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Chapter 6: Osseous Tissue and Bone Structure

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1

The Skeletal System

• Skeletal system includes:
– bones of the skeleton
– cartilages, ligaments, and connective tissues

2

Functions of the Skeletal System

1. Support
2. Storage of minerals (calcium)
3. Storage of lipids (yellow marrow)
4. Blood cell production (red marrow)
5. Protection
6. Leverage (force of motion)

3

Classification of Bones

• Bone are identified by:
– shape
– internal tissues
– bone markings

4

Bone Shapes

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1. Long bones
2. Flat bones
3. Sutural bones
4. Irregular bones
5. Short bones
6. Sesamoid bones

5

Long Bones

(Bone Shapes)

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• Are long and thin
• Are found in arms, legs, hands, feet, fingers, and toes

• Diaphysis:
– the shaft
- A heavy wall of compact bone, or dense bone
- A central space called marrow cavity

• Epiphysis:
– wide part at each end
– articulation with other bones
• Mostly spongy (cancellous)bone
• Covered with compact bone (cortex)

• Metaphysis:
– where diaphysis and epiphysis meet

6

Flat Bones

(Bone Shapes)

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• Are thin with parallel surfaces
• Are found in the skull, sternum, ribs,and scapula

• Resembles a sandwich of spongy bone
• Between 2 layers of compact bone

7

Sutural Bones

(Bone Shapes)

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• Are small,irregular bones
• Are found between the flat bones of the skull

8

Irregular Bones

(Bone Shapes)

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• Have complex shapes
• Examples:
– spinal vertebrae
– pelvic bones

9

Short Bones

(Bone Shapes)

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• Are small and thick
• Examples:
– ankle
– wrist bones

10

Sesamoid Bones

(Bone Shapes)

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• Are small and flat
• Develop inside
tendons near joints of knees, hands, and feet

11

Bone Markings

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• Depressions or grooves:
‐ along bone surface
– Projections:
– where tendons and ligaments attach
– at articulations with other bones

• Tunnels:
– where blood and nerves enter bone

12

Bone Markings

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13

Bone (Osseous) Tissue

• Dense, supportive connective tissue
• Contains specialized cells
• Produces solid matrix of calcium salt deposits
• Around collagen fibers

14

Characteristics of Bone Tissue

• Dense matrix, containing:
– deposits of calcium salts
– bone cells within lacunae organized around blood vessels

• Canaliculi:
– form pathways for blood vessels
– exchange nutrients and wastes

• Periosteum:
– covers outer surfaces of bones
– consist of outer fibrous and inner cellular layers

15

Matrix Minerals

• 2/3 of bone matrix is calcium phosphate, Ca3(PO4)2:
– reacts with calcium hydroxide, Ca(OH)2 to form – crystals of hydroxyapatite, Ca10(PO4)6(OH)2
– which incorporates other calcium salts and ions

16

Matrix Proteins

• 1/3 of bone matrix is protein fibers (collagen)

17

Bone Cells

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• Make up only 2% of bone mass:

- Osteocytes
- Osteoblasts
– osteoprogenitor cells
– osteoclasts

18

Osteocytes

(Bone Cells)

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• Mature bone cells that maintain the bone matrix
• Live in lacunae
• Are between layers (lamellae) of matrix
• Connect by cytoplasmic extensions through canaliculi in lamellae
• Do not divide

Functions
• To maintain protein and mineral content of
matrix
• To help repair damaged bone

19

Osteoblasts

(Bone Cells)

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• Immature bone cells that secrete matrix compounds
(osteogenesis)

20

Osteoid

• Matrix produced by osteoblasts, but not yet
calcified to form bone
• Osteoblasts surrounded by bone become
osteocytes

21

Osteoprogenitor Cells

(Bone Cells)

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• Mesenchymal stem cells that divide to produce
osteoblasts
• Are located in inner,cellular layer of periosteum
(endosteum)
• Assist in fracture repair

22

Osteoclasts

(Bone Cells)

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• Secrete acids and proteindigesting enzymes
• Giant, mutlinucleate cells
• Dissolve bone matrix and release stored minerals
(osteolysis)
• Are derived from stem cells that produce macrophages

23

Homeostasis

• Bone building (by osteocytes) and bone recycling (by osteoclasts) must balance:
– more breakdown than building, bones become weak
– exercise causes osteocytes to build bone

