocalcemia?
Definition
It recruits calcium and phosphate from GI absorption and bone resorption.
Calcitriol inhibits excretion of calcium and phosphate in the kidneys.
Term
How does PTH react to hypocalcemia?
Definition
It recruits calcium and phosphate from bone resorption.
Calcitriol inhibits excretion of calcium in the kidneys but it promotes the excretion of phosphate.
Term
How does calcitonin react to hypercalcemia?
Definition
It stimulates the formation of new bone as Ca3(PO4)2 and increases the excretion of Ca2+ and (PO4)3- from the kidneys
Term
What transporter do thiazides (diuretics) block?
Definition
Na/Cl cotransporter (NCC) in the apical membrane
Term
How might thiazides reduce calcium loss associated with osteoporosis?
Definition
Thiazides promote calcium reabsorption by enhancing the basolateral sodium gradient since the NCC is inhibited. This increases Na/Ca exchange, thus more Ca2+ is transported into the interstitium and back to the blood
Term
What is the normal plasma concentration of calcium?
Definition
10 mg/dl
Term
What are the rapid effects of increased PTH release?
Definition
Increased renal calcium reabsorption and increased bone membrane release
Term
What are the slow effects of increased PTH release?
Definition
Increased osteoclastic bone reabsorption and increased calcium absorption
Term
Where is the renal plasma threshold for phosphate set?
Definition
It is just above the normal plasma concentration of phosphate so the kidney is always excreting the “spilled over” phosphate
Term
What hormone decouples calcium phosphate in the kidney?
Definition
PTH
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