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Physiology of Bone
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Physiology
Professional
01/13/2011

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Cards

Term




Importance of Ca regulation

Definition

Ca plays a key role in many physiological processes:
Contraction of skeletal, cardiac and smooth muscles
Transmission of nerve impulses
Blood clotting
Many others

Term




1,25-dihydroxycholecalciferol

Definition

Effects
Prolonged effect (several week duration of CBP)
Calcium binding protein
Ca++ stimulated ATPase
Alkaline phosphatase

Term




Activation of Vitamin D

Definition

Biologically active 1,25-hydroxycholecalciferol is synthesized in the kidney under PTH stimulation

Synthesis of 1,25-hydroxycholecalciferol is stimulated by parathyroid hormone and inhibited by high plasma Ca and phosphate

Term




Forms of Ca in the extracellular fluid

Definition


1. Ionized free. 50%

2. Ionized complexed with anions (phosphate, citrate). 9%
3. Protein-bound. 41%. Non-diffusible thru the capillary wall.

Term




Phosphate in Plasma

Definition

HPO4- - and H2PO4-
1.3 mmol/L
4 mg/dL

Term




Bone Salts

Definition

Calcium and phosphate
Hydroxyapatite
Ca10(PO4)6(OH)12
Platelike
Also contain Mg, Na, K, CO3
Crystals stacked like bricks in a wall

And fibers run lengthwise like steel cables in concrete

Gives tremendous tensile strength combined with compresional  strength from crystals

Actually stronger than reinforced concrete

Term




Bone Formation

Definition

Osteoblasts lay down osteoid

Collagen

Ground substance

Some cells trapped – osteocytes

Calcium salts deposit

Initially amorphous crystals

Remodeled into  hydroxyapatite

Term




Ca exchange between bone and extracellular fluid

Definition

Exchangeable Ca provides buffering tool to keep normal concentration of the ionized Ca

Exchangeable Ca contains only 0.4–1% of the total bone Ca

Bones are continually being deposited by osteoblasts and absorbed by osteoclasts

Coordinated activity of osteoblasts and osteoclasts allows bones to respond to physiological challenges

Term




Bone Remodeling

Definition

Balance between deposition and reabsorption
Localized mass of osteoclasts bore tunnel
Osteoblasts invade tunnel and rebuild after about 3 weeks and remain for months
Haversian canal remains
Osteon

Term




Reasons for Bone Remodeling

Definition

Adjustments in bone strength and shape
Athletes vs nonathletes
Piezo-electric effect
(-) potential at site of compression
Replace degenerated organic matrix
Maintains supportive collagen fibers
Fracture
Maximal stimulation of osteoblastic activity
Progenitor cells

Term




Effect of PTH on Ca and phosphate concentration in the extracellular fluid

Definition

1. Increased Ca and phosphate absorption from the bone

2. Decreased Ca secretion by the kidneys

3. Increased renal phosphate secretion

Net effect:

    extracellular Ca concentration  

    

    extracellular phosphate

    concentration                            

Term




Renal effects of PTH

Definition

Increased reabsorption of Ca

Diminished reabsorption of phosphate, promoting phosphate secretion with urine

Term




Control of parathyroid secretion

Definition

PTH secretion is stimulated  when Ca concentration in the blood falls down (pregnancy, lactation, rickets)

PTH secretion is inhibited by:
excess quantities of Ca in the diet
bone absorption caused by factors other than PTH

Term




How Parathyroid Hormone Works

Definition

Bone
Osteocytes and osteoblasts form osteocytic membrane system
“bone fluid”
Activation of calcium pump
Osteoclasts
Activation of present osteoclasts
Formation of new osteoclasts

Term




Secretion of calcitonin

Definition

Secretion of calcitonin is stimulated by increased plasma Ca
1. Immediate effect.
Decreased activity of osteoclasts and bone absorption
2. Prolonged effect.
Decreased formation of new osteoclasts

Calcitonin has a weak effect on plasma Ca concentration in adult humans

Term




Pathophysiologies involving Ca++ Homeostasis

Definition

Parathyroid gland hypertropy

1.Rickets
2.Pregnancy
3.Lactation

Parathyroid gland atrophy
1.Excessive vitamin D
2.Excessive calcium
3.Bone absorption

Term




Hypoparathyroidism

Definition

Osteoclasts become almost dormant
Plasma calcium depletes
Tetany
Laryngeal muscles (possibly lethal)
Treatment
PTH administration (expensive)
Vitamin D and Calcium administration

Term




Rickets

Definition

Lack of Calcium and/or phosphate in ECF
Vitamin D deficiency
Lack of adequate sunlight
Condition may appear in spring after winter has allowed for depletion of stored Vit D precursors
Months to become apparent

Tetany may develop in late stages
Treatment
Dietary calcium and phosphate
Vitamin D supplements

 

Term



 

Osteoporosis

Definition

Old age
Common
Diminished organic matrix
Reduced osteoblastic activity
Sometimes caused by increased osteoclast activity (hyperparathyroidism)

 

Term




Osteoporosis, Causes

Definition

Old age
Loss of growth hormone and reduced protein function
Malnutrition
Inactivity
No physical stress to stimulate bone deposition
Loss of estrogen at menopause

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