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Section 2, Chapter 7
Bone Homeostasis
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 it’s 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 it’s stimulated by the
hormone calcitonin.
Nutrients that effect bone homeostasis

Vitamin D – promotes Ca2+ absorption in small intestine
• Vitamin D deficiency = softened and deformed bones
• Osteomalacia in adults
• Rickets in children

Vitamin A – balances bone resorption and deposition
• Vitamin A deficiency = retards bone development

Vitamin C – is required for collagen synthesis.
• Vitamin C deficiency = results in fragile bones
Hormones that affect bone homeostasis
Calcitonin
• Secreted from thyroid gland
• Promotes bone deposition

Parathyroid Hormone (PTH)
• Secreted from parathyroid glands
• Promotes bone resorption

Figure 7.13 Hormonal regulation of
blood calcium and resorption
Hormones that affect bone homeostasis
Growth Hormone (GH)
• Secreted from pituitary gland
• Promotes bone growth at epiphyseal
plates
Pituitary Gigantism
over secretion of GH during childhood
Pituitary Dwarfism
insufficient GH during childhood
Acromegaly
• Over secretion of GH as an adult
• Occurs after epiphyseal plates have sealed
• Enlargement of hands, feet, nose
Hormones that affect bone homeostasis
Sex Hormones (testosterone & estrogen)
• Promotes long bone growth at puberty
• 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.
Incomplete Fractures
Greenstick
fracture

Fissured
fracture
Complete Fractures

Transverse
fracture

Oblique
fracture

Comminuted
fracture

Spiral
fracture
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
Step 2. temporary spongy bone

Step 3. cartilaginous callus

Osteoblasts invade from
periosteum and deposit
temporary spongy bone.

Fibroblasts deposit a mass of
fibrocartilage “cartilaginous
callus”
&
Phagocytes remove hematoma
Osteoclasts remove bony debris
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.
Over time, osteoclasts outnumber osteoblasts, and
more bone is resorbed than can be deposited. Bone
mass decreases as a result.

Osteopenia “low bone mass”
• Progresses towards osteoporosis

Osteoporosis “porous bone”
• Bones develop spaces and canals
• Bones are fragile and easily broken
• Common in menopausal women
(from the low estrogen levels)

Bone loss is rapid in menopausal
women due to reduced estrogen
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

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section 2, chapter 7: skeletal system

  • 1. Section 2, Chapter 7 Bone Homeostasis
  • 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 it’s 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 it’s stimulated by the hormone calcitonin.
  • 3. Nutrients that effect bone homeostasis Vitamin D – promotes Ca2+ absorption in small intestine • Vitamin D deficiency = softened and deformed bones • Osteomalacia in adults • Rickets in children Vitamin A – balances bone resorption and deposition • Vitamin A deficiency = retards bone development Vitamin C – is required for collagen synthesis. • Vitamin C deficiency = results in fragile bones
  • 4. Hormones that affect bone homeostasis Calcitonin • Secreted from thyroid gland • Promotes bone deposition Parathyroid Hormone (PTH) • Secreted from parathyroid glands • Promotes bone resorption Figure 7.13 Hormonal regulation of blood calcium and resorption
  • 5. Hormones that affect bone homeostasis Growth Hormone (GH) • Secreted from pituitary gland • Promotes bone growth at epiphyseal plates Pituitary Gigantism over secretion of GH during childhood Pituitary Dwarfism insufficient GH during childhood Acromegaly • Over secretion of GH as an adult • Occurs after epiphyseal plates have sealed • Enlargement of hands, feet, nose
  • 6. Hormones that affect bone homeostasis Sex Hormones (testosterone & estrogen) • Promotes long bone growth at puberty • 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 “cartilaginous callus” & 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 “low bone mass” • Progresses towards osteoporosis Osteoporosis “porous bone” • Bones develop spaces and canals • Bones are fragile and easily broken • 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