102 11.2 Introduction to the Skeletal System

Created by CK-12 Foundation/Adapted by Christine Miller

11.2.1 Bones
Figure 11.2.1 Bones can be quite jolly.

Skull and Cross-Bones

The skull and cross-bones symbol has been used for a very long time to represent death, perhaps because after death and decomposition, bones are all that remain. Many people think of bones as dead, dry, and brittle. These adjectives may correctly describe the bones of a preserved skeleton, but the bones of a living human being are very much alive. Living bones are also strong and flexible. Bones are the major organs of the .

Overview of the Skeleton System

The  is the organ system that provides an internal framework for the human body. Why do you need a skeletal system? Try to imagine what you would look like without it. You would be a soft, wobbly pile of skin containing muscles and internal organs, but no bones. 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!

Components of the Skeletal System

In adults, the skeletal system includes 206 bones, many of which are shown in Figure 10.2.2 below. Bones are organs made of supportive connective tissues, mainly the tough protein collagen. Bones contain blood vessels, nerves, and other tissues, and they are hard and rigid, due to deposits of calcium and other mineral salts within their living tissues. Spots where two or more bones meet are called . Many joints allow bones to move like levers. Your elbow, for example, is a joint that allows you to bend and straighten your arm.

11.2.2 Skeleton
Figure 11.2.2 The adult skeleton contains 206 bones. A newborn infant has 270 bones, but many of them fuse together by adulthood.

Besides bones, the skeletal system includes cartilage and ligaments.

  • is a type of dense connective tissue, made of tough protein fibres. It is strong, but flexible and very smooth. It covers the ends of bones at joints, providing a smooth surface for bones to move over.
  • are bands of dense fibrous connective tissue that hold bones together. They keep the bones of the skeleton in place.

Axial and Appendicular Skeletons

The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton, both of which are pictured below (Figure 10.2.3 and Figure 10.2.4 respectively).

  • The  forms the axis of the body. It includes the skull, vertebral column (spine), and rib cage. The bones of the axial skeleton — along with ligaments and muscles — allow the human body to maintain its upright posture. The axial skeleton also transmits weight from the head, trunk, and upper extremities down the back to the lower extremities. In addition, the bones protect the brain and organs in the chest.
11.2.3 Axial Skeleton
Figure 11.2.3 The axial skeleton.
  • The  forms the appendages and their attachments to the axial skeleton. It includes the bones of the arms and legs, hands and feet, and shoulder and pelvic girdles. The bones of the appendicular skeleton make possible locomotion and other movements of the appendages. They also protect the major organs of digestion, excretion, and reproduction.
11.2.4 Appendicular Skeleton
Figure 11.2.4 The appendicular skeleton.

Functions of the Skeletal System

The skeletal system has many different functions that are necessary for human survival. Some of the functions, such as supporting the body, are relatively obvious. Other functions are less obvious but no less important. Three tiny bones (hammer, anvil, and stirrup) inside the middle , for example, transfer sound waves into the inner ear.

Support, Shape, and Protection

The skeleton supports the body and gives it shape. Without the rigid bones of the skeletal system, the human body would be just a bag of soft tissues, as described above. The bones of the skeleton are very hard and provide protection to the delicate tissues of internal organs. For example, the encloses and protects the soft tissues of the , and the protects the  of the . The vertebral column, , and sternum (breast bone) protect the , , and major . Providing protection to these latter internal organs requires the bones to be able to expand and contract. The ribs and the cartilage that connects them to the sternum and vertebrae are capable of small shifts that allow breathing and other internal organ movements.

Movement

11.2.5 Movement at Joints animation
Figure 11.2.5 Bones that meet at the elbow and shoulder joint include the scapula, humerus, radius and ulna. These bones provide attachment surfaces for muscles that move the bones at the joint.

The bones of the skeleton provide attachment surfaces for skeletal muscles. When the muscles contract, they pull on and move the bones. Figure 11.2.5, for example, shows the muscles attached to the bones at the elbow and shoulder. They help stabilize the joint and allow the arm to bend at these two joints. The bones at joints act like levers moving at a fulcrum point, and the muscles attached to the bones apply the force needed for movement.

 

 

Hematopoiesis

 is the process by which blood cells are produced. This process occurs in a tissue called , which is found inside some bones, including the , , and . Red marrow synthesizes red blood cells, white blood cells, and platelets. Billions of these blood cells are produced inside the bones every day.

Mineral Storage and Homeostasis

Another function of the skeletal system is storing minerals, especially and . This storage function is related to the role of bones in maintaining mineral . 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. Bone minerals are alkaline (basic), so their release into the blood buffers the blood against excessive acidity (low pH), whereas their absorption back into bones buffers the blood against excessive alkalinity (high pH). In this way, bones help maintain acid-base homeostasis in the blood.

Another way that bones help maintain homeostasis is by acting as an endocrine organ. One endocrine hormone secreted by bone cells is , which helps regulate blood  and fat deposition. It increases secretion, as well as cell’s sensitivity to insulin. In addition, it boosts the number of insulin-producing cells and reduces fat stores.

