Functions of Bones and Blood Cell Formation

Skeletal parts provide shape and support for body structures. They also act as levers that aid body movements, produce blood cells, and store various inorganic salts.

Support and Protection

Bones give shape to structures such as the head, face, thorax, and limbs. They also provide support and protection.

For example, the bones of the feet and legs, pelvis and backbone support the weight of the body. The bones of the skull protect the eyes, ears, and brain. Those of the rib cage and shoulder girdle protect the heart and lungs, while the bones of the pelvic girdle protect the lower abdominal and internal reproductive organs.

Lever Actions

Whenever limbs or other body parts are moved, bones and muscles function together as simple mechanical devices called levers. 

Such a lever has four basic components:

• A rigid bar or rod
• A pivot or fulcrum on which the bar turns; 
• An object or weight that is moved; 
• A force that supplies energy for the movement of the bar.

A playground seesaw is a lever. The board of the seesaw serves as a rigid bar that rocks on a pivot near its center. The person on one end of the board represents the weight that is moved, while the person at the opposite end supplies the force needed for moving the board and its rider.

There are three kinds of levers, and they differ in the arrangements of their parts.

First-Class Lever

A first-class lever is one whose parts are arranged like those of the seesaw. Its pivot is located between the weight and the force, making the sequence of parts weight-pivot-force. Other examples of first-class levers are scissors and hemostats (used to clamp blood vessels closed).

First Class Lever

Second-Class Lever

The parts of a second-class lever are arranged in the sequence pivot-weight-force, as in a wheelbarrow.

Second Class Lever

Third-Class Lever

The parts of a third-class lever are arranged in the sequence pivot-force-weight. This type of lever is illustrated when eyebrow tweezers or forceps are used to grasp an object.

Third Class Lever

The actions of bending and straightening the arm at the elbow involve bones and muscles functioning together as levers. When the arm is bent, the lower arm bones represent the rigid bar. the elbow joint is the pivot, the hand is the weight that is moved, and the force is supplied by muscles on the anterior side of the upper arm.

Function Of Bone

One of these muscles, the biceps brachii, is attached by a tendon to a process on the radius bone in the lower arm, a short distance below the elbow. Since the parts of this lever are arranged in the sequence pivot-forceweight, it is an example of a third-class lever.

function of bone

When the arm is straightened at the elbow, the lower arm bones again serve as a rigid bar, and the elbow joint as the pivot. However, this time the force is supplied by the triceps brachii, a muscle located on the posterior side of the upper arm. A tendon of this muscle is attached to a process of the ulna bone at the point of the elbow. Thus, the parts of the lever are arranged weight-pivot-force, and it is an example of a first-class lever.

Although many lever arrangements occur throughout the skeletal-muscular systems, they are not always easy to identify. Nevertheless, these levers provide advantages in movements. The parts of some levers, such as those that function in moving the limbs, are arranged in ways that produce rapid motions, while others, such as those that move the head, aid in maintaining posture with minimal effort.

Blood Cell Formation

The process of blood cell formation is called hematopoiesis.

Very early in life it occurs in a structure called a yolk sac, which lies outside the body of a human embryo. Later in development, blood cells arc manufactured in the liver and spleen, and still later they are formed in the marrow within bones. Marrow is a soft, netlike mass of connective tissue found in the medullary cavities of long bones, in the irregular spaces of spongy bone, and in the larger haversian canals of bone tissue.

There are two kinds of marrow—red and yellow.

In an adult, red marrow is found primarily in the spongy parts of the bones of the skull, ribs, sternum, clavicles, vertebrae, and pelvis. If the blood cell supply is deficient, yellow marrow may change back into red marrow and become active in blood cell production.

Storage of Inorganic Salts

The intercellular matrix of bone tissue contains large quantities of calcium salts. These are mostly in the form of tiny crystals of a type of calcium phosphate called hydroxyapatite.

Calcium is needed for a number of metabolic processes, including blood clot formation, nerve impulse conduction, and muscle cell contraction. When a low blood calcium condition exists, the parathyroid glands respond by releasing parathyroid hormone.

This hormone stimulates osteoclasts to break down bone tissue, and as a result, calcium salts are released into the blood. On the other hand, if the blood calcium level is excessively high, the thyroid gland responds by releasing a hormone called calcitonin. Its effect is opposite that of parathyroid hormone; it inhibits osteoclast activity and stimulates osteoblasts to form bone tissue. As a result, the excessive calcium is stored in bone matrix.

Blood Cell Formation

In addition to storing calcium and phosphorus, bone, tissue, stores, lesser amounts of magnesium and sodium. Bones also tend to accumulate certain metallic elements such as lead and radium, which are not normally present in the body but are sometimes ingested accidentally.

During childhood, lead poisoning sometimes results from the ingestion of paint chips that contain lead pigments Although modern interior paints do not contain lead, the woodwork and painted plaster of houses constructed prior to 1940 may be covered with many layers of lead-pigmented paints. Such paints may peel off and be eaten by preschool children.

Strontium is another element that may be concentrated in bone tissue. A radioactive isotope of this element, strontium-90, is a by-product of nuclear reactions, such as those that occur in atomic explosions and nuclear power plants. 

If strontium-90 is released into the environment, it may be taken in by plants and animals because it is chemically similar to calcium and can be used metabolically by organisms in the same ways they use calcium. Humans may ingest strontium-90 by drinking milk from cows that fed upon plants exposed to nuclear fallout. When this happens, some of the strontium-90 accumulates in the human bones, and nearby cells are subjected to its radiation. Since radiation from strontium-90 can cause mutations, such exposure may result in the development of abnormal cells associated with bone cancers or leukemias.