Chapter 6 Bones and The Skeletal System

Hypercalcemia excessive calcium binding to cell

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Unformatted text preview: of nervous system and muscle spasm Produced by the following process Calcitriol (Activated Vitamin D) Calcitriol behaves as a hormone UV radiation penetrating to the dermal blood vessels converts a cholesterol derivative (7dehydrocholesterol) to (cholecalciferol) D3 liver converts it to calcidiol kidney converts it to calcitriol stimulates intestine to absorb calcium & phosphate reduces urinary secretion of calcium & phosphate promotes osteoclast activity Abnormal softness of the bones is called rickets in children and osteomalacia in adults Calcitriol Synthesis & Action Calcitonin Secreted by C cells of the thyroid gland when calcium concentration rises too high Functions Important role in children, but little effect in reduces osteoclast activity by as much as 70% in 15 minutes increases the number & activity of osteoblasts adults calcitonin deficiency is not known to cause any disease in adults may be useful in reducing bone loss in osteoporosis Parathyroid Hormone Secreted by the parathyroid glands on the posterior surface of the thyroid gland Released when calcium blood level is too low Functions inhibits activity of osteoblasts stimulates osteoclast multiplication & activity reduces calcium secretion & increases phosphate secretion in the urine stimulates the production of an enzyme in the kidneys that carries out the last step in calcitriol synthesis Control of Calcium Levels 20 or more hormones, vitamins & growth Other Factors Affecting Bone factors not well understood Bone growth especially rapid at puberty Growth ceases when epiphyseal plate hormones stimulate proliferation of osteogenic cells and chondrocytes in growth plate adolescent girls grow faster than boys & reach their full height earlier (estrogen has stronger effect) males grow for a longer time "closes" anabolic steroids may cause premature closure of growth plate producing short adult stature...
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