Skip Navigation

21.2: The Skeletal System

Difficulty Level: At Grade Created by: CK-12
Turn In

Lesson Objectives

  • Give an overview of the human skeleton and its functions.
  • Describe the cells and tissues that make up bones.
  • Explain how bones grow and develop.
  • Distinguish different types of joints and how they move.
  • List common problems that may affect bones and joints.


  • bone marrow
  • bone matrix
  • compact bone
  • joint
  • ligament
  • ossification
  • osteoblast
  • osteoclast
  • osteocyte
  • periosteum
  • skeletal system
  • spongy bone


The skeletal system consists of all the bones of the body. How important are your bones? Try to imagine what you would look like without them. You would be a soft, wobbly pile of skin, muscles, and internal organs, so you might look something like a very large slug. Not that you would be able to see yourself—folds of skin would droop down over your eyes and block your vision because of your lack of skull bones. You could push the skin out of the way, if you could only move your arms, but you need bones for that as well!

The Skeleton

The human skeleton is an internal framework that, in adults, consists of 206 bones, most of which are shown in Figure below. Bones are described in detail in the following sections of this lesson, as well as in the animation “Bones Narrated” at the link below. In addition to bones, the skeleton also consists of cartilage and ligaments. http://www.medtropolis.com/vbody.asp

  • Cartilage is a type of dense connective tissue, made of tough protein fibers, that provides a smooth surface for the movement of bones at joints.
  • A ligament is a band of fibrous connective tissue that holds bones together and keeps them in place.

The human skeleton consists of bones, cartilage, and ligaments.

The skeleton supports the body and gives it shape. It has several other functions as well, including:

  1. protecting internal organs
  2. providing attachment surfaces for muscles
  3. producing blood cells
  4. storing minerals
  5. maintaining mineral homeostasis.

Maintaining mineral homeostasis is a very important function of the skeleton, because just the right levels of calcium and other minerals are needed in the blood for normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood, thus restoring homeostasis.

Structure of Bones

Many people think of bones as being dead, dry, and brittle. These adjectives correctly describe the bones of a preserved skeleton, but the bones in a living human being are very much alive. As shown in Figure below, the basic structure of bones is bone matrix, which forms the underlying rigid framework of bones, formed of both compact and spongy bone. The bone matrix consists of tough protein fibers—mainly collagen—that become hard and rigid due to mineralization with calcium crystals. Bone matrix is crisscrossed by blood vessels and nerves and also contains specialized bone cells that are actively involved in metabolic processes.

You can watch an animated video of bone matrix and other structures of bone at this link: http://www.youtube.com/watch?v=4qTiw8lyYbs.

Bone matrix provides bones with their basic structure. Notice the spongy bone in the middle, and the compact bone towards the outer region. The osteon is the functional unit of compact bone.

Bone Cells

There are three types of specialized cells in human bones: osteoblasts, osteocytes, and osteoclasts. These cells are responsible for bone growth and mineral homeostasis.

  • Osteoblasts make new bone cells and secrete collagen that mineralizes to become bone matrix. They are responsible for bone growth and the uptake of minerals from the blood.
  • Osteocytes regulate mineral homeostasis. They direct the uptake of minerals from the blood and the release of minerals back into the blood as needed.
  • Osteoclasts dissolve minerals in bone matrix and release them back into the blood.

Bones are far from static, or unchanging. Instead, they are dynamic, living tissues that are constantly being reshaped. Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break it down. You can watch an animated video of these processes in bone at http://www.youtube.com/watch?v=yENNqRJ2mu0.

Bone Tissues

Bones consist of different types of tissue, including compact bone, spongy bone, bone marrow, and periosteum. All of these tissue types are shown in Figure below.

  • Compact bone makes up the dense outer layer of bone. It is very hard and strong.
  • Spongy bone is found inside bones and is lighter and less dense than compact bone. This is because spongy bone is porous.
  • Bone marrow is a soft connective tissue that produces blood cells. It is found inside the pores of spongy bone.
  • Periosteum is a tough, fibrous membrane that covers and protects the outer surfaces of bone.

This bone contains different types of bone tissue. How does each type of tissue contribute to the functions of bone?

Growth and Development of Bones

Early in the development of a human fetus, the skeleton is made entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. This is a process in which mineral deposits replace cartilage. As shown in Figure below, ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.

Long bones ossify and get longer as they grow and develop.

In the late teens or early twenties, a person reaches skeletal maturity. By then, all of the cartilage has been replaced by bone, so no further growth in bone length is possible. However, bones can still increase in thickness. This may occur in response to increased muscle activity, such as weight training.


