Question 1

Where is most of the body's calcium stored?

Accepted Answer

Approximately 99% of the body's calcium is stored in the skeleton and teeth, where it combines with phosphate to form mineral salts in a crystal lattice structure. This storage serves as a reservoir that maintains calcium availability for vital physiological functions including blood clotting, neurotransmitter release, and muscle contraction.

Question 2

What are the main roles of calcium in extracellular fluids?

Accepted Answer

In body fluids, calcium functions as an extracellular cation essential for blood clotting, neurotransmitter release, muscle tone maintenance, and nervous and muscle tissue excitability. These roles make calcium critical for proper nervous system signaling and muscular function throughout the body.

Question 3

How does parathyroid hormone regulate calcium levels?

Accepted Answer

Parathyroid hormone (PTH) is the primary regulator of plasma calcium concentration. PTH activates osteoclasts to break down bone matrix, releasing calcium into the blood. In kidneys, PTH enhances calcium reabsorption while decreasing phosphate reabsorption. PTH also stimulates calcitriol release, which increases calcium absorption in the small intestine.

Question 4

What functions do phosphate ions serve in the body?

Accepted Answer

Phosphate ions are important intracellular anions that function as buffers in body fluids and urine. They are covalently bound to organic molecules including lipids, proteins, carbohydrates, nucleic acids, and ATP. About 85% of body phosphate exists as calcium phosphate salts in bones and teeth, while the remaining 15% is distributed in soft tissues and extracellular fluids.

Question 5

Which hormones regulate phosphate levels in blood plasma?

Accepted Answer

Three hormones regulate phosphate levels: PTH, calcitriol, and Fibroblast Growth Factor 23 (FGF23). PTH and FGF23 both inhibit phosphate reabsorption in kidneys, promoting excretion. Calcitriol enhances phosphate absorption from the gastrointestinal tract, while FGF23 decreases blood phosphate by suppressing calcitriol production.

Question 6

How do calcium and phosphate relate to other electrolytes in body fluid balance?

Accepted Answer

Calcium and phosphate work alongside other electrolytes to maintain fluid and electrolyte homeostasis. While calcium functions as an extracellular cation and phosphate as an intracellular anion, they interact with roles of electrolytes sodium and potassium to regulate osmotic balance, nerve transmission, and muscle function across body compartments.

Question 7

What is the relationship between calcium, phosphate, and bone composition?

Accepted Answer

Calcium and phosphate combine to form mineral salts that create the crystal lattice structure of bone and tooth matrix. This mineral composition provides structural strength to skeletal tissues. PTH-mediated bone resorption releases both calcium and phosphate ions simultaneously, linking skeletal integrity to systemic electrolyte regulation.

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