Calcium Homeostasis

Perecentage Drop in Blood Calcium That WIll Trigger PTH

1%

List the Three Ways Calcitrol Works

1. It increases calcium absorption by the small intestine 2. It increases calcium resorption from the skeleton by binding to osteoblasts 3. It weakly promotes the reabsorption of calcium ions by the kidneys, so less calcium is lost in the bones

Four Ways PTH Increases Blood Calcium

1. PTH binds to receptors on the osteoblasts, stimlulating them to secrete RANKL 2. PTH promotes calcium reabsorption by the kidneys, so less calcium is lost in urine 3. PTH promotes the final step of calcitrol synthesis in the kidneys 4. PTH inhibits collagen synthesis by osteoblasts, inhibiting bone deposition

Calcitonin

A hormone produced by the C-cells of the thyroid gland that decreases bloodcalcium levels.

Two Principal Way Calcitionin Works

Osteoclast Inhibition; Osteoblast Stimulation

Calcium Homeostasis Depends on These Three Hormones

calcitrol, calcitonin, parathyroid hormone

Describe Calcitrol Synthesis

Ultraviolet rays act on the epidermal keratinocytes which turn 7-deydrocholesterol into vitamin D3; the liver adds a hydroxyl group converting it to calcidiol; the kidney adds another hydroxyl group converting calcidiol to calcitrol

Growth Factors

at least 12 hormonlike substances produced in bone itself that stimulate neighboring bone cells, promote collagen synthesis, stimulate epiphyseal growth, etc

Hormone

blood-borne chemical messenger from one organ to another

Calcitonin Plays a Greater Role In ___ than ____

children; adults

Cortisol

inhibits osteoclast activity, but if excreted in excess (Cushing disease), can cause osteoporosis

Calcitrol

most active form of vitamin D produced by the sequential action of the skin, liver, and kidneys

RANKL

produced by osteoblasts activates osteoclasts and increases osteoclast number and bone resorption release of RANKL stimulated by hormones and growth factor

Parathyroid Hormone (PTH)

produced by parathyroid glands; raises blood calcium level when it falls below a critical threshold level

Vitamin A

promotes glycosaminoglycan (chondroitin sulfate) synthesis

RANK

receptor on the surfaces of osteoclast-producing stem cells; when activated by RANKL they cause the stem cell to differentiate to osteoclast and liberate calcium and phoshpate ions from bone

Vitamin C (absorbic acid)

reuired for collagen synthesis, bone growth, and fracture repair

What is Common in Rickets and Osteomalacia?

soft bones due to low calcium and phosphate deposition in bones because too little calcitrol; rickets (children), osteomalacia (adults)

Growth Hormone

stimulates bone elongation and cartilage proliferation

Insulin

stimulates bone formation

Testosterone

stimulates osteoblasts and promotes protein synthesis

Estrogen

stimulates osteoblasts in adolescent growth; prevents osteoporosis

Principle Function of Calcitrol

to raise the blood calcium concentration