Care About Your Body and Health

(Acquired immune deficiency syndrome) AIDS, first described in 1981, is caused by a virus that not only damages tissues, but also increases vulnerability to many disorders, especially infections. These result from the progressive destruction of the immune system by the human immunodeficiency virus (HIV). The virus is transmitted when a body fluid of an infected individual-blood, semen, vaginal secretion, or breast milk-is absorbed into the blood-stream of a healthy person. The major exception appears to be saliva, which has never been shown to be a carrier.

When the disease was first discovered in the United States, most cases were transmitted by male homosexual activity, in contrast to the underdeveloped nations, where AIDS was spread mostly through heterosexual intercourse. Now most cases in the U.S. are transmitted by the sharing of hypodermic needles and heterosexual intercourse, with women accounting for a large percentage of new cases. Before development of an HIV screening test in 1986, a number of cases were contracted from contaminated transfusions and the coagulation factors used to treat hemophilia.

Whenever a virus enters the body, a healthy immune system produces a variety of fighter cells, which include the T-cell lymphocytes. HIV invades T-cells and uses their genetic material to multiply itself. The virus eventually destroys the T-cells, producing many new HIV particles in the process. In time, the immune system is over-whelmed by the infection, and the person becomes increasingly susceptible to the infections and other disease that make up the AIDS complex.

It takes an average of 10 years from the time of infection to develop full-blown AIDS, and throughout that entire period, contact with body fluids from the person harboring the virus can spread the infection.

The body contains three types of muscle: cardiac, found only in the heart; involuntary smooth muscles, which are part of various organs; and the skeletal muscles, which are attached to bones and make voluntary movement possible.

Skeletal muscles-the body’s most abundant tissue-are made up of bundles of long fibers bound together by connective tissue. Each fiber is surrounded by tiny capillaries, which deliver a steady supply of oxygen needed for the muscles to function. The fibers in a particular muscle remain constant in number throughout life, but they enlarge when exercised frequently and shrink, or atrophy, with disuse and age.

Every set of muscles is served by one or more nerves. Movement occurs when nerve signals set off specific chemical reactions that cause certain muscles to contract. Most muscle disorders are actually due to nerve problems. The muscle weakness of multiple sclerosis is one example.

Although males and females have the same number-about 600-of skeletal muscles, men are much more muscular than women; 40 percent of a man’s weight is muscle tissue, compared to 23 percent in women. Muscles are attached to tendons, which in turn are attached to bones. Compared to muscles, tendons are relatively inelastic, providing needed stability. Many muscles are arranged in groups, in which one serves as prime mover, or agonist, another as assistant mover, and a third as antagonist to stabilize movement. When bending the knee, for example, the hamstrings are the prime mover, the Sartorius is the assistant mover, and the quadriceps are the antagonist.

The length and placement of the Achilles tendon make it especially vulnerable to injury. The skin not only serves as the body’s protective armor, but it also helps control body temperature, is instrumental in making vitamin D, and is essential to the sense of touch.

Philosophers and scientists alike have observed that human beings and other forms of animal life are made up of the same elements found in ordinary soil and water. But when these 20 or so basic elements combine in thousands of different ways to form a human body, the result is one of the most complex organisms on the planet and a never-ending source of both wonderment and mystery.

About 75 to 80 percent of an adult’s body consists of slightly salty water; the rest is made up of chemical compounds, many of them unique to human beings. These various compounds are arranged to form hundreds of different kinds of cells, the body’s smallest, most basic units.

All human life begins with the fusing of two cells and the subsequent division and multiplication of cells to form a complete body follows the same general blueprint even though no two people are exactly alike.

The average body contains 80 to 100 trillion cells, each programmed to grow, carry out a specific function, and even replicate itself. But, with the exception of blood cells, none function independently; instead, similar cells join together to form specific types of tissues-muscle, nerve, bone, and so forth. Each body organs is made up of a collection of related tissues.