long essay on skeletal system

The Skeletal System Essay

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Introduction

Axial portion of the skeleton, appendicular portion of the skeleton, functions of the skeleton, relationship between the skeletal system and the muscular system, sexual differences in skeletons, clinical conditions and disorders that affect the skeleton, works cited.

Movement is vital for all of you because it provides you with the opportunity to live your lives to the full. Just as other human beings, you fall and stand up to continue moving forward. But what provides you with this opportunity? It is your skeletal system. It does not only facilitate your physical activity but also supports and protects your bodies. This system consists of hundreds of bones that are full of calcium, which makes them strong enough to carry your weight. Bones are connected with the help of joints that facilitate motion. The majority of you were born with about 300 bones that fuse with the course of time so that now you have only 206 bones. They all are divided into two parts: axial and appendicular skeletons.

Your axial portion of skeleton is composed of “the skull, the vertebral column, and the thoracic cage” ( Skeletal System: Bones and Joints 120). Due to its location, it manages to protect your brain and spinal cord from injuries. In addition to that, it supports the organs in the ventral body cavity so that you do not need to carry them in your hands.

Twenty-two bones that are separated into two parts form the skull. You have 8 bones of the cranial cavity that are known as braincase. They surround your brain so that you do not hurt it when fall or receive a headnut. The rest of the bones (there are 14 of them) form your face. They are tightly connected to one another so that your nose is always in the right place. The only exception is the mandible that makes chewing possible. Otherwise, how would you eat? Minimal movement can also be observed within the middle ears. Each of them includes 3 auditory ossicles that are hidden deep in your head.

The vertebral column, or backbone, usually consists of “7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 1 sacral bone, and 1 coccyx bone” ( Skeletal System: Bones and Joints 125). It is the central axis of the skeleton that has four major curvatures. Normally, the cervical and the lumbar regions curve anteriorly. The thoracic, as well as the sacral and coccygeal regions, curves posteriorly. However, considering the way you sit, abnormal curvatures are widespread.

The thoracic or the rib cage protects your organs and supports them. All in all, human beings have 24 ribs that are divided into 12 pairs, but you can recount them to make sure. They are categorized according to their attachment to the sternum. Thus, a direct attachment by costal cartilages is true (1-7); an attachment by a common cartilage is false (8-12); and the absence of attachment resorts to floating ribs (11-12). The sternum, or breastbone, consists of three parts: “the manubrium, the body, and the xiphoid process” ( Skeletal System: Bones and Joints 129).

Your appendicular skeleton consists of the bones of limbs and girdles so that you have:

• “4 bones in the shoulder girdle (clavicle and scapula each side).
• 6 bones in the arm and forearm (humerus, ulna, and radius).
• 58 bones in the hands (carpals 16, metacarpals 10, phalanges 28, and sesamoid 4).
• 2 pelvis bones.
• 8 bones in the legs (femur, tibia, patella, and fibula).
• 56 bones in the feet (tarsals, metatarsals, phalanges, and sesamoid)” (“The Axial & Appendicular Skeleton” par. 4).

What would you be without this part of skeleton? Imagine that it is a big 3D puzzle, gathering all these bones together in a right order, you will build your arms and legs with all details. These are all movable parts that allow you to run, dance, write, and even hug your nearest and dearest. Even though the axial skeleton seems to be more important because it is connected with your brain, the appendicular portion of the skeleton contains about 60% of all your bones, which means that its importance should not be undervalued.

As you have already understood, your skeleton maintains a lot of different functions. Some of them, such as movement and support, were already mentioned. But let us discuss them all in detail.

• Support. Your bodies are supported by the skeleton so that you can change your position to vertical one and stand strait. Without it, you would be able only to lie because of the gravitation. This function is provided by many bones but the long ones seem to be the leaders in this competition. For instance, those that are in legs, support the trunk. Similarly, vertebras support one another so that eventually the firs one provides support to the skull. In addition to that, they support the organs and ensure that they do not change their positions.
• Protection. The skeleton also protects you. For example, the skull prevents fatal brain injuries. The rib cage protects such vital organs as the heart and lungs. It also takes care of your abdominal organs ensuring that they develop normally.
• Movement. The function of bodily motion allowed you to come here today. However, it is critical to remember that it is maintained not only due to the bones but also with the help of the muscular system.
• Mineral and energy storage. From the outer side of your bones, there is a tissue that serves as a storage. It gathers calcium and phosphorus and withdraws them to maintain appropriate blood levels. In addition to that, mature bones store yellow marrow. It consists of fat almost totally and provides you with energy for various activities.
• Blood-cell formation. The inner core of your bones takes part in the formation of blood cell and platelet. It is known as bone marrow or red marrow. Platelet is vital for you because it ensures your ability to heal wounds while blood cells spread oxygen and destroy infectious cells (CAERT 3).

