Exam #4: Calcium and Bone Physiology Flashcards
List the plasma forms of calcium.
There are three forms of Ca++ in the plasma:
1) Free/ ionized Ca++ (50%)
2) Calcium that is bound to protein i.e. albumin (40%)
3) Ca++ bound to small diffusible anions (10%)
In which form is ~ 50% of the total calcium?
Free/ ionized Ca++
*Note that this is the only form of Ca++ that is biologically active & regulated
Which forms of Ca++ are filtered in the glomerulus?
Ca++ that is NOT protein bound i.e. 60% of the total Ca++ in plasma is filtered at the glomerulus
How does pH effect Ca++ in the plasma?
Alkaline pH= more Ca++ bound to protein
Acidic pH= less Ca++ bound to protein
What are the symptoms of hypocalcemia?
Hypocalcemia causes threshold to become CLOSER to membrane potential (more negative), which makes nerves & muscles MORE excitable. Symptoms include:
1) Hypocalcemic tetany
2) Latent tetany revealed by Trousseau or Chvostek sign
3) Paresthesias
4) Laryngospasm
5) Seizures
6) Syncope, CHF, and angina
*Chovstek sign= tapping on the facial nerve that elicits twitching of the facial muscles
Trousseau sign= carpopedal spasm upon inflation of a blood pressure cuff
What are the symptoms of hypercalcemia?
Hypercalcemia causes threshold to more FARTHER from resting membrane potential (more positive), making nerves and muscles LESS excitable. Symptoms include:
1) Fatigue
2) Muscle weakness
3) Constipation
4) Polyuria
5) Kidney stones
6) Coma
7) Cardiac arrest
Describe the regulation of plasma Ca++.
- Ca++ homeostasis involves the coordinated interaction of the: bone, kidneys, and GI tract
- Balance is attained when renal excretion= GI absorption of Ca++
On a daily basis:
- 1,000mg Ca++ ingested but only 350mg absorbed in the GI tract
- 150 mg secreted into the salivary, pancreatic, and intestinal fluids
- Net 200mg is absorbed, which much be excreted by the kidneys to be in balance
*Note that 1,25-dihydroxycholecaliferol (active Vitamin D) increases GI absorption of Ca++
What hormone increased GI absorption of Ca++?
Active form of Vitamin D
Describe the regulation of plasma phosphorus.
GI Tract: most of the dietary phosphorus is absorbed
- Vitamin D controls uptake
Bone: phosphorus is part of the major bone crystalline salt, hydroxapatite
Kidney: at low levels all phosphate is reabsorbed but at higher & higher levels some is excreted
- PTH increases phosphate excretion
- Fibroblast growth factor 23 (FGF 23) is a peptide hormone synthesized by osteoblasts and osteocytes that increases renal excretion of phosphate
Why does hydroxyapatite precipitate in bone and not in other tissues?
Inhibitors found in other body tissues including plasma prevent precipitation
Outline the mechanism of bone growth and calcification.
1) Osteoblasts secrete collagen and ground substance
2) Osteoblasts become trapped in osteoid and become osteocytes
3) Ca++ salts precipitate onto collagen and then hydroxyapatite crystals form
What cells are responsible for bone deposition?
Osteoblasts
Which cell type is responsible for bone resorption?
Osteoclasts
Describe bone remodeling.
Bone remodeling, or the deposition of new bone and reabsorption of old bone, is a continuous process
What is the general function of PTH in bone remodeling?
Stimulation of osteoclasts and bone resorption
How does PTH stimulate bone resorption?
PTH stimulates bone resorption indirectly:
1) Binds osteoBLASTS and causes them to release cytokines 2) Cytokines, osteoprotegerin ligand (OPLG) activate receptors on preosteoCLAST cells
3) PreosteoCLAST cells differentiate into osteoCLASTS that promote bone resorption