9.2: Main Functions for Homeostasis

The bones and joints work together to maintain homeostasis in two main ways. One is by minimizing changes in the body's internal conditions; this is accomplished by providing support and protection from traumatic injury. The second way is by helping to restore errant conditions to proper levels. The skeletal system contributes to this goal by helping the muscles move, storing minerals and fat, and producing blood cells.

Support

The support provided by the skeletal system is important for the same reasons mentioned in Chapter 8 about the muscle system. Recall that some structures of the body are not strong enough to hold themselves up but must be held in position to work properly. For example, the spinal cord, which extends down the back from the base of the head to slightly above the waist, is quite soft and very flexible (Figure 9.2); it cannot stand on end by itself.

Figure 9.2 Skeletal support for the spinal cord. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

If the spinal cord is bent sharply or excessively, the resulting injury can inhibit its impulse conduction and result in permanent paralysis. To prevent such disastrous alterations in the position of the cord, it is encased within the vertebral column. The vertebral column is composed of a row of ring-shaped bones that are firmly attached to each other by joints that allow only slight movement. Therefore, the skeletal system holds the spinal cord in proper position while allowing it to bend an acceptable amount and in a smooth curve.

Similarly, without support from the skeletal components in the thoracic region, the lungs would collapse like leaky balloons and a person would be incapable of breathing. However, the joints among the thoracic bones permit limited flexibility of the thorax, allowing for breathing (Figure 9.1).

Protection from Trauma

The weak, delicate nature of many parts of the body requires that they be protected from injury. Recall that fat in the subcutaneous layer of the integument contributes to such protection. The skeletal system also provides protection from trauma.

The soft and delicate nature of the spinal cord requires not only that it be given support but also that it be protected from pressure and sharp objects. Even a slight squeezing of the cord could crush its nerve cells: Nerve impulses would be blocked, and the victim would be paralyzed. However, the spinal cord is rarely damaged because it is shielded by the vertebrae that encircle it (Figure 9.2).

Consider also the brain, lungs, heart, and bone marrow. Each is essential for life, is easily damaged, and, like the spinal cord, is encased within bones. Through this arrangement, these organs are kept safe from crushing, tearing, cutting, and other forces that may be encountered.

Movement

Movement is a second homeostatic function shared by the muscle and skeletal systems. Recall that movement is one key means by which the body makes adjustments when it detects that a change in conditions is about to take place or has already taken place. By moving, the body can attain what is desirable and avoid what is detrimental. Since muscle cells are the only cells that can furnish motion, one might ask what role the skeletal system plays in movement. The answer to this question has two parts.

The first part is that the skeletal system provides stable anchoring points for muscles (Figure 9.3). These points are needed to make the force generated by muscle contraction effective. If muscles were not firmly attached to other structures, they would slide about inside the body when they contracted, and no helpful actions would be performed.

Figure 9.3 Skeletal components as anchors and levers. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The second part of the answer is that the skeletal system acts as a set of levers to modify the motion provided by the muscles (Figure 9.3). This converts the simple shortening of muscles into the multitude of varied movements that people perform to maintain themselves. Therefore, people can perform bending, twisting, turning, and lengthening actions as well as shortening ones. All these actions can be observed when one watches people perform ordinary tasks such as household chores. A skeletal lever is also able to increase or decrease the distance, speed, and force obtained from the contraction of a muscle so that they better suit the task to be performed. For example, the muscles of the leg can move only a few inches. However, since these muscles are attached to the long bones of the leg, a person can quickly jump far out of the way of an oncoming vehicle.

Mineral Storage

Besides helping to maintain homeostasis in mechanical ways, the skeletal system helps in maintaining homeostasis of certain chemicals through mineral storage.

Minerals are needed so that the body cells can perform properly. For example, calcium is necessary for muscle contraction and nerve impulses and is also important in regulating the speed of many cell activities. Phosphorus is used in the processes that supply energy in all cells, and it is a main ingredient of cell membranes. Each cell must be supplied with the correct level of each mineral at all times. An overabundance can injure or poison cells, and a deficiency can make a cell function abnormally or prevent it from functioning.

The skeletal system helps maintain homeostasis of minerals through two activities. First, extra minerals are taken out of the blood by the bones when their concentration begins to rise above the proper point. This situation often occurs after one has eaten calcium-rich foods such as dairy products. Later, the level of minerals in the blood begins to drop below the proper concentration because they are being used by cells and are being lost in urine and perspiration. Then the bones put back into the blood just enough of the minerals they had stored so that the body cells always have enough. The skeletal system can also store toxic minerals such as lead.

Blood Cell Production

The skeletal system helps prevent changes in the amount of cellular components of the blood (red blood cells, certain types of white blood cells, platelets) by producing them whenever their numbers drop too low. Blood cell production is the one function of the skeletal system that is not performed by the bone material; rather, it is accomplished by specialized bone marrow tissue within the bones (Figure 9.4).

Figure 9.4 Structure of a bone. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Red blood cell production is increased when oxygen in the blood begins to drop. With more red cells, the oxygen is restored to normal levels. As long as oxygen levels remain high, red cell production is slow. Thus, the skeletal system helps keep red blood cell numbers and oxygen levels proper and fairly stable.

White blood cells play a variety of roles, including defending against infection and controlling the inflammatory response. Platelets prevent the loss of blood by helping it clot. The mechanisms controlling the bone marrow so that levels of white blood cells and platelets remain within a normal range are not clearly understood.

The production of blood cells occurs only in red bone marrow. In adults, this marrow is found within the bones of the head, the trunk, the arm bones at the shoulder, and the leg bones at the hip. The rest of the marrow is yellow bone marrow, which stores fat molecules. Yellow marrow is converted to active red marrow when the body needs an extra supply of blood cells and is converted back to yellow marrow once the need has been met.