Have you ever thought of the way our movement are maintained? Even a simple nod of the head requires the cooperation between the skeletal and muscular systems. Muscles ensure movement of our body through the attachment to the bones. All in all, there are about 700 of them, which is an enormous amount that comprises about 50% of your weight.

So what happens in your body when you moves? When you want to move, your brain sends a message for the body to release energy. In medical terms, it is called adenosine triphosphate. Affecting your muscles, it makes them contract or shorten. Shortened muscles pulls bones at their insertion point. Thus, the angle between the bones connected by a joint shortens. Relaxation is maintained when the opposing muscle extends and pulls a bone to its initial position.

Human skeletons seem to be similar, as they contain the same bones. However, you should remember that their characteristics differ depending on the gender. For example, women have lighter pelvis bones that form a shorter cavity with less dimensions. It has less prominent marking for muscles and more circular pelvic brim. The sacral bones of men are longer and narrower, which makes them more massive. Their femur is also longer and heavier. Its texture is rough unlike women’s smooth.

Muscle marking is more developed and shaft is less oblique. The head of men’s femur is larger and trochanters are more prominent. The femoral neck angle in males is more than 125 and in females is less than 125. Women’s sternum is less than twice the length of manubrium and larger in men. Differences in skull include greater capacity, thicker walls, more marked muscular ridges, prominent air sinuses, smoother upper margin of orbit, less vertical forehead, and heavier cheekbones in males.

Hopefully, it will never affect any of you but the skeleton may be affected by tumours that cause bone defects. People may have skeletal developmental disorders including gigantism, dwarfism, osteogenesis imperfecta, and rickets lead to abnormal body sizes, brittle bones, and growth retardation. Bacterial infections cause inflammation and lead to bone destruction.

Decalcification, including the known to you osteoporosis, reduces bone tissue and softens bones. Joint disorders often deal with inflammation. For instance, arthritis. They are often influenced by age and physical activity. In this way, degradation of joints is observed in the elderly but can be delayed due to regular exercises. The abnormal curvatures of the spine may also cause health issues. That is why you should pay attention to your back posture and avoid kyphosis (a hunchback condition), lordosis (a swayback condition), and scoliosis (an abnormal lateral curvature).

CAERT. Structures and Functions of the Skeletal System . 2014. Web.

Skeletal System: Bones and Joints. 2012. Web.

“ The Axial & Appendicular Skeleton. ” TeachPE , 2017. Web.

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6.1 The Functions of the Skeletal System

Learning objectives.

By the end of this section, you will be able to:

List and describe the functions of the skeletal system

• Attribute specific functions of the skeletal system to specific components or structures

The skeletal system is the body system composed of bones, cartilages, ligaments and other tissues that perform essential functions for the human body. Bone tissue, or osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the internal support structure of the body. In the areas of the skeleton where whole bones move against each other (for example, joints like the shoulder or between the bones of the spine), cartilages, a semi-rigid form of connective tissue, provide flexibility and smooth surfaces for movement. Additionally, ligaments composed of dense connective tissue surround these joints, tying skeletal elements together (a ligament is the dense connective tissue that connect bones to other bones). Together, they perform the following functions:

Support, Movement, and Protection

Some functions of the skeletal system are more readily observable than others. When you move you can feel how your bones support you, facilitate your movement, and protect the soft organs of your body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilages of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones facilitate movement by serving as points of attachment for your muscles. Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (see Figure 6.1.1 ).

Mineral and Fat Storage, Blood Cell Formation

On a metabolic level, bone tissue performs several critical functions. For one, the bone tissue acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and are involved in the transmission of nerve impulses.

Bones also serve as a site for fat storage and blood cell production. The unique connective tissue that fills the interior of most bones is referred to as bone marrow . There are two types of bone marrow: yellow bone marrow and red bone marrow. Yellow bone marrow contains adipose tissue, and the triglycerides stored in the adipocytes of this tissue can be released to serve as a source of energy for other tissues of the body. Red bone marrow is where the production of blood cells (named hematopoiesis, hemato- = “blood”, -poiesis = “to make”) takes place. Red blood cells, white blood cells, and platelets are all produced in the red bone marrow. As we age, the distribution of red and yellow bone marrow changes as seen in the figure ( Figure 6.1.2 ).

Career Connection – Orthopedist

An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery ( Figure 6.1.3 ).

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Section Review

The major functions of the skeletal system are body support, facilitation of movement, protection of internal organs, storage of minerals and fat, and blood cell formation.

Review Questions

Critical thinking questions.

• Suppose your red bone marrow could not be formed. What functions would your body not be able to perform?
• Suppose your osseous tissue could not store calcium. What functions would your body not be able to perform?