Sexual Dimorphism of the Human Skeleton

The human skeleton is not as as that of many other primate species, although human female skeletons tend to be smaller and less robust than human male skeletons within a given population. There are also subtle differences between males and females in the morphology of the skull, teeth, longs bones, and pelvis. The greatest difference is in the pelvis, because the female pelvis is adapted for child birth.  Take a look at the pelvises in Figure 11.2.6 and 11.2.7.  How are they different?

11.2 Male Pelvis
Figure 11.2.6 The male pelvis.
11.2 Female Pelvis
Figure 11.2.7 The female pelvis

11.2 Summary

  • The is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.
  • are organs made of supportive connective tissues, mainly the tough protein . Bones also contain blood vessels, nerves, and other tissues. Bones are hard and rigid, due to deposits of calcium and other mineral salts within their living tissues. Besides bones, the skeletal system includes cartilage and ligaments.
  • The skeleton is traditionally divided into two major parts: the (which includes the skull, spine, and rib cage) and the (which includes the appendages and the girdles that attach them to the axial skeleton).
  • The skeletal system has many different functions, including supporting the body and giving it shape, protecting internal organs, providing attachment surfaces for skeletal muscles, allowing body movements, producing blood cells, storing minerals, helping to maintain mineral , and producing endocrine hormones.
  • There is relatively little in the human skeleton, although the female skeleton tends to be smaller and less robust than the male skeleton. The greatest sex difference is in the pelvis, which is adapted for childbirth in females.

11.2 Review Questions

  1. What is the skeletal system? How many bones are there in the adult skeleton?
  2. Describe the composition of bones.
  3. Besides bones, what other organs are included in the skeletal system?
  4. Identify the two major divisions of the skeleton.
  5. List several functions of the skeletal system.
  6. If a person has a problem with blood cell production, what type of bone tissue is most likely involved? Explain your answer.
  7. What are three forms of homeostasis that the skeletal system regulates? Briefly explain how each one is regulated by the skeletal system.
  8. What do you think would happen to us if we did not have ligaments? Explain your answer.
  9. What is a joint? How is cartilage related to joints? Identify one joint in the human body and describe its function.

11.2 Explore More

What can you learn from ancient skeletons? – Farnaz Khatibi, TED-Ed, 2017.

Kathy Reichs on Forensic Anthropology, Cornerstobe Publishing, 2012.

https://www.youtube.com/watch?v=7tKPju8nYi8

Sexual dimorphism in non-human primates – Video Learning – WizScience.com, 2015.

Attributions

Figure 11.2.1

Skull_and_Crossbones.svg by Unknown author on Wikimedia Commons is from The Unicode Standard (this image shows the character U+2620.) All graphic representations of Unicode characters are in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 11.2.2

Skeleton by OpenStax on Wikimedia Commons is used under a  CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 11.2.3

Axial_skeleton_diagram_blank.svg by Quico/ Qllach on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (This is a derivative work from Axial skeleton diagram.svg, by Mariana Ruiz Villarreal [LadyofHats].)

Figure 11.2.4

Appendicular_skeleton_diagram_blank.svg by by Quico/ Qllach on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (This is a derivative work from Appendicular_skeleton_diagram.svg, by Mariana Ruiz Villarreal [LadyofHats].)

Figure 11.2.5

Animation_triceps_biceps by Niwadare on Wikimedia Commons is used under a CC BY-SA 4.0  (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 11.2.6

Male pelvisGray241 by Henry Vandyke Carter (1831-1897) on Wikimedia Commons is in the  public domain (https://en.wikipedia.org/wiki/Public_domain). (Bartleby.comGray’s AnatomyPlate 241)

Figure 11.2.7

Female pelvisGray242 by Henry Vandyke Carter (1831-1897) on Wikimedia Commons is in the  public domain (https://en.wikipedia.org/wiki/Public_domain). (Bartleby.comGray’s AnatomyPlate 242)

 

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 27). Figure 7.2 Axial and appendicular skeleton [digital image].  In Anatomy and Physiology (Section 7.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/7-1-divisions-of-the-skeletal-system

Cornerstobe Publishing. (2012, November 28). Kathy Reichs on forensic anthropology. YouTube. https://www.youtube.com/watch?v=L101Bvj0lAA

TED-Ed. (2017, June 15). What can you learn from ancient skeletons? – Farnaz Khatibi. YouTube. https://www.youtube.com/watch?v=T24hdchCVIg

VanDyke Carter, H. (1858). Illustration plates 241 and 242. In H. Gray,  Anatomy of the Human Body. Lea & Febiger. Bartleby.com, 2000. www.bartleby.com/107/.

Wiz Science. (2015, September 4). Sexual dimorphism in non-human primates – Video Learning – WizScience.com. YouTube. https://www.youtube.com/watch?v=7tKPju8nYi8

 

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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