Anatomy & Physiology – The Skeletal System
It’s estimated that more than two and a half million red blood cells are produced each second by the bone marrow to replace old blood cells destroyed by the liver. Bones also act as a storage area for minerals, including calcium and phosphorus. When the blood has an excess of those minerals, they will get stored in the bones. When the supply in the blood gets low, the minerals are taken out of the bones and put into the blood.
The primary function of the skeletal system is to support the body and enable movement by providing the structure where muscles can attach to bones with tendons . Bones also help protect internal organs and tissues from damage.
Muscles are connected to bones by tendons and bones are connected to each other by ligaments . The place where bones meet is a joint . Movement occurs when muscles connected to two different bones contract to pull them together.
Skeletal Divisions
The human skeleton is divided into two distinct parts. The axial skeleton consists of bones that form the axis of the body: the skull, sternum, ribs, and vertebrae. These bones support and protect the organs of the head, neck, and trunk.
The appendicular skeleton is composed of bones in the arms, hands, legs, feet, shoulders, and pelvis.
Types of Bone
Bones fall into one of four general categories: long bones , short bones , flat bones , and irregular bones .
Long bones are longer than they are wide and work as levers. These include bones of arms, hands, legs, and feet.
Short bones are the cube shaped bones found in the wrists and ankles.
Flat bones have wide surfaces to help protect organs and the attachment of muscles. These include the ribs, cranial bones, and the shoulder girdle.
Irregular bones are those that don’t fall into one of the first three categories. These include the vertebrae and some skull bones.
Bone Composition
The outer surface of bone, the periosteum , is a thin, dense membrane that contains nerves and blood vessels that nourish the bone.
Bones can be composed of compact (dense) or spongy (cancellous) bone. Most bones contain both types. The compact bone forms the protective exterior portion while the cancellous bone is inside (underneath) the compact bone, where it serves to protect the marrow.
Bone tissue is composed of several types of bone cells embedded in inorganic salts such as calcium and phosphorus, which give it strength , and collagen fibers and ground substance, which give it flexibility .
Babies have around 300 bones at birth. Many of these eventually fuse to form the 206 bones in an adult body . Some of a baby’s bones are made partially or entirely of cartilage, which is soft and flexible. As the child grows older, the cartilage is slowly replaced by bone. By the time a person is 25, the cartilage to bone formation is complete. Once that happens, the bones are as big as they will ever be, and people are as tall as they ever will be.
Bone is constantly being broken down and rebuilt by the body. Osteoclasts are bone absorbing cells that break down bone and discard the worn cells. After a few weeks, the osteoclasts disappear and osteoblasts repair the bone. During this cycle, calcium is deposited and withdrawn from the blood.
While the skeletal system as a whole supports the body, facilitates movement, and aids in certain metabolic processes, different areas of the skeleton have exclusive functions.
The Spine and Ribs
The Spine
The spine has 26 separate bones called vertebrae which enable you to stand upright, twist, and bend. It also protects the spinal cord. Each vertebra is shaped like a ring but there are different types of vertebrae in the spine and each play a different role.
The top seven vertebrae are called the cervical vertebrae and are located right below your brain in the back of your neck. They support your head and neck.
Below the cervical vertebrae are the 12 thoracic vertebrae , which anchor the ribs in place.
The next five are the lumbar vertebrae , after which is the sacrum (pronounced say-krum), which is made up of five vertebrae fused together to form one single bone.
Lastly, at the bottom of the spine is the coccyx , or tail bone, which is comprised of four fused vertebrae.
The lower sections of the spine are designed to be weight bearing, enabling humans to stand, walk upright, and keep balance while doing so.
In between each vertebra are small disks made of cartilage, which keep the vertebrae from rubbing against one another and act as your spine’s natural shock absorbers to prevent damage as we move around.
The Thorax
The thorax consists of thoracic vertebrae, sternum (breastbone), ribs, and costal cartilages. The ribs act like a cage to protect the heart, lungs and liver. The ribs come in identical left-right pairs. Most people have 12 pairs but some people are born with one or more extra ribs, while others may have one fewer pair.
All 12 pairs of ribs are attached in the back to the spine, where they are held in place by the thoracic vertebrae . The first seven pairs of ribs are attached in front to the sternum , the bone in the center of your chest that holds those ribs in place.
The remaining sets of ribs are not directly attached to the sternum. Cartilage holds the next three pairs to the ribs above them. The last two sets are called floating ribs because they aren’t connected to the sternum or the ribs above them, just to the spine in the back.
The Skull
The skull protects the brain as well as giving the face structure. The skull contains 8 cranial bones and 14 facial bones. All of the bones of the skull are joined together by sutures (except the mandible).
