Skeletal system anatomy and physiology

Definitions & Key takeaways

The skeletal system is made up of bones, cartilage, and other connective tissues that provide support and structure to the body. The primary functions of the skeletal system are to protect internal organs, provide support for the body, allow for movement, produce blood cells, and store minerals such as calcium and phosphorus.

Bones are living tissues that are constantly undergoing remodeling, a process in which old bone tissue is broken down and new bone tissue is formed. Bone tissue is composed of cells called osteoblasts, osteocytes, and osteoclasts, as well as a matrix of collagen and mineral salts such as calcium and phosphorus.

Cartilage is a type of connective tissue that provides cushioning and support to joints. It is composed of cells called chondrocytes and a matrix of collagen and proteoglycans. Other connective tissues include ligaments and tendons, which help to connect bones to other bones and muscles, respectively.

Chapters:

Watch video only

Introduction0:00–0:22

We all have 206 bones, and together they make up the skeleton which gives the body structure, protects important organs like the brain and heart, and allows muscles to facilitate movement.
Without bones you’d be a shapeless, immobile blob. Normally, the skeleton can be broken down into the axial and the appendicular skeleton.

Skeleton0:22–1:09

The axial skeleton consists of bones located along the vertical axis of your body. It contains 80 bones; 22 bones that make up the skull, 33 vertebrae, 24 ribs, and the sternum.
The appendicular skeleton consists of bones in your limbs as well as the bones that attach the limb bones to the axial skeleton, like the pelvis and the scapulae.
The appendicular skeleton is made up of around 126 bones; 4 bones in both shoulders, 6 bones in the arms, 54 bones in the hands, 2 hip bones that form the pelvic girdle, 8 bones in the legs, and 52 bones in the feet.

Long bones1:09–1:47

Now there are 5 types of bones based on their shape - long bones, short bones, flat bones, sesamoid bones, and irregular bones.
Long bones are longer than they are wide, and they’re in the limbs and include the humerus, radius, and ulna in the arms; as well as the metacarpals and phalanges of the hand and fingers.
Long bones also include the femur, tibia, and fibula in the leg as well as metatarsals and phalanges of the feet and toes.
During childhood and adolescence, long bones continues to grow and are the bones that are responsible for your height. Unlike long bones, the short bones have a similar length and width, and that gives them a round or cube-like appearance.

Short bones1:47–2:03

They include the carpal bones of the wrist and tarsal bones of the ankle and their main functions are to support the hand and foot.

Flat bones2:03–2:22

Flat bones are thin bones, and some of them are curved. They include bones of the skull, the two shoulder blades or scapulae, sternum, and ribs.
Their main job is to serve as armor plating that protects vital organs like the brain, heart, and lungs. Sesamoid bones, are embedded in tendons and they’re like giant sesame seeds - in shape.

Sesamoid bones2:22–2:55

Most of these bones can be found in the metacarpal phalangeal joints in the hand and metatarsal phalangeal joints in the feet.
The larger sesamoid bones include the pisiform bone that’s in the wrist, and the patella which is the kneecap. These bones increase the angle between the bone and the tendon of muscles which gives the muscles more leverage.
The sesamoid bones also provide support and protects the tendon from wear and tear. The last type of bones are the irregular bones which are basically the misfits that don’t fit into any of the previous categories.

Irregular bones2:55–3:11

This includes the facial bones, the mandible, the vertebrae of the vertebral column, and the sacrum and coccyx. Now, some bones have surface structures that help them function.

