HUMAN BIOLOGY

90 Chapter 5

What are the parts of the skeletal system?

• In the skeletal system, bones are organized into two parts—
  the axial skeleton and the appendicular skeleton.


• Ligaments connect bones at joints, and tendons attach bones
  to muscles (or to other muscles).


• Bones contribute to homeostasis by providing body support,
  enabling movement, and storing minerals.


• Some bones contain red marrow where blood cells form. In
  adults, some other bones contain fatty yellow marrow where
  no blood cells form.

taKe-hoMe Message

appendicular skeleton

Portion of the skeleton that

includes bones of the limbs,

shoulders, and hips.

axial skeleton Portion of

the skeleton that forms the

body’s vertical axis—the

skull bones, vertebral col‑

umn, and rib cage.

bone marrow Connective

tissue in some bones, where

blood cells form.

ligament Connective tissue

that connects bones at joints.

tendon Straplike connec‑

tive tissue that attaches

muscles to bones or to

other muscles.

the skeletal system: the body’s bony Framework

the skeletal system consists of bones,

ligaments, and tendons

The skeletal system consists of bones along with joints, carti-

lages, and straplike ligaments that hold our bones together.

The bones are organized into an axial skeleton and an

appendicular skeleton (Figure 5.4). The bones of the axial

skeleton form the body’s vertical, head-to-toe axis. The

appendicular (“hanging”) skeleton includes bones of the

limbs, shoulders, and hips. Ligaments connect bones at

joints. Ligaments are composed of elastic connective tissue,

so they are stretchy and resilient like thick rubber bands.

Tendons are cords or straps that attach muscles to bones or

to other muscles. They are built of connective tissue packed

with collagen fibers, which make tendons strong.

bones have several important functions

Bones contribute to homeostasis in many ways (Table

5.1). For instance, bones that support and anchor skeletal

muscles help maintain or change the positions of our body

parts. Some form hard compartments that enclose and

protect other organs; for example, the skull encloses and

protects the brain, and the rib cage protects the lungs. As

noted in Section 5.1, bones also serve as a “pantry” where

the body can store calcium. Because the calcium in bone is

in the form of the compound calcium phosphate, bone also

is a storage depot for phosphorus.

n    Bones are the major organs of the skeletal system. Their

functions include providing a hard surface against which

muscles can exert force to move body parts.

n    Link to Muscle tissue 4.3

An adult’s skeleton contains 206

bones. These bones vary in size and

shape, from bones in the ear that are

the size of a watch battery to massive

thighbones (Figure 5.3). Some, like

the thighbone, are long and slender.

Others, like the ankle bones, are quite

short. Still others, such as the ster-

num (breastbone), are flat, and still

others, such as spinal vertebrae, are

“irregular.” All bones contain bone

tissue, however, and other connec-

tive tissue lines their surfaces and

internal cavities. At joints there is

cartilage where one bone meets or

“articulates” with another. Other tis-

sues associated with bones include

nervous tissue and epithelium, which

occurs in the walls of blood vessels that carry substances to

and from bones. Clearly, bones are complex organs!

Some bones, such

as long bones, have

cavities that contain

bone marrow, a type

of connective tis-

sue. In children most

marrow-containing

bones have red bone

marrow, in which

blood cells form. With

time, however, much

of this red marrow

is replaced by fat-

rich yellow marrow,

where no blood cells

form. For this reason,

most of an adult’s

blood cells form in

red bone marrow in

irregular bones, such

as the hip bone, and in

flat bones, such as the

sternum. If you lose

a lot of blood, yellow

marrow in your long

bones can convert to

red marrow, which

makes red blood cells.

1. Movement. Bones interact with skeletal muscles to maintain or
   change the position of body parts.


2. Support. Bones support and anchor muscles.


3. Protection. Many bones form hard compartments that enclose
   and protect soft internal organs.


4. Mineral storage. Bones are a reservoir for calcium and
   phosphorus. Deposits and withdrawals of these mineral ions
   help to maintain their proper concentrations in body fluids.


5. Blood cell formation. Some bones contain marrow where
   blood cells are produced.

Table 5.1 Functions of Bone

Figure 5.3 Animated! The femur
(thighbone) is a typical long bone.

5.2

nutrient

canal into

and from

marrow

(for blood

vessels

and nerves)

marrow

cavity

compact

bone

tissue

spongy

bone

tissue

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