24

Compact Bone

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25

Osteon

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• The basic unit of mature compact bone
• Osteocytes are arranged in concentric lamellae
• Around a central canal containing blood vessels

26

Perforating Canals

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• Perpendicular to the central canal
• Carry blood vessels into bone and marrow

27

Circumferential Lamellae

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• Lamellae wrapped around the long bone
• Binds osteons together

28

Spongy Bone

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• Does not have osteons
• The matrix forms an open network of trabeculae
• Trabeculae have no blood vessels

29

Red Marrow

• The space between trabeculae is filled with red bone marrow:
– which has blood vessels
– forms red blood cells
– and supplies nutrients to osteocytes

30

Yellow Marrow

• In some bones, spongy bone holds yellow bone marrow:
– is yellow because it stores fat

31

Weight–Bearing Bones

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• The femur transfers weight from hip joint to knee joint:
– causing tension on the lateral side of the shaft
– and compression on the medial side

32

Periosteum and Endosteum

• Compact bone is covered with membrane:
– periosteum on the outside
– endosteum on the inside

33

Periosteum

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• Covers all bones:
– except parts enclosed in joint capsules

• It is made up of:
– an outer, fibrous layer
– and an inner,cellular layer

Functions of Periosteum
1. Isolate bone from surrounding tissues
2. Provide a route for circulatory and nervous
supply
3. Participate in bone growth and repair

34

Perforating Fibers

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• Collagen fibers of the periosteum:
– connect with collagen fibers in bone
– and with fibers of joint capsules,attached tendons,and ligaments

35

Endosteum

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• An incomplete cellular layer:
– lines the marrow cavity
– covers trabeculae of spongy bone
– lines central canals

• Contains osteoblasts,osteoprogenitor cells, and osteoclasts

• Is active in bone growth and repair

36

Bone Development

• Human bones grow until about age 25

• Osteogenesis:
– bone formation

• Ossification:
– the process of replacing other tissues
with bone

37

Calcification

• The process of depositing calcium salts
• Occurs during bone ossification and in other
tissues

38

Ossification

• The 2 main forms of ossification are:
– intramembranous ossification
– endochondral ossification

39

Intramembranous Ossification

• Also called dermal ossification:
– because it occurs in the dermis
– produces dermal bones such as mandible and
clavicle

• There are 3 main steps in intramembranous
ossification

40

Intramembranous Ossification: Step 1

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• Mesenchymal cells aggregate:
– differentiate into osteoblasts
– begin ossification at the ossification center
– develop projections called spicules

41

Intramembranous Ossification: Step 2

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• Blood vessels grow
into the area:
– to supply the osteoblasts

• Spicules connect:
– trapping blood vessels inside bone

42

IntramembranousOssification: Step 3

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• Spongy bonedevelops and is remodeled into:
– osteons of compact bone
– periosteum
– or marrow cavities

43

Endochondral Ossification

• Ossifies bones that originate as hyaline
cartilage

• Most bones originate as hyaline cartilage

• Growth and ossification of long bones occurs in 6 steps

• Appositional growth:
– compact bone thickens and strengthens long bone with layers of circumferential lamellae

44

Endochondral Ossification: Step 1

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• Chondrocytes in the center of hyaline cartilage:
– enlarge
– form struts and calcify
– die, leaving cavities in cartilage

45

Endochondral Ossification: Step 2

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• Blood vessels grow around the edges of the cartilage

• Cells in the perichondrium change to osteoblasts:
– producing a layer of superficial bone around the shaft which will continue to
grow and become compact bone (appositional growth)

46

Endochondral Ossification: Step 3

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• Blood vessels enter the cartilage:
– bringing fibroblasts that become osteoblasts
– spongy bone develops at the primary ossification center

47

Endochondral Ossification:

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Step 4
• Remodeling creates a marrow cavity:
– bone replaces cartilage at the metaphyses

48

Endochondral Ossification: Step 5

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• Capillaries and osteoblasts
enter the epiphyses:
– creating secondary ossification centers

49

Endochondral Ossification: Step 6

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• Epiphyses fill with spongy bone:
– cartilage within the joint cavity is articulation cartilage
– cartilage at the metaphysis is epiphyseal
cartilage

50

Endochondral Ossification: Step 7

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• As long as the epiphyseal cartilage continues to grow at its epiphyseal surface, the bone will continue to increase in length.