Answers for Critical Thinking Questions

• Without red bone marrow, you would not be able to produce blood cells. The red bone marrow is responsible for forming red and white blood cells as well as platelets. Red blood cells transport oxygen to tissues, and remove carbon dioxide. Without red blood cells, your tissues would not be able to produce ATP using oxygen. White blood cells play a role in the immune system fighting off foreign invaders in our body – without white blood cells you would not be able to recover from infection. Platelets are responsible for clotting your blood when a vessel ruptures. Without platelets you would bleed to death and die.
•  The calcium in osseous tissue provides mineral support to bones. Without this calcium, the bones are not rigid and cannot be supportive. The calcium in osseous tissue is also an important storage site, that can release calcium when needed. Other organ systems rely on this calcium for action (specifically, muscle contraction and neural signaling). Without calcium storage, blood calcium levels change dramatically and affect muscle contraction and neural signaling.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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The Anatomy of the Skeletal System

The skeletal system comprises 206 bones and has two main parts—the axial skeleton and the appendicular skeleton. The skeletal system includes your bones, ligaments that attach bone to bone, and cartilage that provides padding between your bones.

This article discusses the anatomy of the skeletal system—what it's made of, how it's organized, conditions that affect it, and tests that assess it.

SDI Productions / Getty Imgaes

Skeletal System: Labeled Diagram of Major Organs

In addition to the bones, organs of the skeletal system include ligaments that attach bones to other bones and cartilage that provides padding between bones that form joints throughout your body.

The bones are divided into two main categories—the axial skeleton , which contains the bones that support the middle of your body, and the a ppendicular skeleton, which includes bones that make up your appendages—arms and legs—and bones that attach your limbs to your axial skeleton.

Axial Skeleton

The axial skeleton forms the "axis" that runs down the center of the body. There are 80 bones that make up the axial skeleton.

Skull (Cranium)

Your skull ( cranium ) is made up of cranial and facial bones. Cranial bones protect your brain, while facial bones make up your facial structure. Skull bones include the following:

Cranial bones include:

Facial bones include:

• Maxillae (upper jaw)
• Mandible (lower jaw)
• Zygomatic (cheekbones)
• Inferior nasal conchae

Auditory Ossicles

The auditory ossicles consist of a total of six tiny bones, with three in each ear. They are located in the inner ear and are structures that help create sound in your body. The auditory ossicles include the following:

The hyoid bone is a horseshoe-shaped bone located in the throat. It is part of bodily functions like speaking, swallowing, and airway maintenance.

Vertebral column

The vertebral column (spine) protects your spinal cord, supports your head, and allows bodily movement. It contains the sacrum (made up of four bones) and coccyx (the tailbone, which is made up of five bones), and 24 vertebrae, including:

• Cervical vertebrae : Seven bones in the neck region
• Thoracic vertebrae : Twelve bones attached to the ribs
• Lumbar vertebrae : Five bones in the low back region

The thorax contains the sternum (breastbone) and the thoracic (rib) cage . The thoracic cage comprises 12 pairs of ribs connecting to the thoracic vertebrae and the sternum. Your rib cage protects your heart.

Appendicular Skeleton

The appendicular skeleton includes 126 bones that comprise your appendages—your arms and legs—and the bones that attach your limbs to your axial skeleton.

Upper Extremities

Your upper extremities refer to your shoulders and arms. Bones in the upper extremities include:

• Scapula (shoulder blade)
• Clavicle (collarbone)
• Humerus (upper arm)
• Radius and ulna (forearm bones)
• Carpals (eight tiny bones in the wrist)
• Metacarpals (in the palm)
• Phalanges (bones of the fingers)

Lower Extremities

Bones in the lower extremities make up your hips and legs and include:

• Femur (thigh bone)
• Patella (kneecap)
• Tibia and fibula (lower leg bones)
• Tarsals (eight tiny bones in the ankle)
• Metatarsals (in the middle of the foot)
• Phalanges (bones of the toes)

Which Bones Are Most Commonly Broken?

The most commonly fractured bones include the distal radius (on the thumb side of your wrist), the ankle, the femur (thigh bone), the humerus (upper arm bone), and the metacarpals (bones of the palms).

With osteoporosis, the most commonly fractured bones are the vertebrae (in the spine).

What Is the Purpose of the Skeletal System?

The primary purpose of the skeletal system is to give the body its shape and to provide attachment points for the muscles that move the body.

Other purposes of the skeletal system include:

• Storing minerals (such as calcium) and fats
• Producing red blood cells
• Protecting internal organs

Calcium in Your Bones

Most of the body's calcium is stored in your bones.

