Chapter 4
Skeletal System
4.1 Bone as Living Tissue
A. Functions of the Skeletal System
(approximately 206 individual bones)
1. Support – provides shape and support to
the trunk and limbs
2. Protection – ribs protect vital organs, skull
protects brain, spinal column protects cord
3. Movement – muscles contract, pulling on
attached bone – providing movement
4. Storage – repository for minerals,
especially phosphorus and calcium
a. phosphorus needed for bone and teeth
health, also for fat conversion reactions
b. calcium needed for neuromuscular
system and blood clotting
c. medullary cavity = hollow space in
long bones
- aka marrow cavity
- stores the bone marrow (2 types)
1. yellow marrow = storehouse
for fat
2. red marrow = red blood cell
formation
5. Blood Cell Formation – (hematopoiesis)
occurs in red marrow
- red marrow found in flat and short
bones, vertebrae, sternum, ribs, and long
bones
B. Structures and Classifications of Bones –
shape is determined by function
1. Composition of Bones
a. Osteocytes are mature bone cells
b. of overall bone weight, 60 – 70% is
from mineral content (not cells)
- calcium carbonate
- calcium phosphate
Remaining weight is collagen
(proteins) and water
c. minerals and water provide bone
strength, collagen provides flexability
- child’s bones have higher collagen
and therefore are more flexible than
adult
2. Organization of Bones (two types of tissues)
a. cortical bone – dense, stiff, high mineral
content
b. trabecular bone – spongy (porous,
honeycombed) more flexible
c. bone function determines composition of
the bone
- outer covering is cortical, inner bone
trabecular
- long bones = shaft of cortical, ends
have some trabecular
- vertebral column = cortical covering,
trabecular insides for shock absorption
3. Shape categories of Bones
(traditionally 4 categories)
a. Long bones – round shaft with bulbous
ends
- shaft is cortical bone surrounding a
medullary cavity (canal)
- ends are trabecular bone encased in
cortical bone
- major arm and leg bones
b. Short bones – shaped like a cube
- mainly trabecular bone
- wrists and ankles
c. Flat bones – thin with large surface area,
generally curved
- two thin layers of cortical bone with a
layer of trabecular bone in between
- protect underlying organs and provide
muscle attachments
- skull, sternum, and scapula (shoulder
blade)
d. Irregular bones – bones that do not fit
the above categories
- individualized bones to fit specific
functions
- spinal column, hip girdle
4. Anatomical Structures of Long Bones
(pages 114 and 115)
a. shaft – diaphysis
- cortical bone
- surrounded by periosteum – blood &
lymph vessels, nerves, for growth,
repair, nutrition
- medullary cavity (canal) in the center
of the diaphysis
+ at 5 years of age, filled with yellow
marrow, lined with endosteum
b. bulbous ends – epiphyses
- trabecular bone, filled with red marrow
(hematopoesis)
- surrounded by cortical bone (thin layer)
- epiphyses is protected by articular
cartilage
c. bone nourishment and waste removal
- Haversian canal – major passageways
running lengthwise in the bone
- lacunae – cavities with Osteocytes
+ Laid out in concentric circles
around the Haversian canals
+ The concentric circles are called
lamellae.
- each Haversian canal and its
surrounding lacunae form an OSTEON
or Haversion system
+ within each Osteon, are tiny
sideways channels called canaliculi
+ canaliculi connect lacunae for
nutrient supply and waste removal
+ perforating canals (Volkmann)
also run sideways and connect
Haversian Systems.
Bone growth in width
http://highered.mheducation.com/sites/0072495855/student_view0/chapter6/animation__bone_growth_in_widt
h.html
C. Growth and Development of Bones
1. Osteoblasts and Osteoclasts
a. Osteoblasts build new bone
b. Osteoclasts reabsorb or eliminate weak or
damaged bone
2. Bone Formation
a. bone modeling is the creation of new bone
by the osteoblasts
b. in fetus – first hyaline cartilage is formed
then:
1st step = bone matrix shell covers the
cartilage (osteoblasts)
2nd step = osteoclasts reabsorb the
cartilage and create the medullary
cavity
This process of bone formation is called ossification
3. Longitudinal Growth
a. length grows at epiphyseal plates located
at end of long bones
b. after growth period, plate fuses and
lengthening stops
4. Circumferential Growth
a. growth in width occurs throughout
lifespan
b. osteoblasts in the periosteum create new
bone on the outside
c. osteoclasts in the endosteum reabsorb
layers of bone in the medullary cavity
5. Adult Bone Development
a. as humans age they lose collagen in the
bone = less flexible, more brittle
b. bone mineral peaks:
- females = 25 – 28 years
- males = 30 – 35 years
c. females tend to have smaller bones than
men so mineral density loss is more of a
problem in women.
D. Remodeling of Bones – osteoblast/osteoclast activity
continues throughout adulthood changing size, shape
and density
1. Hypertrophy of Bones (subjected to larger forces)
a. increased physical activity will cause an
increase in bone density
b. impact activities are particularly effective in
osteoblast activity
c. only 15% of total body weight is bone
(regardless of weight of the person)
2. Atrophy of Bones
a. bone loss in less active, sedentary individual
b. swimmers have bone loss due to buoyancy
c. astronauts experience rapid bone loss in space
– major factor preventing mission to Mars