American Medical Association Family Medical Guide - American Medical Association [84]
Muscles
The more than 600 muscles in your body are composed of bundles of interlocking fibers that have the ability to contract (shorten) and relax (lengthen). The skeletal muscles are attached (directly or with a tendon) to two or more bones; when these muscles contract, the bones move. A group of muscles can often work together—one contracts, another relaxes, and nearby muscles provide stability.
Front view
Back view
Immune System
Your immune system provides your body with a wide variety of mechanisms—both internal and external—that protect you from disease-causing microorganisms such as viruses and illnesses such as cancer. The immune system is a complex, tightly orchestrated network of proteins, cells, organs, and the lymphatic vessels, all of which work together to keep you healthy.
Lymphatic system
The major organs of the immune system are called lymph nodes. Lymph nodes—clustered in your neck, armpits, and groin—contain white blood cells called lymphocytes that mount the response against potentially harmful invading microorganisms such as viruses or bacteria. Like all blood cells, white blood cells are produced in the soft marrow inside bones. White blood cells leave the bone marrow and are carried in the blood to the lymph nodes.
A system of fluid-filled ducts (called lymphatic vessels) channels white blood cells from the lymph nodes back into the bloodstream. White blood cells patrol the entire body—circulating in the blood, lymph nodes, and lymphatic vessels—to watch for harmful microorganisms and to remove damaged cells.
The thymus, an organ that lies behind the breastbone, is where a group of lymphocytes called T cells grows to maturity. The spleen, a fist-sized organ in the upper left corner of the abdomen, contains large numbers of white blood cells, including many lymphocytes. The tonsils and nearby adenoids (not shown) and the appendix are clumps of lymphoid tissue that provide lines of defense at sites in the body where potentially harmful microorganisms are likely to enter or multiply.
Bones
Your bones and muscles work together to support your body and enable you to move. The average human skeleton has 206 bones—32 bones in each arm, 31 in each leg, 29 in the skull, 26 in the spine, and 25 in the chest. In some people, the number of bones varies slightly from the norm—for example, about 5 percent of us have an extra pair of ribs, and some of us may have a few extra bones in our hands or feet or may be missing one or more bones.
Bones meet at joints, of which there are several types. Fixed joints, such as those in the skull, hold the bones firmly together. Partly movable joints, such as those between the bones of the spine, allow limited flexibility. Freely movable joints, such as in the jaw, hip, knee, or shoulder, provide variable flexibility in several planes of movement.
The skeletons of men and women differ very little. Men’s bones are generally slightly larger and heavier than women’s bones. The cavity in the female pelvis, surrounded by the hipbones and sacrum, is wider than the cavity in the male pelvis, to accommodate the passage of a baby during delivery.
Heart and Blood Vessels
Blood delivers life-sustaining oxygen and other vital nutrients to cells throughout the body and carries away wastes produced by the cells. The pumping action of the heart keeps the blood in constant circulation, sending it to the lungs to pick up a fresh supply of oxygen and then pushing it back out to the organs and tissues. Every minute, the heart pumps about 5 quarts of blood through the entire circulatory system.
Heart
The heart is a muscular