51

Bone Growth in Length

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• Epiphyseal plate or cartilage growth plate
- cartilage cells are produced by mitosis on epiphyseal side of plate
- cartilage cells are destroyed and replaced by bone on diaphyseal side of plate

• Between ages 18 to 25, epiphyseal plates close.
- cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)

• Growth in length stops at age 25

52

Zones of Growth in Epiphyseal Plate

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• Zone of resting cartilage
- anchors growth plate to bone

• Zone of proliferating cartilage
- rapid cell division (stacked coins)

• Zone of hypertrophic cartilage
- cells enlarged & remain in columns

• Zone of calcified cartilage
- thin zone, cells mostly dead since matrix calcified
- osteoclasts removing matrix
- osteoblasts & capillaries move in to create bone over calcified cartilage

53

Bone Growth in Width

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• Only by appositional growth at the bone’s surface

• Periosteal cells differentiate into osteoblasts and form bony ridges and then a tunnel around periosteal blood vessel.

• Concentric lamellae fill in the tunnel to form an osteon.

54

Epiphyseal Lines

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• When long bone stops growing, after puberty:
– epiphyseal cartilage disappears
– is visible on X‐rays as an epiphyseal line

55

Mature Bones

• As long bone matures:
– osteoclasts enlarge marrow cavity
– osteons form around blood vessels in compact
bone

56

Blood Supply of Mature Bones

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• 3 major sets of blood vessels develop

1) Nutrient artery and vein:
– a single pair of large blood vessels
– enter the diaphysis through the nutrient foramen
– femur has more than 1 pair

2) Metaphyseal vessels:
– supply the epiphyseal cartilage
– where bone growth occurs

3) Periosteal vessels provide:
– blood to superficial osteons
– secondary ossification centers

57

Lymph and Nerves

• The periosteum also contains:
– networks of lymphatic vessels
– sensory nerves

58

Remodeling

• The adult skeleton:
– maintains itself
– replaces mineral reserves

• Remodeling:
– recycles and renews bone matrix
– involves osteocytes, osteoblasts, and osteoclasts

• Bone continually remodels, recycles, and replaces
• Turnover rate varies
• If deposition is greater than removal, bones get stronger
• If removal is faster than replacement, bones get weaker

59

Effects of Exercise on Bone

• Mineral recycling allows bones to adapt to stress
• Heavily stressed bones become thicker and stronger

60

Bone Degeneration

• Bone degenerates quickly
• Up to 1/3 of bone mass can be lost in a few weeks of inactivity

• What you don’t use, you lose
• Stresses applied to bones during physical activity are essential to maintain bone strength and mass

61

Effects of Hormones and Nutrition on Bone

• Normal bone growth and maintenance
requires nutritional and hormonal factors

62

Minerals

Effects of Hormones and Nutrition on Bone

• A dietary source of calcium and phosphate salts:
– plus small amounts of magnesium, fluoride, iron,
and manganese

63

Vitamins

Effects of Hormones and Nutrition on Bone

• Vitamin C is required for collagen synthesis,
and stimulates osteoblast differentiation

• Vitamin A stimulates osteoblast activity

• Vitamins K and B12 help synthesize bone proteins

64

Calcitriol

Effects of Hormones and Nutrition on Bone

• The hormone calcitriol:
– is made in the kidneys
– helps absorb calcium and phosphorus from digestive tract
– synthesis requires vitamin D3 (cholecalciferol)

65

Other Hormones

Effects of Hormones and Nutrition on Bone

• Growth hormone and thyroxine stimulate
bone growth
• Estrogens and androgens stimulate osteoblasts
• Calcitonin and parathyroid hormone regulate
calcium and phosphate levels

66

Hormones for Bone Growth and Maintenance

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67

The Skeleton as Calcium Reserve

• Bones store calcium and other minerals
• Calcium is the most abundant mineral in the
body

68

Chemical Composition of Bone

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69

Functions of Calcium

• Calcium ions are vital to:
– membranes
– neurons
– muscle cells, especially heart cells

70

Calcium Regulation

• Calcium ions in body fluids:
– must be closely regulated

• Homeostasis is maintained:
– by calcitonin and parathyroid hormone
– which control storage, absorption, and excretion

