The human body is the physical and material structure of the human being. An adult has 206 bones, while that of a newborn is made up of about 303 bones since some, especially those in the head, fuse during the growth stage.
It is made up of a head, trunk and limbs; The arms are the upper extremities and the legs are the lower extremities; It is worth mentioning that the trunk is divided into thorax and abdomen and is what gives movement to the upper and lower extremities and the head.
The human body is organized in different hierarchical levels. Thus, it is composed of devices; These are made up of systems, which in turn are composed of organs made up of tissues, which are made up of cells made up of molecules.
The body has more than fifty trillion cells. These are grouped into tissues, which are organized into organs, and these into eight devices or systems: locomotor (muscle and bone), respiratory, digestive, excretory, circulatory, endocrine, nervous and reproductive.
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In an adult man, muscles make up about 40% of body weight. A muscle is an organ made up of contractile fibers (muscle fibers). They may be related to the skeleton (skeletal muscles) or be part of the structure of organs or apparatus (visceral muscles).
The human body has about 650 muscles of different types, which are wrapped by a membrane known as aponeurosis. Muscles are the organs with the greatest adaptability, since their shape and content can be modified to a great degree. By doing physical exercise, for example, muscles can be developed and strengthened.
Muscles perform several functions. They are responsible for producing movement, providing joint stability, maintaining posture, transforming mechanical energy into chemical energy, providing heat, stimulating blood vessels and reporting on the physiological state of the body, among other activities.
Depending on their location, muscles can be cutaneous (they are located immediately under the skin) or deep (they have at least one bony insertion). Depending on their shape, there are circular or sphinerian muscles and square muscles.
Another classification of muscles occurs according to their nature. In this case, we can talk about striated muscles (voluntary control and very slow contraction), smooth muscles (involuntarily controlled, they do not contain striations) and cardiac muscles (involuntary control and modified striated nature).
Muscles can suffer from various diseases, which are known as myopathies. These disorders can have various causes and must be treated in different ways. Among them are muscular dystrophies, atrophies, inflammatory myopathies, myasthenia and certain tumors.
The longest muscle is the sartorius, which runs from the pelvis to below the knee: it exceeds 40 cm in length. The smallest muscle is in the middle ear, inserted into the tiny bone called stapes: its length does not reach 1 mm. The largest muscle is the gluteus maximus, which is part of the buttock. The fastest muscle is the levator palpebrae: it can contract up to five times per second. The most powerful muscle is the masseter, responsible for moving the jaw during chewing: it develops a force equivalent to 100 kg.
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Recommended link with anatomy images.