All babies are born with spaces between the bones in their skulls, which allow the bones to move and even overlap as the baby moves through the birth canal. As the baby grows, the spaces close up and special joints called sutures connect the bones.
The lower jaw is the only bone in the head that can move.
Joints
The spot where two bones meet is called a joint . Bones are held together at the joints by ligaments , which act like very strong rubber bands. The difference between tendons and ligaments is that tendons connect bones to muscles and ligaments connect bones to bones to help stabilize joints.
Some joints, like the hips, elbow, knee, and shoulder move freely. The spine has limited movement. And others, like the sutures in the skull, are fixed and are immobile.
The most flexible joints are called hinge joints , which bend in a certain direction, much like the hinge on a door. Elbows, knees, and fingers are examples.
Another type of joint is called the ball and socket . These joints allow 360o movement, such as in the hips and shoulders. Ball and socket joints work by having the round end of one bone fitting into a cup-like area of another bone.
To keep the joints lubricated and moving freely, the body produces synovial fluid , which acts as a kind of oil.
Extremities
The arm is made up of three bones: the humerus , which is in the upper arm, and the radius and ulna , which are below the elbow. At the end of the radius and ulna are the eight wrist bones. Each arm is attached to a shoulder blade, or scapula .
The center part of your hand is made up of five separate bones. Each finger on your hand has three bones and the thumb has two.
Your legs are attached to a circle of bones called your pelvis, a bowl shaped structure that supports the spine. It has two large hip bones in front, and behind it are the sacrum and the coccyx. The pelvis serves as a ring of protection around parts of the digestive, urinary, and reproductive system.
The leg bones are large and strong to help support the weight of your body. The bone in your thigh is the femur , and it is the longest bone in the body. The patella protects the knee joint. Below the knee are the tibia and fibula , which connect to large bones in feet, called the talus . The various bones in the foot help give us the balance needed to stand and walk.
Arm Bones
Pelvic Girdle
This is also known as the hip bone and transfers the weight of the upper body to the hips. Each side of the girdle contains a pair of coxal bones, and this bone is further divided (on each side) into three bones: the ilium , ischium , and pubis . All of these bones put together along with the sacrum and coccyx make the pelvic girdle appear bowl shaped.
Lower Limb
This consists of the thigh, the leg, and the foot. There are 30 bones in each limb. These limbs are quite a bit larger than the bones in the upper limb simply because these limbs must support and move all of the body weight.
The Knee
The Foot
Complete List of Bones
Cranial and Facial Bones (22):
Frontal bone.
Parietal bone (2).
Temporal bone (2).
Occipital bone.
Sphenoid bone.
Ethmoid bone.
Mandible.
Maxilla (2).
Palatine bone (2).
Zygomatic bone (2).
Nasal bone (2).
Lacrimal bone (2).
Vomer.
Inferior nasal conchae (2).
Ear Bones (6):
Malleus (2).
Incus (2).
Stapes (2).
Throat Bone (1):
Hyoid bone.
Shoulder Bones (4):
Scapula or shoulder blade (2).
Clavicle or collarbone (2).
Chest Bones (25):
Sternum (1).
Ribs (2 x 12).
Vertebral Bones (26):
Cervical vertebrae (7).
Thoracic vertebrae (12).
Lumbar vertebrae (5).
Sacral vertebrae (1).
Coccygeal vertebrae (1).
Arm and Forearm Bones (6):
Humerus (2).
Radius (2).
Ulna (2).
Hand Bones (54):
Carpal (wrist) bones:
Scaphoid bone (2).
Lunate bone (2).
Triquetral bone (2).
Pisiform bone (2).
Trapezium (2).
Trapezoid bone (2).
Capitate bone (2).
Hamate bone (2).
Metacarpus (palm) bones:
Metacarpal bones (5 x 2).
Digits of the hands (finger bones or phalanges):
Proximal phalanges (5 x 2).
Intermediate phalanges (4 x 2).
Pelvic Bones (2):
Hip bone (innominate bone or coxal bone) (2).
Leg Bones (8):
Femur (2).
Patella (2).
Tibia (2).
Fibula (2).
Foot Bones (52):
Tarsal (ankle) bones:
Calcaneus (heel bone) (2).
Talus (2).
Navicular bone (2).
Medial cuneiform bone (2).
Intermediate cuneiform bone (2).
Lateral cuneiform bone (2).
Cuboid bone (2).
Metatarsus bones:
Metatarsal bone (5 x 2).
Digits of the feet (toe bones or phalanges):
Proximal phalanges (5 x 2).
Intermediate phalanges (4 x 2).