The Human Body: Systems Working Together

In recent years, stocks of high-fat fish have been declining in the waters off the west coast of the United States. This has caused the population of seals that eat the fish to decline as well.


An abscess is an accumulation or collection of pus* that can result from an infection in any part of the body. An abscess is often surrounded by red, swollen tissue.


Until he turned 13, Johnny's skin had always been clear. Soon after the start of eighth grade, though, Johnny noticed the first few pimples on his face.

Addison's Disease

Many people know that U.S. President John F.


In the early 1980s, doctors in New York and California began noticing a very unusual disease in a small number of young men. The men, who were mostly homosexual, were developing unusual infections and cancers, and some of them were dying.


Albinism (AL-bi-niz-im) is a disorder that occurs when a person inherits various genes* that are defective in their ability to make the pigment melanin (MEL-a-nin). Melanin is the main substance that determines a person's skin, hair, and eye color.


Heavy drinking places a heavy burden on the body. It has been estimated that 25 to 40 percent of the patients in general hospital beds in the United States are there because of illnesses caused by alcohol.


The human body is filled with special cells and organs that stand ready to fight invaders. Together they are called the immune system.

Altitude Sickness

Altitude sickness is caused by a lack of oxygen. Altitude sickness can affect anyone at high altitude.

Alzheimer's Disease

Joe's grandmother seems to do the strangest things. She repeats stories she told him in almost the exact words she used during his last visit.


Amebiasis is a disease caused by an ameba, or microscopic parasite*, called Entamoeba histolytica. At one stage in the ameba's life cycle, it is enclosed in a protective wall called a cyst.


Many movies and television programs show people losing their memory after a whack on the head. They awake mumbling about their identity.

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (a-my-o-TROF-ik LAT-er-al skle-RO-sis) (ALS) is also called Lou Gehrig's disease after the famous baseball player who died after developing the condition. ALS is uncommon.


When Carrie turned thirteen, she was determined to finally lose all of what she called her "baby fat." She longed to look like the skinny models who graced the covers of the fashion magazines. Carrie stopped eating the well-balanced meals her mother made.


Aneurysms are sometimes called "silent killers" because they may go undetected for years until they break open. The wall of a section of an artery*, vein*, or other blood vessel may become weak and begin to bulge, like an underinflated balloon whose air is squeezed from the ends to the middle.

Animal Bites

Animal bites can range from mild to serious. When the skin is not broken, bites usually are not dangerous.

Anxiety Disorders

Everyone worries now and then about things such as passing an upcoming math test or being chosen for a team. However, people who worry excessively may be experiencing an anxiety (ang-ZY-eh-tee) disorder.


Acute* appendicitis occurs when the vermiform (VER-mi-form) appendix becomes infected with bacteria. Vermiform means shaped like a worm.


For many years, Jenny's father was a dedicated runner. He got up early each morning for a 3-mile jog, and on weekends he sometimes ran as far as 10 miles or competed in road races.


The world is full of roundworms. Hundreds of thousands of different kinds inhabit the Earth, and a few types inhabit the bodies of humans, as well.


When Stacy was young, her parents noticed that she seemed to get tired more quickly than her friends while playing. She also had repeated coughing spells, and her breathing was sometimes noisy.


The eyeball usually is round and nearly the same size as a ping-pong ball. The front part contains a clear layer of tissue called the cornea and the lens that help to focus the light that enters the eye.