Physiology of bone and calcium homeostasis Flashcards
Describe how soft tissue and bone growth are measured?
Soft tissue: measured by weight
Bone: measured by height
What is required for tissue and bone growth?
GH and IGFs
Thyroid hormone
Insulin
Sex steroids
Bone growth: calcium and protein
Describe the ECM of bone?
Calcium phosphate crystals (hydroxyapatite) precipitate and attach to collagen fibre lattice
Describe the structure of bone?
Outer cortical bone: privdes strength
Inner trabecular bone: open, cell-filled spaces between calcified struts of lattice
Central bone marrow
When does bone growth occur, in terms of the state of the matrix?
When the matrix is deposited faster than it is resorbed
What are osteobalsts?
Bone-forming cells
Modified fibroblasts
What do osteoblasts produce?
Produces enzymes and osteoid (collagen and protein mix) to which hydroxyapatite binds
What does the epiphyseal plate signify in terms of bone growth?
Site of linear growth of long bone
What are chondrocytes?
Where are they found?
Collagen-producing cells of cartilage
Found near epiphysis
Describe the columns created by chondrocytes?
Continuously dividing columns of chondrocytes that increase in length
Collagen layer thickens and old chondrocytes disintegrate, leaving behind spaces which are filled with cartilage
Describe the actions of osteoblasts?
Lay down bone on top of cartilage base, where old chondrocytes degenerated
Revert to less active osteocytes when work is complete
Briefly describe the effect of osteoblast and osteoclast action on blood Ca levels?
Osteoblasts deposit calcium in bone > decrease blood Ca levels
Osteoclasts resorb bone and release Ca > increase blood Ca levels
Describe the action of osteoclasts?
Secrete acid and protease enzymes to dissolve bone matrix
Cause Ca to enter blood
Describe the morphology of osteoclasts?
Large
Mutlinucleate
Derived from haematopoietic stem cells
Which organs/structures are involved in control of blood calcium?
Bone
Kidneys
Intestine
Which hormones are involved in the maintenance of blood calcium levels?
Parathyroid hormone
Vitamin D3/Calcitriol
Calcitonin
What are the three major pools of calcium in the body?
Bone matrix
Extracellular (ionised Ca)
Intracellular (free Ca)
Which of the three pools of calcium can move to other locations?
ECF
Briefly describe how calcium is transferred between ECF and ICF?
ECF > ICF: electrochemical gradient
ICF > ECF: active transport
Briefly describe what stimulates Ca to move between ECF and bone matrix?
Calcitonin: ECF > bone
PTH: bone > ECF
Calictriol: bone > ECF
Describe what stimulates Ca to move between ECF and the intestines?
PTH and calcitriol stimulate absorption
(1/3 absorbed)
Describe how calcium moves between ECF and the kidneys?
ECF > kidneys: passive filtration
Kidneys > ECF: stimulated by PTH, inhibited by calcitonin
Where in the kidneys does calcium reabsorption occur?
Distal nephron
How is excess calcium removed from the body?
Most in urine
Some in faeces
Where is PTH released from?
When is it released?
Parathyroid glands
Released in response to decreased blood calcium
Describe the actions of PTH on bone, kidneys and intestine?
BONE: stimulates osteoclasts via RANKL > bone resorption
KIDNEYS: increase calcium reabsorption, decrease phosphate reabsorption
INTESTINE: activates Vitamin D > increases calcium and phosphate reabsorption
Describe the onset of action for PTH?
2-3 hours for bone
1-2 days for intestinal absorption
Minutes for kidney transport
Where is calcitonin released from?
When is it released?
Released from C cells of thyroid gland
Released when plasma Ca is extremely high
Describe the target tissues of calcitonin?
Bone and kidney (NOT intestines)
Describe the effect of calcitonin release?
BONE: decrease bone resorption
KIDNEY: increase calcium excretion
Describe the role of the thyroid and parathyroid glands in control of plasma calcium?
Describe how Vitamin D is activated?
Describe the effect of PTH on Vitamin D activation?
PTH activates kidney enzymes to activate Vitamin D
Describe how plamsa calcium and phosphate levels affect Vitamin D activation?
Low plasma calcium> PTH release > Vitamin D activation
Low plasma phosphate > Vitamin D activation
Describe the effect of Vitamin D on plasma calcium and phosphate?
Promotes intestinal absorption of calcium and phosphate > increase plasma calcium and phosphate
Describe the body’s response to a decrease in plasma calcium?
Which conditions may cause PTH hypersecretion?
Which may cause PTH hyposecretion?
Hypersecretion: tumour
Hyposecretion: autoimmune attack of glands (rare)
Describe the effects of PTH hypersecretion and hyposecretion?
Hypersecretion: hypercalcemia, hypophosphatemia
Hyposecretion: hypocalcemia, hypophosphatemia, neuromuscular excitability
Describe the effect of Vitamin D deficiency?
Impaired intestinal calcium absorption > low blood caclium > PTH release > bone resorption > continues > bone demineralisation
Rickets in children
Osteomalcia in adults
What is osteoporosis?
Long term imbalance in resorption and formation of bone
Resorption > formation
Reduced bone architecture
Describe the risk factors for osteoporosis?
Age > 50 years
Post-menopause (oestrogen withdrawal)
Low dietary calcium
Lack of exercise
Smoking
