1 - PTH and Calcitonin Flashcards
What percentage of total body Ca is stored in the cells?
In the ECF?
0.1%
1%
rest is in the bones
What percentage of total body Phos is stored in the cells?
In the ECF?
14-15%
1%
Rest in the bones
What is so special about ionized calcium?
It can diffuse through the cell membrane
What percentage of plasma calcium is ionized?
50%
What are the immediate effects of decreasing phosphate in the ECF?
Major increases or decreases in plasma Phos have no immediate effects
______calcemia causes tetany
When it occurs in the hand, this is called:
hypocalcemia
carpopedal spasm
What percentage of daily calcium intake is excreted in the feces?
90% (900mg)
Why is Vitamin D important in Ca reabsorption?
divalent cations (like Ca) have a really hard time being absorbed from the intestines
Calcium is excreted in the _______
Phos is excreted in the _______
feces
urine
What is the most important factor controlling Ca reabsorption in the distal portions of the nephron?
PTH
Which bone type accounts for 80% of bone mass and forms the hard outer layer
cortical
Which bone type accounts for 20% of bone mass and is found in the interior of skeletal bones
trabecular
bone is composed of a tough organic matrix that is greatly strengthened by deposits of ________
calcium salts
cortical bone is about 30% matrix and 70% calcium salts!
The organic matrix of the bone is comprised of ___ and ____
collagen
ground substance (gelatinous)
The crystalline salts deposited in bone are made up of:
calcium and phosphate
The collagen fibers in bone have _____ strength
The calcium salts have _____ strength
tensile
compressional
Why doesn’t hydroxyapatite precipitate in the ECF if Ca and Phos are both present?
Pyrophosphate
inhibitor present in almost all tissues to prevent precipitation
What’s the difference between an osteoblast and an osteocyte?
When new bone is formed, osteoblasts lay down collagen and ground substance
The tissue formed becomes osteoid, and some of the osteoblasts get trapped
these osteoblasts become quiescent and are called osteocytes
Calcium should never precipitate in nonosseous tissues. What are some examples where it does?
Arteriosclerosis
Valves
Degeneratign tissues and old blood clots
The bones can buffer an increase or decrease in calcium within 30 minutes. How?
They maintain exchangeable calcium that maintains a constant equilibrium with the ECF
PTH causes increased bone resorption, but it doesn’t communicate with osteoclasts. How is this acheived?
Osteoblasts are responsible for secreting both the osteoclast growth factor (RANKL) and the osteoclast inhibitor (OPG)
Osteoblasts usually keep these in equilibrium
VItamin D and PTH stimulate production of mature osteoclasts by binding to osteoblasts and inhibiting OPG formation and stimulating RANKL formation
Why do bones thicken when exposed to heavy loads?
bone adjusts its strength in proportion to the degree of bone stress
bone is deposited in proportion to the compressional load that the bone must carry
Why is mechanical fixation so helpful in bone healing?
it means the patient can start beraing weight on the limb right away, which increases calcium deposition and prevents decalcification of the effected bone
Vitamin D itself doesn’t effect calcium absorption or bone resorption. What does?
Its final active product, 1,25 - dihydroxycholecalciferol
formulated in the liver and kidneys into this active form
In the absence of _______, Vit D loses pretty much all of its effectiveness
kidneys
What hormone is required in order to convert Vit D to 1,25 D
PTH
Why do calcium levels affect Vit D synthesis?
PTH secretion is supressed by elevated [Ca]
less PTH means less Vit D
Vitamin D acts to promote:
Intestinal Calcium reabsorption (increases the amount of calbindin in epithelial gut cells)
phosphate absorption (increased calcium absorption acts as a transport mediator for the phos)
The _____ cells in the Parathyroid secrete PTH
chief, not oxyphil
PTH causes resorption of both calcium and phos from the bones. Why does an increase in PTH cause an increase in serum Ca but a decrease in serum phos?
PTH increases renal phos excretion at a much higher rate than the bone resorption
How does osteolysis occur?
Osteoblasts and osteocysts form a matrix on the surface of the bone called the osteocytic membrane which separates the bone from the ECF
pumps in the membrane keep the calcium concentration lower on the bone side
when the pump becomes excessively activated (by PTH) calcium concentration in the bone falls and salts are released from the bone
PTH has two effects on osteoclasts:
- immediate activation of existing osteoclasts
- formation of new osteoclasts
What are some conditions that cuase hypertrophy of the parathyroid?
Conditions that lower calcium, even if it’s in minute amounts
Rickets
Pregnancy
Lactation
If the calcium receptors in the parathyroid sense excess calcium, what do they do?
CRP is a G protein-coupled receptor
activation stimulates release of calcium which DECREASES PTH secretion
PTH isn’t turned on by a lack of Ca, it’s turned off by a presence of Ca
Activation is a failure of inhibition
Calcitonin _______ the calcium concentration
decreases
It is the opposite of PTH
List the three effects of PTH
- stimulates bone resorption, release Ca into ECF
- Decreases excretion of Ca, increases excretion of Phos
- converts Vit D into its active form
Calcitonin decreases plasma [Ca] by:
- Halts osteolysis, which increases bone [Ca] which moves the balance back toward calcium deposition
- decreases the formation of osteoclasts
- Very, very minimal effect of renal excretion
Why does calcitonin have a weaker immediate effect than PTH?
No effect on renal excretion, through which PTH has overriding effect
Does have a much bigger effect in children because the rate of bone resorption and formation is already so much higher
What is renal rickets?
Osteomalacia from renal disease
Primary hyperparathyroidism is associated with ______ calcium levels
Secondary hyperparathyroidism is associated with ______ calcium levels
increased
decreased (PTH is elevated as an attempt to increase ca levels)
Children with rickets have a deficit of Vitamin D which leads to decreased plasma [Ca], but it also leads to decreased Phos levels. Why?
Vitamin D promotes phosphate absorption and calcium absorption, so the amount of both decreases
But PTH is secreted to increase the level of Ca, and this further lowers the level of Phos
There is not a regulatory check for low levels of Phos
What is usually the culprit in osteoporosis: osteoblasts or osteoclasts?
Osteoblasts
Why does a lack of Vit C cause osteoporosis?
Osteoblasts depend on Vit C to form osteoid
What is tooth occlusion?
The upper and lower teeth fit together so that even small particles of food can be caught
What are the differences between dentin and bone?
Dentin doesn’t have osteoclasts or osteoblasts, no space for blood vessels or nerves
dentin is deposited and nourished by a layer of cells called odontoblasts instead
What is cementum?
Bony substance that lines the tooh socket
holds the tooth in place
increases in thickness and strength with excessive strain, and age
What kind of cells line the pulp?
Odontoblasts
What is the clinical term for baby teeth?
Deciduous teeth
What is the final number of teeth in the adult?
28 + 4 wisdom teeth
Which part of the tooth has deposition and absorption rates comparable to the jaw bone it sits in?
cementum
Why does flourine help prevent caries?
flourine ions replace many of the hydoxyl ions in the hydroxyapatite crystals, making the enamel much less soluble
flourine promotes deposition of calcium when small pits do form in the teeth
How effective is flourine?
makes teeth about three times as resistant to caries
