Revealing The Skeleton: Your Bones and Their Functions

Ling Beisecker
Revealing The Skeleton: Your Bones and Their Functions

When does a skeleton laugh? When something tickles its funny bone!

Okay, there may not technically be a “funny bone,” but with a bit of luck, that joke bumped against your humerus.

The Bare Bones

Able to move easily through space, transmit force, receive weight and adapt to stress, and NOT collapse under significant force, bones are simply amazing. After being born with about 300 bones, some fuse together leaving the average adult body roughly 206 bones.

Interestingly, about 20% of the adult skeleton is replaced each year. Moderate physical activity and weight-bearing activities foster bone maintenance and maintain bone strength.

Axial and Appendicular Skeleton

The bones of the human skeleton are separated into the axial skeleton and the appendicular skeleton. The axial skeleton consists of about 80 bones and forms the long axis of the body, including important bones like the skull, vertebral column, and rib cage. The axial skeleton protects, supports, and carries other body parts.

The appendicular skeleton is comprised of about 126 bones and consists of bones of the upper and lower limbs and the pectoral and pelvic girdles. Because the appendicular skeleton is not fused like the axial skeleton, it has a greater range of motion and is involved in movement of the body (locomotion) as well as protecting major organs.

Cell Composition

Bones are made from three types of cells: osteoblasts, osteocytes, and osteoclasts. Osteoblasts create a matrix that is mineralized to become bone. Osteocytes are mature bone cells inside the matrix’s lacunae.

Lacunae are the spaces located between the bone layers, or lamellae. Osteoclasts absorb bone tissue during growth and health and fundamentally act as the cleanup crew for maintenance, repair and remodeling of bones.

Compact and Cancellous Bones

Compact bones are strong and weight-bearing, and they form the outer shell of bones. They are hard, nearly solid masses of bony tissue in concentric layers that are made up of canals, or tubes, that allow blood vessels and nerves to pass through the bone. These canals known as Haversian canals make up the Haversian system.

Cancellous bones are often referenced as spongy bones because of their meshwork of bony bars, or trabeculae, with interconnecting spaces that contain bone marrow. These are layered areas that contribute to the overall strength of the bone, as well as keeping it light to reduce the weight of the skeleton — all of which make locomotion easier.

These are often located beneath compact bone and can be found at the end of long bones, like the head of the femur, or at the center of bones.

Five Types of Bones

  1. Flat bones act as a shield for vulnerable areas such as the brain, heart, and pelvic organs. In adults, the highest number of red blood cells are formed in flat bones. Flat bones also have large areas for muscle attachment and are often thin in shape and structure. Areas of the skull, thoracic cage, pelvis, and scapulae are examples of flat bones.
  2. Long bones are classified by their dimensions, with their length surpassing their width. Typically located in the appendicular skeleton, long bones support the weight of the body as well as help with movement. Interestingly, they are often somewhat curved for increased strength. The femur (also the longest bone in the body), humerus, radius, ulna, tibia, fibula and the bones in the fingers and toes are examples of long bones.
  3. Short bones provide stability and some movement. These bones are often found in the wrist and ankle joints.
  4. Irregular bones are categorized because of their variation in shape and structure. Often complex in design to protect specific internal organs, their shape is based on the function they fulfill. For instance, the vertebral column, sacrum and facial bones like the jawbone (mandible) are all classified as irregular bones.
  5. Sesamoid bones are embedded in tendons and are found in areas impacted by considerable friction, tension and physical stress. The most common example is the patella, or kneecap, which protects the tendons from stress and wear and tear.

Common Bone Conditions

A fracture (when a bone breaks), potentially due to enduring a great impact, is the most common condition that affects the skeletal system. Other common conditions include osteoporosis, leukemia, types of bone loss (osteopenia, osteitis deformans and osteomalacia) and irregularities of the spinal curve (scoliosis, kyphosis and lordosis).

Six Basic Functions of the Skeletal System

While they are a primary part of the musculoskeletal system, bones are also important for the circulatory, nervous, immune, and endocrine systems. Bones are often classified into six functions: support, protection, movement, storage, blood cell production and endocrine regulation.

  1. Support: bones are strong and act as a framework for the body.
  2. Protection: reduce risk of injury to internal organs; for instance, the rib cage protects the heart and lungs.
  3. Movement: skeletal muscles attach to bones and muscle contractions move the bones. Additionally, bones can act as levers at points of attachment for muscles, which can impact the magnitude and direction of the forces generated by muscles.
  4. Storage: bone tissue stores several minerals, including calcium and phosphorus. Bones are important for the nervous system because the skeleton holds 99% of the body's calcium, the most abundant mineral in the body that is critical for working neurons. Bones are also key players in releasing minerals, like calcium, into the blood when needed to balance the body.
  5. Blood cell production: red bone marrow and yellow bone marrow is found in the cavities of certain bones. Red bone marrow forms most of the red and white blood cells (hematopoiesis) in the body and helps destroy old blood cells, which is crucial for the circulatory and immune system. Yellow bone marrow consists of adipose cells and stores a few blood cells, lipids such as fats, and can be key players when needing chemical energy from reserve.
  6. Endocrine regulation: hormones like osteocalcin, which help regulate bone formation as well as act as a defense against obesity, glucose intolerance, and diabetes mellitus, are secreted by bone cells; therefore, bones also have a part to play in the endocrine system.

While this article focused on the bones and their functions, it is important to note that lifestyle changes, like adding yoga to your daily regimen, can have great impact to foster long-term bone health. Feel free to add any cool bone facts in the comments below!