Bone Anatomy3:11–4:48

For example, bones can have tubercles which are small bumps on the bone that serve as an attachment site for muscles. A large tubercle is called a tuberosity, and an example is the deltoid tuberosity on the humerus which is where the deltoid muscle attaches.
Holes in the bone that allow blood vessels or nerves to pass through are called foramen. An example of that is the foramen magnum in the occipital bone of the skull, which allows the spinal cord to exit the skull.
Bones can also have canals, which are tunnels within the bone that allow structures like blood vessels or nerves to travel through.
An example is the optic canal in the sphenoid bone which allows the optic nerve, to travel from the brain to the eyes. Another name for a canal is a meatus, like the external auditory meatus of the ear, located in the temporal bone which lets sound pass through to the eardrum.
Some bones have a fossa which is a depression within the bone, where another structure rests. One example is the hypophyseal fossa or sella turcica on the sphenoid bone which is like a tiny seat where the pituitary gland rests.
Also, there are sinuses and cavities, which are empty spaces within a bone or formed by multiple bones coming together. Examples include the nasal cavity which is formed by the maxilla, the nasal bone and palatine bone, as well as the paranasal sinuses, like the maxillary sinus, is located within the maxillary bone.
Alright, now all bones share some similarities internally, so if we look at a cross section of a bone we can see that it has a hard-external layer known as the cortical bone, and a softer internal, layer of spongy bone that looks like honeycombs.

Bone Histology4:48–5:40

The cortical bone is covered by a membrane of fibrous connective tissue called the periosteum, and that’s where the muscles, tendons, and ligaments attach.
If we zoom into a cross section of the cortical bone, we can see that it has many pipe like structures called osteons running through the length of the bone.
Each pipe has an empty center called a haversian canal which contains the nerves and blood vessels that supply the osteon.
Adjacent osteons are connected to one another by horizontal channels called volkmann’s canals which carry smaller blood vessels that branch from the main one.

Osteoblasts5:40–6:10

The thick walls of the osteon are made up of tightly packed, concentric layers of bone matrix which are made of type I collagen that’s reinforced with hydroxyapatite, a mineral made of calcium and phosphate.
At the periphery of the osteon is a ring of cells called osteoblasts which synthesize type I collagen and hydroxyapatite, creating new layers of matrix.
Some of these osteoblasts gets trapped in the new layers of matrix and mature into osteocytes within tiny cavities called lacunae.

Osteocytes6:10–6:30

Osteocytes are less able to create bone matrix and unlike osteoblasts, they can’t multiply. Instead, osteocytes become mechanoreceptors that detect stress and damage, and they help control the process of remodeling which is when old bone is broken down and replaced with new bone.
Now, dispersed among the osteoblasts are some larger cells called osteoclasts. Osteoclasts secrete enzymes to help with the breakdown of bone matrix, which releases the calcium and phosphate that’s in the hydroxyapatite, back into the blood.

Osteoclasts6:30–6:56

This is useful for bone remodeling, but also when there’s low calcium level in the blood, because the bones can be thought of as a place where we store lots and lots of calcium.
Now, under the layer of tough cortical bone is the spongy bone, or trabecular bone. The trabecular bones is composed of similar material as the cortical bone.

Spongy/trabecular bone6:56–8:10

But the organizational structure is more loose - with networks of branching rods called trabeculae. This makes spongy bone less dense and softer than the cortical bone and also gives it a “hole-ridden” appearance kind of like a sponge.
Within the spongy bone layer is the bone marrow which is composed of a mix of hematopoietic stem cells that eventually differentiate into red blood cells, white blood cells, and platelets.
The marrow also contains adipocytes or fat cells which store fat - a great source of energy. Marrow can be divided into either red marrow, which contains mostly hematopoietic cells, or yellow marrow which has a higher percentage of adipocytes.In bones that form the appendicular skeleton, like long femur bones, the red marrow is mostly located at the tips of the bone, while the shaft of the bone contains a hollow medullary cavity filled with lots of yellow marrow.
In contrast, in bones that form the axial skeleton, there are no medullary cavity and they are mostly packed with red marrow.
Alright, as a quick recap, the human skeleton consists of around 206 bones which divided into the axial and appendicular skeleton.

Review8:10–8:52

There are long, short, flat, sesamoid, and irregular bones. Bones give shape to your body, provide support to your internal organs and allows movement to occur.
They also produce platelets, red blood cells, and white blood cells as well as serve as a storage site for calcium and phosphate.
Each bone has an outer layer made of tough cortical bone and an inner layer made of soft spongy bone. Red bone marrow is found in the spongy bone and yellow marrow is found mainly in the medullary cavity of long bones.