The Structural System
Lesson 14, Page 3 of 16

True. One of the functions of the bones is to store calcium, releasing it into the bloodstream when necessary, to keep enough calcium in the blood for vital bodily functions such as muscle contraction and nerve conduction.

In fact, the bones are often thought of as a calcium bank. When there is sufficient calcium in the diet, particularly when we are young, our bodies store calcium in the bones making them stronger. When we get older, and especially when we don't get enough calcium in our diets, our bodies make withdrawals from the calcium bank in order to keep the 1% in the blood constant.

That is why it is extremely important for young people to get sufficient calcium in their diets. They are making deposits to their "calcium bank" that will be needed as they grow older. Unfortunately, young people do not generally get enough calcium, and to make matters worse, they often engage in lifestyle habits, such as soft drink consumption, that lead to excess loss of calcium from the body.

Because of calcium's ability to buffer acids, the body also uses calcium from the bones to help maintain the proper pH of the blood. In order to maintain life, the pH of the blood must be keep constant at all costs. When the pH of the blood begins to fall (i.e., the blood starts becoming more acidic) the body will increase osteoclastic activity pulling calcium out of the bones into the bloodstream to buffer the acidic condition. This calcium is eventually lost through the urine; and when it becomes excessive it can lead to a weakening of the bones. This is the primary cause of osteoporosis, which has become an epidemic in our society. (We will discuss osteoporosis later in this lesson.) The cause of this excess loss of calcium is the consumption of too many acid-producing foods, common in today's "modern" diet, with the biggest culprits being animal protein (including dairy), products containing refined sugar and refined "enriched" flour, and carbonated beveragesparticularly cola drinks.


Cartilage, commonly called gristle, is a dense, rubbery connective tissue which connects and cushions the bones and gives support and shape to rigid structures such as the ear, nose and windpipe. Cartilage is composed of specialized cells, called chondrocytes, surrounded by a gelatinous matrix of collagen, a tough protein. Just as there are different types of bone, there are also different types of cartilage. For example, hyaline cartilage is the connective tissue that covers the moving ends of bones.

A characteristic of cartilage is that it tends to harden with age. This is due to cross linkages in its microscopic matrix, caused mainly by free radical damage, and is one of the reasons that free radical scavengers, or antioxidants, are purported to slow down some of the effects of aging.


Muscle tissue is a special type of tissue that has the ability to contract, usually as a result of a stimulus from a nerve. The three types of muscles are smooth, cardiac and skeletal. Smooth muscle is found in the lining of organs, such as the stomach and intestines, and is responsible for involuntary movement—movement that we do not have conscious control over. Cardiac muscle is a special kind of involuntary muscle found only in the heart and is responsible for pumping the blood through the body. The most abundant type of muscle is skeletal muscle, which is responsible for voluntary movements such as walking and talking. Some of the major skeletal muscles are illustrated below:

muscles front muscles back
Muscles can only contract and relax. They do not forcefully expand. Opposing movements, such as flexion and extension of the arm, are performed by the actions of opposing muscle groups located on either side of the joint. The biceps muscle of the inside of the arm, for example, is responsible for the flexion or bending of the elbow (bringing the bones closer together), while the triceps muscle, located on the back side of the arm, is responsible for the extension of the elbow (straightening out the arm.)

True or False: One of the primary functions of the muscles is movement.


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