Skeletal System Associated Conditions

Various conditions and injuries can affect the skeletal system. Examples include:

• Fractures (broken bones)
• Ligament sprains
• Osteoarthritis
• Osteoporosis
• Osteogenesis imperfecta
• Osteomalacia
• Paget's disease of bone
• Osteomyelitis
• Avascular necrosis (osteonecrosis)
• Marfan syndrome
• Ehlers-Danlos syndrome
• Osteopetrosis
• Ankylosing spondylitis
• Bone marrow diseases

Skeletal System Tests

Many different tests can help diagnose conditions that affect the skeletal system.

Imaging Tests

Healthcare providers use various imaging tools to get detailed pictures of your bones. Depending on the reason for imaging, a healthcare provider will conduct one or more of the following tests:

• X-rays : This common test can help diagnose conditions that affect the bones and joints, such as fractures or arthritis.
• Computed tomography (CT scan), computerized axial tomography (CAT scan) : This test provides three-dimensional pictures that help diagnose fractures that aren't clear on X-rays, or other bone conditions, such as cancer.
• Magnetic resonance imaging (MRI) : This type of imaging often helps diagnose conditions that affect soft tissues of the skeletal system (ligaments, cartilage).
• Bone scintigraphy (bone scan) : These scans can provide detailed information about a bone injury or condition, such as the staging of bone cancer.
• Positron emission tomography (PET scan) : This test, which uses an injected radioactive tracer, can help stage bone (and other types of) cancer.
• Bone density test : These tests are primarily for determining how dense bones are—the key factor in diagnosing osteoporosis.

Other Skeletal System Tests

Healthcare providers may perform additional tests if they need further information about your bones or skeletal system. These tests may include:

• Joint aspiration : This test involves removing a sample of fluid from a joint to help diagnose infection.
• Biopsy : For the skeletal system, this procedure can involve removing a small sample of bone or bone marrow so that it can be tested for conditions such as cancer.
• Blood tests : These tests help diagnose infections that can affect the skeletal system.

The skeletal system is made up of your bones, ligaments, and cartilage. Though its main function is to provide structural support for the body, it also stores important minerals—such as calcium—forms red blood cells, and protects your internal organs. The skeletal system can break down into two main categories—the axial skeleton, which forms the "long axis" of the body, and the appendicular skeleton, which forms your arms and legs.

Many different injuries and diseases can affect the skeletal system. Imaging procedures, such as X-rays, MRI, and bone density tests, and other tests, such as blood work or tissue biopsy, can help to diagnose these conditions.

National Cancer Institute. Cranium .

National Cancer Institute SEER Training Modules. Axial skeleton (80 bones) .

Fisher E, Austin D, Werner HM, et al. Hyoid bone fusion and bone density across the lifespan: prediction of age and sex .  Forensic Sci Med Pathol . 2016;12(2):146-157. doi:10.1007/s12024-016-9769-x

Osmosis from Elsevier. Bones of the vertebral column .

National Cancer Institute SEER Training Modules. Appendicular skeleton (126 bones) .

American Academy of Orthopaedic Surgeons. Osteoporosis and spinal fractures .

National Cancer Institute SEER Training Modules. Introduction to the skeletal system .

MedlinePlus. Calcium and bones .

National Institute of Arthritis and Musculoskeletal and Skin Diseases. Muscle and bone diseases .

MedlinePlus. Diagnostic imaging .

InformedHealth.org. Understanding tests used to detect bone problems .

Johns Hopkins Medicine. Joint aspiration .

By Aubrey Bailey, PT, DPT, CHT Dr, Bailey is a Virginia-based physical therapist and professor of anatomy and physiology with over a decade of experience.

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skeleton summary

Learn about the parts and functions of the skeletal system.

skeleton , Bony framework of the body. It includes the skull, vertebral column , collarbone, shoulder blades, rib cage, pelvic girdle and the bone s of the hands, arms, feet, and legs. The skeleton supports the body and protects its internal organs. It is held together by ligaments and moved at the joint s by the muscle s, which are attached to it. The skeletal system includes both bones and cartilage.

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Skeletal System

Reviewed by: BD Editors

The skeletal system provides support and protection for the body’s internal organs and gives the muscles a point of attachment. Humans have an endoskeleton , where our bones lie underneath our skin and muscles. In other animals, such as insects, there is an exoskeleton on the outside of the body.

In humans, the skeletal system consists of bones, joints and associated cartilages. An adult human has 206 bones in their body and variety of different joints.

Image shows a human skeleton with the major bones labeled.

The human skeleton can be divided into two components: the axial skeleton and the appendicular skeleton. The axial skeleton is formed around the central axis of the body and thus includes the skull, spine , and ribcage. It protects the brain, spinal cord, heart, lungs, esophagus and major sense organs like the eyes, ears, nose, and tongue. The appendicular skeleton is related to the limbs and consists of the bones of the arms and legs, as well as the shoulder and hip girdles.

