2. Homeostasis of Bone Tissue
Calcium is constantly exchanged between the blood and bone.
Bone resorption = Osteoclasts breakdown bone releasing
calcium into the blood. Bone resorption occurs when
blood [Ca2+] is low and itâs stimulated by parathyroid
hormone (PTH).
Bone deposition = Osteoblasts deposit new bone from
calcium in the blood stream. Bone deposition occurs
when blood [Ca2+] is high and itâs stimulated by the
hormone calcitonin.
3. Nutrients that effect bone homeostasis
Vitamin D â promotes Ca2+ absorption in small intestine
âą Vitamin D deficiency = softened and deformed bones
âą Osteomalacia in adults
âą Rickets in children
Vitamin A â balances bone resorption and deposition
âą Vitamin A deficiency = retards bone development
Vitamin C â is required for collagen synthesis.
âą Vitamin C deficiency = results in fragile bones
4. Hormones that affect bone homeostasis
Calcitonin
âą Secreted from thyroid gland
âą Promotes bone deposition
Parathyroid Hormone (PTH)
âą Secreted from parathyroid glands
âą Promotes bone resorption
Figure 7.13 Hormonal regulation of
blood calcium and resorption
5. Hormones that affect bone homeostasis
Growth Hormone (GH)
âą Secreted from pituitary gland
âą Promotes bone growth at epiphyseal
plates
Pituitary Gigantism
over secretion of GH during childhood
Pituitary Dwarfism
insufficient GH during childhood
Acromegaly
âą Over secretion of GH as an adult
âą Occurs after epiphyseal plates have sealed
âą Enlargement of hands, feet, nose
6. Hormones that affect bone homeostasis
Sex Hormones (testosterone & estrogen)
âą Promotes long bone growth at puberty
âą Sex hormones also stimulate ossification at
epiphyseal plates.
Effects of Exercise on bone
homeostasis
Contracting muscles pull on bones
and promotes bone thickening
Figure 7.12 The thickened bone on
the left is better able to withstand
forces from muscle contractions.
9. When a bone breaks blood vessels rupture and the periosteum tears.
The repair of a broken bone occurs in 5 general steps.
Step 1. hematoma formation
Blood soon forms a
hematoma (blood clot).
Hematoma in foot
10. Step 2. temporary spongy bone
Step 3. cartilaginous callus
Osteoblasts invade from
periosteum and deposit
temporary spongy bone.
Fibroblasts deposit a mass of
fibrocartilage âcartilaginous
callusâ
&
Phagocytes remove hematoma
Osteoclasts remove bony debris
11. Step 4. bony callus
Step 5. bone remodeling
Osteoblasts replace the
cartilaginous callus with bone,
forming a bony callus
Osteoclasts remove excess
bone, remodeling the bone
the bone close to its original
shape.
12. Over time, osteoclasts outnumber osteoblasts, and
more bone is resorbed than can be deposited. Bone
mass decreases as a result.
Osteopenia âlow bone massâ
âą Progresses towards osteoporosis
Osteoporosis âporous boneâ
âą Bones develop spaces and canals
âą Bones are fragile and easily broken
âą Common in menopausal women
(from the low estrogen levels)
Bone loss is rapid in menopausal
women due to reduced estrogen
13. Ways to delay or prevent osteoporosis:
1. Exercise daily.
2. Consume enough calcium and
vitamin D every day.
3. Do not smoke.
End of Chapter 7,
Section 2