KEY CONCEPTS
• Calcium and phosphate ions in blood are lost in urine
• Ions must be replaced to maintain homeostasis
• If not obtained from diet, ions are removed
from the skeleton, weakening bones
• Exercise and nutrition keep bones strong

71

Calcitonin and Parathyroid Hormone Control

• Bones:
– where calcium is stored

• Digestive tract:
– where calcium is absorbed

• Kidneys:
– where calcium is excreted

72

Parathyroid Hormone (PTH)

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• Low calcium ion levels in the blood cause the parathyroid glands in neck to secrete Parathyroid Hormone (PTH)

• Increases calcium ion levels by:
– stimulating osteoclasts to release stored calcium ions from the bone
– increasing intestinal absorption of calcium
- kidneys retain calcium ions

– decreases calcium loss in urine
- calcium is absorbed quickly in intestines

73

Calcitonin

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• High calcium ion levels in blood cause Calcitonin to be secreted by C cells (parafollicular cells) in thyroid

• Decreases blood calcium ion levels by:
– inhibiting osteoclast activity
- rate of intestinal absorption decreases
- Kidneys allow calcium loss

– increasing calcium excretion at kidneys and increased calcium loss in urine
- Calcium is absorbed slowly in intestines
- Calcium is stored in bone matrix

74

Fractures

• Fractures:
– cracks or breaks in bones
– caused by physical stress

• Fractures are repaired in 4 steps

75

Fracture Repair: Step 1

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• Bleeding:
– produces a clot (fracture hematoma)
– establishes a fibrous network

• Bone cells in the area die

76

Fracture Repair: Step 2

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• Cells of the endosteum and periosteum:
– Divide and migrate into fracture zone

• Calluses stabilize the break:
– external callus of cartilage and bone surrounds break
– internal callus develops in marrow cavity

77

Fracture Repair: Step 3

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• Osteoblasts:
– replace central cartilage of external callus
– with spongy bone

78

Fracture Repair: Step 4

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• Osteoblasts and osteocytes remodel the fracture for up to a year:
– reducing bone calluses

79

Pott’s fracture

The Major Types of Fractures

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- occurs at the ankle and affects both bones of the leg

80

Comminuted fractures

The Major Types of Fractures

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- shatter the affected area into a multitude of bony fragments.

81

Transverse fractures

The Major Types of Fractures

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- break a bone shaft across its long axis

82

Spiral fractures

The Major Types of Fractures

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- twisting stresses that spread along the length of the bone

83

Displaced fractures

The Major Types of Fractures

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- produce new and abnormal bone arrangements, non-displaced fractures retain the normal alignment of the bones or fragments

84

Colles’ fracture

The Major Types of Fractures

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- break in the distal portion of the radius (usually from reaching to cushion a fall)

85

Greenstick fracture

The Major Types of Fractures

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- one side of the shaft is broken, and the other side is bent.

86

Epiphyseal fractures

The Major Types of Fractures

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- where bone matrix is undergoing calcification and chondrocytes are dying. A clean transverse fracture along this line can generally heal well. Unless carefully treated, fractures between the epiphysis and the epiphyseal cartilage can permanently stop growth at this site.

87

Compression fractures

The Major Types of Fractures

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- occur in the vertebrae subjected to extreme streses

88

Age and Bones

• Bones become thinner and weaker with age
• Osteopenia begins between ages 30 and 40
• Women lose 8% of bone mass per decade, men 3%

89

Effects of Bone Loss

• The epiphyses, vertebrae, and jaws are most
affected:
– resulting in fragile limbs
– reduction in height
– tooth loss

90

Osteoporosis

• Severe bone loss
• Affects normal function
• Over age 45, occurs in:
– 29% of women
– 18% of men

91

Hormones and Bone Loss

• Estrogens and androgens help maintain bone mass
• Bone loss in women accelerates after menopause

92

Cancer and Bone Loss

• Cancerous tissues release osteoclastactivating
factor:
– that stimulates osteoclasts
– and produces severe osteoporosis

93

Name the five primary functions of the skeletal system. p170

The 5 primary functions of the skeletal system are: - support,
- storage of minerals and lipids,
- blood cell production,
- protection,
- and leverage.