Skeletal System Function

The first and most apparent function of the skeletal system is to provide a framework for the body. The presence of a firm bony skeleton allows the organism to have a distinctive shape adapted towards a particular lifestyle. For instance, in a fast-moving animal like the cheetah, the skeleton contains long, thin limb bones and an extremely flexible spine. The structure of the skeleton also allows it to absorb the impact of running at high speeds.

The bones of birds are hollow, light and create a streamlined body adapted for flight. Many animals even have sexual dimorphism in their skeletons. In humans, while this dimorphism is fairly limited, there are differences in the angle of the pelvic bones, to accommodate pregnancy.

Integration with the Muscular System

The skeletal system also provides an important form of attachments to the muscular system. Bones and exoskeletons are hard and do not bend or move when muscles are flexed. This means that the contraction of muscle cells will lead to the shortening of muscles, while the bone retains its shape. This basic structure allows muscles to move different parts of the body, using forces generated while pulling on the skeletal system.

The next obvious function of the skeletal system is the role it plays protecting the fragile internal organs. In humans, this is seen in the skull, which surrounds the brain completely. It is also exhibited by the ribcage, which surrounds the lungs and heart but still allows for expansion. Even invertebrates like snails and prawns often have hard exoskeletons to protect themselves from predators.

The rigid endoskeleton allows the body to rise up above the ground or stand upright, and bears the weight of the organism, and provides the scaffolding for movement. Muscles generate the force required to move bones at joints. Muscle fibers contain actin and myosin, two protein filaments that can slide past each other to change the length of the muscle. When a nerve impulse arrives at the neuromuscular junction, it signals the muscle to contract. The force generated by the contracting muscle either pulls two bones together or apart, based on the nature of the interaction between the muscle and joint.

Blood Cell Production

The central part of a bone contains the bone marrow , the primary site for blood cell production in adult humans. There are two types of bone marrow in adults. Around 50% is red bone marrow containing hematopoietic stem cells and supportive tissue. The rest is yellow bone marrow made of fat and its proportion increases with age.

Bone marrow will revert to a higher proportion of red marrow if the body suffers an injury and needs to create more red blood cells. The bone marrow composition also changes during pregnancy and lactation in mammals. Over the course of gestation, blood volume increases by about 1.5 liters, and even the concentration of red blood cells and white blood cells increase.

Production of other Cell Types

In addition to producing red blood cells, bone marrow within the skeletal system is the production site of a number of other cells. These include lymphocytes , which are immune cells that travel the lymphatic system. In addition to providing immune functions, the skeletal system is also responsible for hosting stem cells which can differentiate into muscle cells, cartilage-producing cells, and cells that create bone (osteoblasts).

Osteoblasts in bone also have an endocrine function, secreting a hormone called osteocalcin. It requires vitamin K to be synthesized and is an anabolic hormone. It mediates an increase in insulin levels and increases the sensitivity of the body to insulin. Osteocalcin contributes to an increase in bone mass and bone mineralization.

Storing Minerals

The bones of the skeletal system act as a storehouse for calcium ions , changing the quantum of mineralized deposits within bones to maintain plasma calcium ion concentration within a narrow range. Calcium ions can affect crucial sodium ion channels in the plasma membrane of every cell, thereby affecting overall homeostasis.

For this reason, changes to the concentration of calcium ions have particularly adverse effects on excitable cells in the nervous system, and in cardiac, skeletal and smooth muscle. Different interacting hormones maintain the balance of calcium ions in the plasma and bones, especially the parathyroid hormone secreted from the parathyroid glands in the neck.

Skeletal System Parts

The anatomy of the skeletal system is complex, and it includes hundreds of bones in the human body . The anatomy of the system varies widely between organisms, as evolution has selected for various adaptations in certain species which change the structure and function of their bones.

Bones serve a variety of functions, but the most important is supporting movement of the limbs and body. Two bones or cartilages are held together at a joint through tough connective tissues called ligaments. Muscles are securely attached to bones through flexible but inelastic connective tissue called tendons. Muscles, joints, tendons, and ligaments are part of the intricate machinery that allows the movement of different bones.

Functionally, joints can be divided into three classes based on the range of movement they allow in the associated bones. Immovable joints are formed when two bones are held together by fibrous connective tissue with no synovial fluid. These kinds of joints hold the bones of the cranium together.

Partially movable joints are also called cartilaginous joints and are present in the spine and ribs. The third type of joints are called synovial joints and have a fluid-filled synovial cavity that allows the interfacing bones the largest range of movement. Based on the structure of the synovial joints, they can be classified into 6 types, including the hinge joints of the fingers and the ball and socket joints of the hips and shoulders.

