The Skeletal System

Skeleton and its Functions

The skeleton is among the busiest and the most durable of all living organs of the body. Human bones have been found that are nearly a million years old. Apart from forming the supporting framework of the body, the bones are manufacturing units for red blood cells (which transports oxygen to the various tissues), white blood cells (which fight infection), and platelets (which are essential for clotting of blood). Bones also act as the body’s storehouse for reserve nourishment. The marrow of the bone stores fats, proteins, calcium and phosphorus. By an intricate self-regulating system the calcium ingested through milk and eggs is deposited in the bones while the calcium deposited earlier is withdrawn for the current use of the body. The skeleton protects the delicate organs of the body. The cranium protects the brain, the ribs enclose the heart and lungs while the spinal cord is protected by the vertebrae. As a supporting structure, bone construction can be compared to reinforced concrete. On a weight-to-weight basis, bones are stronger than steel. The leg bones which support the entire body weight are light in keeping with the engineering principle that a hollow column is stronger than a solid one of the same weight.


The separate unit of the skeletal framework is the bone. Bones are connected to each other by ligaments. About two-thirds of the weight of bone tissue is mineral, made up of calcium, phosphate and carbonate. The rest is organic material consisting largely officious protein collagen. When boiled in water, collagen yields gelatin. Together, the mineral and collagen form the bone matrix. If a long bone is soaked in acid, the mineral component dissolves, leaving behind the organic matter. There are two forms of bone, compact and spongy. The compact bone forms the hard outer layer. Within this layer lies the spongy bone, which in turn houses the weak tissue of the bone marrow, the nerves, and the blood vessels. The adult human skeleton consists of 206 named bones.

  • Synovial Joint: A joint in which there is a space between articulating bones is called a synovial joint. Synovial joints contain a synovial cavity, cartilage, synovial membrane and ligaments. All synovial joints are freely movable. Types of movement at synovial joints include gliding movements, angular movements, rotation and special movements. Types of synovial joints include gliding joint, hinge joint, pivot joint, ball-and-socket joint.
  • Knee Joint : The knee joint is the largest joint of the body. It is formed by three joints.
  1. Gliding joint : Between the patella (knee cap) and femur.
  2. Hinge joints: Laterally between the tibia and femur , medially between the tibia and femur.

The knee joint allows flexion, extension, medial and lateral rotation. These joints do not allow any movement as their fit between bones is very tight. In case of sutures, there is complete fusion of bones across the suture line. e.g. bones of the skull. These joints allow very slight movement. e.g. between bodies of vertebrae. These are essentially synovial joints capable of free movement. e.g. elbow joint, hip joint. Synovial cavity absent: Bones held very closely by fibrous connective tissue al lows little or no movement e.g. bones of the skull. Roots of the teeth with sockets of the maxillae and mandible. Synovial cavity absent. Bones held together tightly by cartilage. Allows little or no movement e.g., joint between the first rib and the sternum. Between bodies of vertex Synovial vertebrae. cavity present. The cavity, together with the arrangement of bones and ligaments, allows free movement. e.g. elbow, shoulder, hip, neck, wrist joint. Bones and joints are incapable of moving the body on their own. Movement is an essential body function brought about by the contraction and relaxation of muscles. Muscle tissue is highly specialized to actively generate force and constitutes about 40 to 50% of the total body weight.

Function of Muscles

Through contraction, muscle tissue performs three important functions. They bring about motion. Apart from bringing about obvious movements such as walking and running, localized movements such as holding a pen, nodding the head or chest movements involved with breathing are also the outcome of integrated functioning of bones, joints and skeletal muscles. Some of the less noticeable movements produced by muscles include beating of the heart, churning of the food in the stomach, peristalsis, contractions of the gall bladder and the urinary bladder. It helps to maintain body posture. The muscle tissue enables the body to maintain posture. The contraction of skeletal muscles holds the body in stationary positions such as standing and sitting. Some of the Function are described below:

  • Heat production : Almost 85% of all body heat is generated by muscle contractions. Hence muscles are important in maintaining normal body temperature. The immediate, direct source of energy for muscle contraction is ATP, adenosine-triphosphate which is derived from the complete breakdown of pyruvic acid to CO2 and H20 in the presence of oxygen (Kreb’s cycle). Muscle fibres generate ATP continuously by the metabolism of fats and glycogen. Nerves convey impulses for muscular contraction, while blood provides nutrients and oxygen for contraction.

Classification of Muscles

All muscle tissues are classified in the following way:

  1. Skeletal, striated, voluntary muscle tissue.
  2. Cardiac, striated, involuntary muscle tissue.
  3. Smooth, non-straited, involuntary muscle tissue.

The principal characteristics of muscle tissues are summarized below. Skeletal muscles produce movements by exerting force on tendons, which in turn pull on bones or other structures, such as the skin. Most movements are coordinated by several skeletal muscles acting in groups and these are generally arranged in opposing pairs at joints. A muscle that contracts to bring about a certain movement is called the agonist or the prime mover.

Oxygen Debt

During exercise, blood flow to muscles is in- creased in order to increase the available oxygen supply. However, if the muscular exertion is very great, oxygen cannot be supplied to muscle fibres fast enough and all the ATP required for further muscle contraction is not generated. At such times, ATP is generated in the absence of oxygen (anaerobic-glycolysis) during which most of the pyruvic acid is converted to lactic acid. Although 80% of the lactic acid diffuses from the skeletal muscles and is transported to the liver for conversion back to glucose or glycogen, some lactic acid accumulates in muscle tissue. Thus, when ad- equate oxygen is available, lactic acid is Catabolized completely into carbon dioxide and water. The additional oxygen that must be taken into the body after vigorous exercise to restore al systems to their normal states is called oxygen debt. The debt is paid back by laboured breathing that continues after exercise has stopped. Muscle glycogen is restored through diet, and may take several days, depending on the in- density of exercise the antagonist relaxes to yield to the movement of the agonist. This can be illustrated by considering the flexing and extension of the forearm at the elbow as an example. During flexing, the biceps contract and act as the agonist. At the same time the triceps relax and function as the antagonist. During extension of the arm the reverse occurs. The triceps contract and the biceps relax. Lever Systems in producing a body movement, bones act as levers and joints function as fulcrums. A lever is acted upon at two different points by two different forces – the resistance and the effort. In anatomy, the part of the body to be moved may be regarded as the resistance or load. The muscular contraction applied to the bone at the insertion of the muscle may be regarded as the effort. In the example shown alongside of the biceps flexing the forearm at the elbow, the elbow is the fulcrum and the weight of the forearm is the resistance. The shortening due to the force of contraction of the biceps pulling the for earn-up is the effort. Levers are categorised into three classes according to the position of the fulcrum, effort and the resistance.

Muscle Fatigue

The inability of a muscle to maintain its strength of contraction or tension is called muscle fatigue. It occurs when a skeletal muscle or a group of skeletal muscles is continuously stimulated for an extended period of time. As a result, the strength of contraction becomes progressively weaker until the muscle no longer responds. In short, the muscle is unable to produce sufficient energy to meet its needs. Its cause may be due to insufficient oxygen, depletion of glycogen and or lactic acid build up. A lever may be defined as a rigid rod that moves about on some fixed point called a fulcrum.

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