94

Identify the six broad categories for classifying a bone according to shape. p173

The 6 broad categories for classifying bones according to shape are:
- flat
- irregular
- long
- sesamoid
- short
- sutrural

95

Define bone marking. p173

A bone marking, (surface feature) is an area on the surface of a bone structured for a specific function, such as:
- joint formation,
- muscle attachment,
- or the passage of nerves and blood vessels.

96

Mature bones cells are known as _____, bone-building cells are called __________, and ______ are bone-resorbing cells. p175

Mature bones cells are known as OSTEOCYTES, bone-building cells are called OSTEOBLASTS, and OSTEOCLASTS are bone-resorbing cells.

97

How would the compressive strength of a bone be affected if the ration of collagen to hydroxyapatite increased? p175

IF the ration of collagen to hydroxyapatite in a bone increased, the bone would become less strong (as well as more flexible.)

98

If the activity of osteoclasts exceeds the activity of osteoblasts in a bone, how will the mass of the bone be affected? p175

because osteoclasts break down or demineralize bone, the bone would have a reduced mineral content (less mass), as a result, it would be weaker.

99

Compare the structures and functions of compact bone and spongy bone. p179

Compact bone consists of osteons (Haversian systems) with little space between them. Compact bone lies over spongy bone and makes up most of the diaphysis. It functions to protect, support, and resist stress. Spongy bone consists of trabeculae with numerous red marrow-filled spaces. Spongy bone makes up most of the structure of short, flat, and irregular bones and is also found at the epiphyses of long bones. Spongy bone functions in storing marrow and providing some support.

100

A sample of bone has lamellae, which are not arranged in osteons, Is the sample most likely taken from the epiphysis or diaphysis? p179

The presence of lamellae that are not arranged in osteons is indicative of spongy bone, which is located in an epiphysis.

101

During intramembraneous ossification, which type of tissue is replaced by bone? p183

During intramembranous ossification, fibrous connective tissue is replaced by bone.

102

In endochondral ossification, what is the original source of osteoblasts? p183

In endochondral ossification, cells of the inner layer of the pericondrium differentiate into osteoblasts, and a cartilage model is gradually replaced by bone.

103

How could x-rays of the femur be used to determine whether a person has reached full height? p183

Long bones of the body, such as the femur, have an epiphyseal cartilage, a plate of cartilage that separates the epiphysis from the diaphysis so long as the bone is still growing lengthwise. An x-ray would indicate whether the epiphyseal cartilage is still present. If it is, growth is still occurring; if it is not, the bone has reached its adult length.

104

Describe bone remodeling. p184

Bone remodeling refers to the process whereby old bone is continuously being destroyed by osteoclasts while new bone is being constructed by osteoblasts.

105

Explain how heavy-metal ions could be incorporated into bone matrix. p184

The biochemistry of some heavy-metal ions, such as strontium, cobalt, uranium, and plutonium, is very similar to that of calcium. Osteoblasts cannot differentiate these abnormal heavy-metal ions from normal calcium ions, so the heavy metal ions become incorporated into the bone matrix. Over time, these dangerous ions can be released into circulation during normal bone remodeling.

106

Why would you expect the arm bones of a weight lifter to be thicker and heavier than those of a jogger? p186

The larger arm muscles of the weight lifter would apply more mechanical stress to the bones of the upper limbs. in response to that stress, the bones would grow thicker.

107

A child who enters puberty several years later than the average age is generally taller than average as an adult, Why? p186

Growth continues throughout childhood. At puberty, a growth spurt occurs and is followed by the closure of the epiphyseal cartilages. The later puberty begins, the taller the child will be when the growth spurt begins, so the taller the individual will be when growth is completed.

108

A 7yr old child has a pituitary gland tumor involving the cells that secrete growth hormone (GH), resulting in increased levels of GH. How will this condition affect the child's growth? p186

increased levels of growth hormone prior to puberty will result in excessive bone growth, making the individual taller.

109

Identify the hormones involved in stimulating and inhibiting the release of calcium ions from bone matrix. p188

Parathyroid hormone (PTH) influences osteoclast activity to cause a release of stored calcium ions from the bone. Under the influence of calcitonin, osteoclast activity is inhibited, while osteoblasts continue to lock calcium ions in the bone matrix. Therefore, PTH serves to increase blood calcium levels by causing its release from bone, and calcitonin decreases blood calcium levels by causing calcium to remain in bone.