Cellular Composition

Each bone is made of complex sets of cells, tissues and a specialized extracellular matrix . The two main cell types are called osteoblasts and osteoclasts with mostly opposing functions. While osteoblasts are involved in the formation of bone, osteoclasts are associated with a reduction in bone mass. The extracellular matrix of the bone consists of collagen and other organic fibers as well as the inorganic component containing calcium salts such as hydroxyapatite. In the interior of bones, a soft tissue called the bone marrow plays an important role in immunity and hematopoiesis. The bone is also richly supplied with nerves and blood vessels.

Skeletal System Structure

In general, the skeletal system is structured to provide support against gravity and protect an animal’s internal organs. While this article mainly discusses the human skeletal system, most animals have some sort of skeleton. Some animals, like sponges, can have an extremely simplified skeleton made of calcium deposits within the animal. Others, like the turtle, have drastically modified their skeletal system to provide extra protection.

While this article mostly discusses an  endoskeleton,  many animals use an  exoskeleton for the same purposes. Instead of bones being on the inside, the bones, protective plates, or chitinous skeleton actually surrounds the muscles. While this may seem completely different, the structure of the system is still very similar. The only difference is that muscles and tendons connect to the inside of the system, rather than to the surface of bones.

The structure of the skeletal system reflects an animal’s evolution, as well as the needs it has to survive. For example, humans have a tailbone. This is an evolutionary relic, from the time when our ancestors had tails and were swinging from the trees. As we became bipedal, we lost the need for a tail, and it was reduced to a single, nonfunctional bone. Likewise, all animals are constantly adapting and changing their skeletal system through evolutionary time.

Skeletal System Diseases

Diseases of the skeletal system could be confined to one section of the skeleton such as changes to the curvature of the spine, or they could be a genetic disorder affecting all bones and joints such as arthritis or osteoporosis.

The spine in healthy individuals is S-shaped, with a convex curvature for the thoracic region and the concave tilt in the cervical and lumbar regions. This shape for the spine is ideally suited for an upright walking posture. If either the thoracic or lumbar regions have a change in curvature or there is sideways bend to the spine, it can lead to back pain, difficulty with breathing, digestion, mobility, and reproduction.

Curvature of the Spine

The bulk of the weight of the upper body is transmitted along the central axis towards the legs. When the bones or muscles of the back or not functioning optimally, it can lead initially to accommodative changes in posture and thereafter to pain, injury or permanent deformity. Since the spine surrounds the spinal cord, abnormalities in the skeletal structure of the spine can affect the nervous system, either manifesting as pain, tingling or numbing in the extremities. Additionally, the spine supports the ribcage, enclosing the heart, lungs, and diaphragm. Thus, spinal deformities can also lead to shortness of breath, palpitations or even cardiac arrhythmias.

Kyphosis is the term for the convex curve of the thoracic region and excessive curvature in this region is called hyperkyphosis. Extreme hyperkyphosis presents as a hunchback. This could arise from genetic factors or poor posture due to obesity or osteoporosis or arthritis.

The normal concave structure of the lumbar region is called lordosis , and when the region is overly arched, it is called lumbar hyperlordosis. In hyperlordosis, shoulders appear to be pushed back, while the abdominal region seems to be jutting forward.

Image shows a person with hyperlordosis. In a healthy spine, the midpoint of the spine (A) would be directly above the knee (B).

Hyperlordosis can arise from genetic factors, poor posture, or even deficient muscle strength. When the spine has a sideways tilt, or a lateral bend, it is called scoliosis and could be associated with both hyperkyphosis and hyperlordosis.

Osteoporosis

Osteoporosis is a condition marked by bone resorption. This reduces bone mass and density, thereby enhancing the probability of fractures from even minor stressors such as sneezing. Although osteoporosis is commonly associated with aging, smoking, obesity, diet, some medications and alcohol consumption can contribute to the progression of the ailment.

Weight training, exercise, and a diet containing adequate calcium, iron, phosphorous as well as Vitamin D, help in enhancing bone density and bone mass. There is some evidence that the pH of blood plays a role in the release of calcium stores from bones and the extent of bone mineralization since calcium salts are often used as buffers in acidic environments in the body. A whole-foods, plant-based diet has been shown greatly reduce blood acidification. As a result, it also lowers cases of osteoporosis.

Arthritis includes a number of joint disorders that are characterized by stiffness, inflammation, and pain. While there is a range of potential causes, arthritis usually worsens with age, affecting the joints that are used most frequently – especially the joints in fingers, hips, and knees. Arthritis, therefore, causes disability, restricts movement and impairs fine motor skills.

