Skeletal System
The skeleton is responsible for supporting and giving shape to the body. It also protects the internal organs and works together with the muscular and articular systems to allow movement.
Other functions are the production of blood cells in the bone marrow and storage of mineral salts, such as calcium.
Bone is a living structure, very resistant and dynamic as it has the ability to regenerate when it suffers a fracture.
Bone Structure
The bone structure is made up of different types of connective tissue (dense, bony, adipose, cartilaginous and blood), in addition to nervous tissue.
Long bones are formed by several layers, see the table below:
| Bone layer | Description |
|---|---|
| Periosteum | It is the outermost layer, being a thin, fibrous membrane (dense connective tissue) that surrounds the bone, except in the articulating regions (epiphyses). It is in the periosteum that the muscles and tendons are inserted. |
| compact bone | Compact bone tissue is composed of calcium, phosphorus and collagen fibers that give it strength. It is the most rigid part of the bone, formed by small channels that circulate nerves and vessels. Between these channels are spaces where osteocytes are found. |
| spongy bone | spongy bone tissue is a less dense layer. In some bones only this structure is present and may contain bone marrow. |
| medullary canal | It is the cavity where the bone marrow is found, generally present in long bones. |
| Bone marrow | Red marrow (blood tissue) produces blood cells, but in some bones it ceases to exist and there is only yellow marrow (adipose tissue) that stores fat. |
Skeleton division
The human skeleton is made up of 206 bones of different sizes and shapes. They can be long, short, flat, sutural, sesamoid or irregular.
Each of them has its own functions and for this reason, the skeleton is divided into axial and appendicular.
Axial Skeleton
The bones of the axial skeleton are in the central part of the body, or near the midline, which is the vertical axis of the body.
The bones that make up this part of the skeleton are:
- the head (skull and facial bones)
- the spine and vertebrae
- the thorax (ribs and sternum)
- the hyoid bone
Skull and Facial Bones
The head is made up of 22 bones (14 in the face and 8 in the cranium); and there are also 6 bones that make up the inner ear.
The skull is extremely resistant, its bones are closely connected and without movement. It is responsible for protecting the brain, in addition to containing the sense organs.
Vertebral Column
The spine is made up of vertebrae that are connected to each other by joints, which makes the spine very flexible. It has curves that help balance the body and cushion shocks during movements.
It consists of 24 independent vertebrae and 9 that are fused. See the table below to see how they are grouped:
| Vertebrae | Features |
|---|---|
| Cervical | There are 7 vertebrae in the neck, with the first (atlas) and second (axis) facilitating the movements of the skull. |
| Thoracic or dorsal | There are 12 and they articulate with the ribs. |
| Lumbar | These 5 vertebrae are the largest and support the most weight. |
| Sacrum | These 5 vertebrae are called sacral, they are separated at birth and later fuse together to form a single bone. It is an important support point for the pelvic girdle. |
| Coccyx | There are 4 small coccygeal vertebrae that, like the sacral vertebrae, become united into a single bone in early adulthood. |
Chest
The thorax consists of 12 pairs of ribs connected to each other by the intercostal muscles. They are flat, curved bones that move during breathing. The ribs are connected to the thoracic vertebrae at the back.
Anteriorly, the first seven pairs of ribs (called true) are attached to the sternum, the next three (false) are attached to each other, and the last two pairs (floating) are not attached to any bone. The sternum is a flat bone that is attached to the ribs by cartilage.
Hyoid bone
The hyoid bone is U-shaped and acts as a support point for the muscles of the tongue and neck.
Appendicular Skeleton
The appendicular skeleton includes the “appendages” of the body. They correspond to the bones of the upper and lower limbs.
In addition, the appendicular skeleton has the bones that connect it to the axial skeleton, the so-called shoulder and pelvic girdles, as well as ligaments , joints and articulations.
Shoulder Girdle
The shoulder girdle is formed by the clavicles and scapulae.
The clavicle is long and narrow, articulates with the sternum and at the other end with the scapula, which is a flat, triangular bone articulated with the humerus (shoulder joint).
Upper Limbs
The upper limbs correspond to the arms, where we have the humerus, which is the longest bone in the arm. It articulates with the radius, which is the shortest and lateral, and also with the ulna, a flat and very thin bone.
There are 27 bones in the hand, divided into carpals (8), metacarpals (5) and phalanges (14).
Pelvic Girdle
The pelvic girdle is formed by the hip bones, the iliac bones (consisting of the fused ilium, ischium and pubis) and are firmly attached to the sacrum.
The union of the iliac bones, sacrum and coccyx forms the pelvis, which in women is wider, less deep and has a larger cavity. It is this formation that allows the pelvis to open at the time of birth to allow the baby to pass through.
Lower Limbs
The bones of the lower limbs are responsible for supporting the body and allowing movement. To do this, they have to support weight and maintain balance.
See the characteristics of the bones of the lower limbs in the table below:
| Bones of the lower limb | Features |
|---|---|
| Femur | It is the longest bone in the body. It has a rounded head to fit into the pelvis. |
| Patella | It is a sesamoid bone, articulated with the femur. |
| Tibia | Supports almost all the weight on the lower body. |
| Fibula | It is a weaker bone, connected to the tibia it helps to move the foot. |
| Foot bones | The feet have 26 bones divided into: tarsals (7), metatarsals (5) and phalanges (14). |
Bone Ossification and Remodeling
The process of bone formation begins around the first 6 weeks of life and ends in early adulthood. However, the bone continually undergoes a remodeling process, where part of the existing tissue is reabsorbed and new tissue is formed.
In the embryo, the skeleton is basically formed of cartilage , but this cartilaginous matrix is calcified and the cartilaginous cells die.
Young cells, called osteoblasts, act by producing collagen and mineralizing the bone matrix. They are formed in connective tissue and occupy the cartilaginous matrix.
However, this process produces gaps and small channels that trap osteoblasts in the bone matrix. This action transforms osteoblasts into osteocytes, which are the cells present in the already formed bone.
Another type of bone cells, osteoclasts, are responsible for absorbing the formed bone tissue . Osteoclasts act on the central portion of the bone matrix and form the medullary canal.
Fractures
In situations where bones are subjected to pressure greater than their resistance, they can break.
Fractures can also occur due to stress, when small amounts of pressure are applied repeatedly to the area. Another situation that can cause fractures is due to disease, such as osteoporosis , a condition in which the bone undergoes demineralization, losing calcium to the blood.
A blood clot forms on the surface of the fracture site, cells die and the bone matrix is destroyed.
Intense vascularization takes over the area and there is proliferation of precursor cells of bone cells, creating repair tissue; a bone callus is formed in this region .
Depending on the treatment and activities carried out by the person, over time, the callus will be replaced by spongy bone and, later, by compact bone, reconstituting the tissue as it was before.