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If you’ve ever been curious about what holds your body upright or allows you to walk down the hall, this lesson is for you. Here, we’ll be exploring the four main organs of the skeletal system and examining their job in providing structure and movement for the body.

What Is the Skeletal System?

Take a minute to check in with your body. Where are you sitting? Can you feel the chair under your hip bones? Now consider why you don’t just flop over. What is preventing you from turning into a pool of jelly on the floor? The answer is your skeletal system, or all the bones and connective tissue in your body that helps to hold you up.However, you might be surprised to learn that the skeletal system does more than just provide structure. The skeletal system is imperative for movement as well.

It also helps create the red blood cells that carry oxygen around your body, allowing you to make energy and stay alive. Today, we’re going to look at the four main organs of the skeletal system: bones, cartilage, ligaments, and tendons.

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Take a minute and press on one of the bones in your body. Bones seem pretty solid, and even with a rough fall, they still stay intact most of the time. However, bones aren’t just solid pieces of tissue.

Bones are living, breathing organs that grow and change, just like the rest of your body.Bones are made of cells, primarily osteocytes. Most bones have a hollow center filled with bone marrow.

The osteocytes are arranged into groups called osteons that surround the bone marrow in circles. The bone cells secrete a calcium matrix, made of calcium, proteins, and phosphorus that create the solid bone we are familiar with.

Structure and Support

The main job of the bones is to provide structure and support for your body. Bone cells called osteoblasts produce new bone and keep it hard. This hard structure is used to keep us standing and support our organs.

Take your rib cage for example. Inside your rib cage are delicate lungs. Lungs have no muscle and are made of a soft, spongy tissue. Your rib cage prevents any impact to your chest from puncturing the lungs. Your skull serves a similar purpose for your brain. The brain is a soft, spongy tissue in need of a solid case to protect it, much like your phone case protects your phone from damage.


Now, let’s look at the skeletal muscles.

Our skeletal muscles are anchored to our bones for movement. When the muscles contract, or shorten, they cause our bones to move with them, since they are anchored together. The effect is movement of our appendages.Picture doing curls at the gym.

Your bicep muscle is attached to the bone in your forearm and upper arm. When it contracts, it pulls the two bones together, causing your forearm to raise. Without bones, the muscles would just shorten, but we wouldn’t go anywhere.

Blood Cell Production

Inside the hard, calcified bone is a spongy tissue called bone marrow. Bone marrow is the site of production for all blood cells.

Bone marrow contains special cells called stem cells. These cells are capable of becoming any type of blood cell.The stem cells divide and are instructed by chemicals in the environment and interactions with other cells about which blood cell type to become. This process is incredibly important because blood cells don’t divide on their own.

The only way they are replaced is through stem cell division in the bone marrow, and for some white blood cells, the spleen or thymus.


Unlike the hard bones holding up our body, cartilage is flexible. This tissue is made by cells called chondroblasts, which create a matrix of proteins and carbohydrates. Have you ever heard of a cartilage piercing? This popular piercing goes through the upper ear, which is made of cartilage. If you touch your upper ear, you’ll notice it’s firm, but still bends. This is because it is made of cartilage.

Parts of our ribcage are also made of cartilage, providing protection, but still allowing it to expand during respiration.Cartilage also serves as a cushion between our joints. You might be familiar with the medical condition where cartilage wears out between our joints, particularly in the knees. Years of high impact sports can crush and eventually wear out cartilage.


Ligaments are a type of connective tissue that connect bone to other bones. This type of tissue is important for keeping the skeletal system strong and stable. Joints, or connections between bones, allow us to move our appendages, but they also are weak points liable to dislocations or other damage. Ligaments help keep our bones put together.

If you’re a fan of soccer, you are probably familiar with the injury of an anterior cruciate ligament (ACL) tear. This injury occurs when the ligaments connecting the femur in your upper leg to the tibia in your lower leg tear apart. The injury occurs in the knee and is typically caused by twisting the knee or suddenly stopping.


Tendons, made of cells called fibroblasts, protein, and carbohydrates, connect muscle to bone.

Remember how our bones are involved in muscle movement? The tendon is what links the muscle to the bone, anchoring it during contraction. Tendons are what allow your bones to move during muscle contraction.Tendons also are susceptible to injury. People who perform repetitive movements all day, such as typing, writing, or drawing can wear out their tendons in a condition called tendonitis. During tendonitis, the tendons become inflamed and sore.

People with tendonitis may need to discontinue the activity, take medications, or even have surgery.

Lesson Summary

The skeletal system is used for structure and support in the body and is composed of bones, cartilage, ligaments, and tendons. Bones are calcified structures made of mostly osteocytes that provide structure, protection, aid in movement, and create blood cells. Cartilage is a flexible tissue made of chondroblasts, proteins, and carbohydrates. It creates flexible structures and cushions the junction between bones. Ligaments connect bone to bone and keep the weak areas of the skeletal system stable. Tendons connect muscle to bone and are important for movement.

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