Interesting Facts

• Three bones in the inner ear, called malleus, incus and stapes, are the smallest bones in the human body. The thigh bone, or femur, is the largest bone.
• The hyoid bone situated behind the lower jaw is also called a ‘floating’ bone because it is not part of any joint, and is not directly attached to any other bone.
• The position of the hyoid bone makes it extremely resistant to fractures. However, autopsies that reveal a broken hyoid bone indicate death from strangulation.
• Newborn babies have about 300 bones. Many of these bones fuse together to form the 206 bones of the adult.
• Teeth are part of the skeletal system. However, they are not bones.

1. Which of these is an inflammatory condition?

2. Meat, carbonated beverages, cheese, eggs and even milk have been linked to a decrease in blood pH. How might these things affect bone health?

3. Why is the hyoid bone called a floating bone?

4. Why do babies have more bones than adults?

5. Which of the following are functions of the skeletal system?

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Module 7: Bone Tissue and The Skeletal System

The functions of the skeletal system, learning objectives.

• Define bone, cartilage, and the skeletal system
• List and describe the functions of the skeletal system

Figure 1. Bones Support Movement. Bones act as levers when muscles span a joint and contract. (credit: Benjamin J. DeLong)

Bone , or  osseous tissue , is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints),  cartilage , a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The  skeletal system  is the body system composed of bones and cartilage and performs the following critical functions for the human body:

• supports the body
• facilitates movement
• protects internal organs
• produces blood cells
• stores and releases minerals and fat

Support, Movement, and Protection

The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body.

Figure 2. Bones Protect Brain. The cranium completely surrounds and protects the brain from non-traumatic injury.

Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin.

Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers and joints serve as fulcrums (Figure 1).

Unless a muscle spans a joint and contracts, a bone is not going to move. For information on the interaction of the skeletal and muscular systems, that is, the musculoskeletal system, seek additional content.

Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (Figure 2).

Career Connection: Orthopedist

An  orthopedist  is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery (Figure 3).

Figure 3. Arm Brace. An orthopedist will sometimes prescribe the use of a brace that reinforces the underlying bone structure it is being used to support. (credit: Juhan Sonin)

While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development.

Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly.

Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery.

Mineral Storage, Energy Storage, and Hematopoiesis

Figure 4. Head of Femur Showing Red and Yellow Marrow. The head of the femur contains both yellow and red marrow. Yellow marrow stores fat. Red marrow is responsible for hematopoiesis. (credit: modification of work by “stevenfruitsmaak”/Wikimedia Commons)

On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and potassium. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses.

Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow (Figure 4). There are two types of bone marrow: yellow marrow and red marrow.  Yellow marrow  contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy.  Red marrow  is where  hematopoiesis —the production of blood cells—takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow.

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Bones development and repair and the effects of disease and aging on locomotion.

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Author: Roberto Grujičić, MD • Reviewer: Dimitrios Mytilinaios, MD, PhD Last reviewed: October 30, 2023 Reading time: 8 minutes

Bones make up the skeletal system of the human bod y. The adult human has two hundred and six bones. There are several types of bones that are grouped together due to their general features, such as shape, placement and additional properties. They are usually classified into five types of bones that include the flat, long, short, irregular, and sesamoid bones.

The human bones have a number of important functions in the body. Most importantly, they are responsible for somatic rigidity, structural outline, erect posture and movement (e.g. bipedal gait ). Due to their rigidity, bones are the main 'protectors' of the internal organs and other structures found in the body.

This article will describe all the  anatomical and important histological facts about the bones.

What is a bone?

Short bones, irregular bones, sesamoid bones, clinical aspects.

A bone is a somatic structure that is composed of calcified connective tissue . Ground substance and collagen fibers create a matrix that contains osteocytes . These cells are the most common cell found in mature bone and responsible for maintaining bone growth and density. Within the bone matrix both calcium and phosphate are abundantly stored, strengthening and densifying the structure.

Each bone is connected with one or more bones and are united via a joint (only exception: hyoid bone ). With the attached tendons and musculature, the skeleton acts as a lever that drives the force of movement. The inner core of bones ( medulla ) contains either red bone marrow (primary site of hematopoiesis) or is filled with yellow bone marrow filled with adipose tissue .

The main outcomes of bone development (e.g. skull bones development ) are endochondral and membranous forms . This particular characteristic along with the general shape of the bone are used to classify the skeletal system. The bones are mainly classified into five types that include:

Types of bones

These bones develop via endochondral ossification , a process in which the hyaline cartilage plate is slowly replaced. A shaft, or diaphysis , connects the two ends known as the epiphyses (plural for epiphysis). The marrow cavity is enclosed by the diaphysis which is thick, compact bone . The epiphysis is mainly spongy bone and is covered by a thin layer of compact bone; the articular ends participate in the joints.