A joint is a place where two or more bones of the skeleton meet. With the help of muscles, joints work like mechanical levers, allowing the body to move with relatively little force. The surfaces of bones at joints are covered with a smooth layer of cartilage that reduces friction at the points of contact between the bones.

Types of Joints

There are three main types of joints: immovable, partly movable, and movable. For a video about these types of joints, go to the following link: http://www.youtube.com/watch?v=SOMFX_83sqk.

  • Immovable joints allow no movement because the bones at these joints are held securely together by dense collagen. The bones of the skull are connected by immovable joints.
  • Partly movable joints allow only very limited movement. Bones at these joints are held in place by cartilage. The ribs and sternum are connected by partly movable joints.
  • Movable joints allow the most movement. Bones at these joints are connected by ligaments. Movable joints are the most common type of joints in the body, so they are described in more detail next.

Movable Joints

Movable joints are also known as synovial joints. This is because the space between the bones is filled with a thick fluid, called synovial fluid, that cushions the joint (see Figure below).

A movable, or synovial, joint is protected and cushioned by cartilage and synovial fluid.

There are a variety of types of movable joints, which are illustrated in Figure below. The joints are classified by how they move. For example, a ball-and-socket joint, such as the shoulder, has the greatest range of motion, allowing movement in several directions. Other movable joints, including hinge joints such as the knee, allow less movement.

You can watch an animation of movable joints and how they function at this link: http://www.youtube.com/watch?v=zWo9-3GJpr8.

Types of Movable Joints in the Human Skeleton. Movable joints can move in a variety of ways. Try moving each of the joints indicated in the diagram. Can you tell how their movements differ? Other joints in the human skeleton that are not depicted here include saddle, elipsoid, and plane joints.

Skeletal System Problems

Despite their hardness and strength, bones can suffer from injury and disease. Bone problems include fractures, osteoarthritis, and rickets.

  • Fractures are breaks in bone, usually caused by excessive stress on bone. Fractures heal when osteoclasts form new bone. The animation at this link shows how this happens: http://www.youtube.com/watch?v=qVougiCEgH8.
  • Osteoarthritis is a condition in which cartilage breaks down in joints due to wear and tear, causing joint stiffness and pain.
  • Rickets is softening of the bones in children that occurs because bones do not have enough calcium. Rickets can lead to fractures and bowing of the leg bones, which is illustrated in the Figure below.

The bones of a child with rickets are so soft that the weight of the body causes them to bend.

Lesson Summary

  • The adult human skeleton includes 206 bones and other tissues. It supports the body, protects internal organs, produces blood cells, and maintains mineral homeostasis.
  • Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break down bone to maintain mineral homeostasis. Bone tissues include compact bone, spongy bone, bone marrow, and periosteum.
  • Bones become increasingly ossified and grow larger during fetal development, childhood, and adolescence. When skeletal maturity is reached at about age 20, no additional growth in bone length can occur.
  • Joints are places where two or more bones of the skeleton meet. With the help of muscles, joints allow the body to move with relatively little force. Some joints can move more than others.
  • Skeletal system problems include fractures, osteoarthritis, and rickets.

Lesson Review Questions


1. What is cartilage? What is its role in the skeletal system?

2. List three functions of the human skeleton.

3. Identify the three types of specialized bone cells and what they do.

4. Define immovable joint, and give an example of bones that are connected by this type of joint.

5. Describe the movement of a pivot joint, such as the elbow.

Apply Concepts

6. A newborn baby has a soft spot on the top of its head. Over the next few months, the soft spot gradually hardens. What explains this?

7. Jana is 17 years old and 172 cm tall. She plays basketball and hopes to grow at least 4 cm more before she turns 18 and goes to college. Jana recently injured her leg, and her doctor took an X-ray of it. Based on the X-ray, the doctor determined that Jana had reached skeletal maturity. How much taller is Jana likely to grow? Explain your answer.

Think Critically

8. Explain how bones maintain mineral homeostasis in the body.

9. Compare and contrast the structure and function of compact bone and spongy bone.

10. Osteoporosis is a disease in which osteoclasts are more active than osteoblasts. How is this likely to affect the bones? Why would a person with osteoporosis have a greater-than-normal risk of bone fractures?

Points to Consider

Human organ systems work together to carry out many of their functions. The skeletal and muscular systems are no exception.

  • Do you know how the skeletal and muscular systems work together?
  • How do you think muscles are able to move bones?

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

Show Hide Details
Files can only be attached to the latest version of section
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original