110

Why does a child who has rickets have difficulty walking? p188

The bones of children who have rickets are poorly mineralized and as a resulte are quite flexible. under the weight of the bondy, the leg bones bend. The instability makes walking difficult and can lead to other problems of the legs and feet.

111

What effect would increased PTH secretion have on blood ion calcium levels? p188

Parathyroid hormone (PTH) stimulates osteoclasts to release calcium ions from bone and enhances calcitriol's effect on the intestinal absorption of calcium. Increase PTH secretion would result in an increase in the level of calcium ions in the blood.

112

How does calcitonin help lower the calcium ion concentration of blood? p188

Calcitonin lowers blood calcium levels by inhibiting osteoclast activity and increasing the rate of calcium excretion by the kidneys.

113

List the steps involved in fracture repair, beginning at the onset of the bone break. p192

Immediately following a fracture, extensive bleeding occurs at the site of injury. after several hours, a large blood clot called a fracture hematoma develops. Next, an internal callus forms as a network of spongy bone unties the inner edges, and an external callus of cartilage and bone stabilizes the outer edges. The cartilaginous external callus is eventually replaced by one, and the struts of spongy bone now unite the broken ends. With time, the swelling that initially marks the location of the fracture is remodeled, and little evidence that a break occurred remains.

114

At which point in fracture repair would you find an external callus? p192

An external callus forms early in the healing process, when cells from the endosteum and periosteum migrate to the area of the fracture. These cells form an enlarged collar (external Callus) that encircles the bone in the area of the fracture.

115

Define osteopenia. p193

Osteopenia is inadequate ossification and is common to the aging process. It results as a consequence of decreasing osteoblast activity accompanied with normal osteoclast activity.

116

Why is osteoporosis more common in older women that in older men? p193

In women, the sex hormones known as estrogens play an important role in moving calcium into bones. after menopause, the level of these hormones decreases dramatically; as a result, older women have difficulty replacing the calcium in bones that is being lost due to normal aging. In men, the level of sex hormones (androgens) does not decrease until much later in life.

117

Which of the following is NOT a function of the skeletal system?

protection
contraction
support
blood cell production

contraction

118

The femur and the humerus are examples of __________.

long bones

119

The carpals or wrist bones are examples of __________.

short bones

120

What is the term for the extended tubular shaft of a long bone?

diaphysis

121

Which of the following types of cells are the mature bone cells that maintain the bone matrix?

osteocytes

122

Which of the following statements about bone tissue is FALSE?

It is made primarily of cells.
It contains collagen.
It is made primarily of calcium phosphate.
It contains four different cell types.

It is made primarily of cells.

123

Which of the following types of bone cells is responsible for removing and recycling bone?

osteoclasts

Osteoclasts are responsible for removing and recycling bone.

124

Which of the following are NOT structural components of compact bone?

trabeculae
osteons
concentric lamellae
central canals

trabeculae

125

Which component of bone is responsible for blood cell formation?

Red bone marrow

Red bone marrow is responsible for blood cell formation.

126

What is the name of the membrane that covers the outer surface of the bones?

periosteum

127

Which of the following forms the flat bones of the skull?

intramembranous ossification

128

In which of the following does bone replace existing cartilage?

endochondral ossification

129

Which of the following allows a bone to increase in diameter or width?

appositional growth

Appositional growth allows a bone to increase in diameter.

130

What is the term for the process in which the organic and mineral components of bone are continuously recycled and renewed?

remodeling

131

Which of the following is an effect of stress on a bone?

The bone will become thicker.

Bones become thicker and stronger in response to stress.

132

Text: __________ is required for collagen synthesis, and a deficit results in a condition called scurvy.

Vitamin C

133

Which two hormones play opposing roles in regulating the calcium level in blood and body fluids?

calcitonin and parathyroid hormone

134

Which of the following is the term for a fracture in which the broken bone breaks through the skin?

open or compound

An open or compound fraction occurs when the broken bone projects through the skin.

135

Which of the following is the last step of fracture repair?

remodeling to return the bone to its normal shape

Remodeling is the last step of fracture repair.

136

What is the term for a reduction in bone mass that is sufficiently large that it compromises the normal function of the bone?

osteoporosis

Osteoporosis is a reduction in bone mass that is sufficiently large to compromise the normal function of bone. It results in weakening of the bones, making them likely to break.


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