The metaphysis is situated on the border of the diaphysis and the epiphysis at the neck of the bone and is the place of growth during development.

Some examples of this type of bones include:

• The humerus
• The  fibula
• The  metacarpal bones
• The metatarsal bones
• The  phalanges
• The  radius and ulna . 

The short bones are usually as long as they are wide. They are usually found in the carpus of the hand and tarsus of the foot. 

In the short bones, a thin external layer of compact bone covers vast spongy bone and marrow , making a shape that is more or less cuboid . 

The main function of the short bones is to provide stability and some degree of movement.

Some examples of these bones are:

• The scaphoid bone
• The lunate bone
• The calcaneus
• The navicular bone

In flat bones, the two layers of compact bone cover both spongy bone and bone marrow space. They grow by replacing connective tissue. Fibrocartilage covers their articular surfaces. This group includes the following bones:

• The  skull bones
• The  sternum
• The  scapulae

The prime function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Also, due to their flat shape, these bones provide large areas for muscle attachments. 

Due to their variable and irregular shape and structure, the irregular bones do not fit into any other category. In irregular bones, the thin layer of compact bone covers a mass of mostly spongy bone .

The complex shape of these bones help them to protect internal structures. For example, the irregular pelvic bones protect the contents of the pelvis. 

Some examples of these types of bones include:

• The bones of the spine (i.e. vertebrae )
• The bones of the pelvis ( ilium , ischium and pubis )

Sesamoid bones are embedded within tendons . These bones are usually small and oval-shaped. 

The sesamoid bones are found at the end of long bones in the upper and lower limbs, where the tendons cross.

Some examples of the sesamoid bones are the patella bone in the knee  or the pisiform bone of the carpus.

The main function of the sesamoid bone is to protect the tendons from excess stress and wear by reducing friction.

Learn the basics of the skeletal system with this interactive quiz.

The bones mainly provide structural stability to the human body. Due to the development of the complex bony structures (e.g. spine) the humans are able to maintain erect posture, to walk on two feet (bipedal gait) and for all sorts of other activities not seen in animals. 

Due to their rigid structure, bones are key in the protection of internal organs and other internal structures. Some bones protect other structures by reducing stress and friction (e.g. sesamoid bones) while some bones join together to form more complex structures to surround vital organs and protect them (e.g. skull, thoracic cage, pelvis). 

Bones also harbor bone marrow which is crucial in production of blood cells in adults. In addition, the bone tissue can act as a storage for blood cells and minerals.

Common bone diseases often affect the bone density, e.g. in young children due to malnutrition. For example, rickets is a bone deformity seen in young children who lack vitamin D. Their legs are disfigured and they have trouble walking. The damage is irreversible though surgery may help. Osteomalacia and osteoporosis are diseases seen mainly in adulthood.

Osteomalacia is the improper mineralization of bone due to a lack of available calcium and phosphate. The bone density decreases and the bones become soft. Osteoporosis has been noted in all ages but mostly in postmenopausal and elderly women. A progressive decrease in bone density increases the risk of fracture. Patients who are on long-term steroid medication are in particular risk.

• Kyung Won Chung and Harold M. Chung , Gross Anatomy, Sixth Edition, Wolters Kluwer: Lippincott, Williams and Wilkins, Chapter 1, p.1-2.

Illustrators:

• Hamate bone (ventral view) - Yousun Koh
• Patella (lateral-right view) - Yousun Koh

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Critical Thinking Questions

The skeletal system is composed of bone and cartilage and has many functions. Choose three of these functions and discuss what features of the skeletal system allow it to accomplish these functions.

What are the structural and functional differences between a tarsal and a metatarsal?

What are the structural and functional differences between the femur and the patella?

If the articular cartilage at the end of one of your long bones were to degenerate, what symptoms do you think you would experience? Why?

In what ways is the structural makeup of compact and spongy bone well suited to their respective functions?

In what ways do intramembranous and endochondral ossification differ?

Considering how a long bone develops, what are the similarities and differences between a primary and a secondary ossification center?

What is the difference between closed reduction and open reduction? In what type of fracture would closed reduction most likely occur? In what type of fracture would open reduction most likely occur?

In terms of origin and composition, what are the differences between an internal callus and an external callus?

If you were a dietician who had a young female patient with a family history of osteoporosis, what foods would you suggest she include in her diet? Why?

During the early years of space exploration our astronauts, who had been floating in space, would return to earth showing significant bone loss dependent on how long they were in space. Discuss how this might happen and what could be done to alleviate this condition.

An individual with very low levels of vitamin D presents themselves to you complaining of seemingly fragile bones. Explain how these might be connected.

Describe the effects caused when the parathyroid gland fails to respond to calcium bound to its receptors.

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