The human body is remarkably well designed. Most of its organs Organ Systems Although each organ has its specific functions, organs also function together in groups, called organ systems (see table ). Doctors categorize disorders and their own medical specialties according... read more have a great deal of extra capacity or reserve: They can still function adequately even when damaged. For example, more than two thirds of the liver Liver The wedge-shaped liver is the largest—and, in some ways, the most complex—organ in the body. It serves as the body's chemical factory, performing many vital functions, from regulating the levels... read more must be destroyed before serious consequences occur, and a person can usually live with only one lung Overview of the Respiratory System To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with... read more or one kidney Kidneys The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies... read more . However, other organs can tolerate little damage before they malfunction and symptoms occur. For example, if an artery in the brain becomes blocked or ruptures (stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more ) and even a small amount of tissue in a vital part of the brain is destroyed, a person may be unable to speak, move a limb, or maintain balance. If a heart attack Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or a heart attack (myocardial infarction), depending on the location and amount... read more 
destroys a small amount of tissue in the part of the heart that creates or carries the signals to beat, the heart rate may become dangerously slow and the person may even die.
Disease often affects anatomy, and changes in anatomy can cause disease. If the blood supply to a tissue is blocked or cut off, the tissue dies (called infarction), as in a heart attack Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or a heart attack (myocardial infarction), depending on the location and amount... read more (myocardial infarction) or stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more (cerebral infarction). An abnormal heart valve Overview of Heart Valve Disorders Heart valves regulate the flow of blood through the heart's four chambers—two small, round upper chambers (atria) and two larger, cone-shaped lower chambers (ventricles). Each ventricle has... read more 
can cause heart malfunction. Trauma to the skin Structure and Function of the Skin The skin is the body’s largest organ. It serves many important functions, including Protecting the body against trauma Regulating body temperature Maintaining water and electrolyte balance Sensing... read more 
may damage its ability to act as a barrier, which may lead to infection. Abnormal growths, such as cancer Overview of Cancer A cancer is an abnormal growth of cells (usually derived from a single abnormal cell). The cells have lost normal control mechanisms and thus are able to multiply continuously, invade nearby... read more , can directly destroy normal tissue or produce pressure that ultimately destroys it.
Because of the relationship between disease and anatomy, methods of seeing into the body have become a mainstay in the diagnosis and treatment of disease. The first breakthrough came with x-rays Plain X-Rays X-rays are high-energy radiation waves that can penetrate most substances (to varying degrees). In very low doses, x-rays are used to produce images that help doctors diagnose disease. In high... read more , which enabled doctors to see into the body and examine internal structures without surgery. Another major advance was computed tomography Computed Tomography (CT) In computed tomography (CT), which used to be called computed axial tomography (CAT), an x-ray source and x-ray detector rotate around a person. In modern scanners, the x-ray detector usually... read more 
(CT), which combines x-rays and computers. A CT scan produces detailed cross-sectional (two-dimensional) images of the body's interior.
Other methods of producing images of internal structures include ultrasonography Ultrasonography Ultrasonography uses high-frequency sound (ultrasound) waves to produce images of internal organs and other tissues. A device called a transducer converts electrical current into sound waves... read more 
, which uses sound waves; magnetic resonance imaging Magnetic Resonance Imaging (MRI) In magnetic resonance imaging (MRI), a strong magnetic field and very high frequency radio waves are used to produce highly detailed images. MRI does not use x-rays and is usually very safe... read more 
(MRI), which uses the movement of atoms in a magnetic field; and radionuclide imaging Radionuclide Scanning In radionuclide scanning, radionuclides are used to produce images. A radionuclide is a radioactive form of an element, which means it is an unstable atom that becomes more stable by releasing... read more , which uses radioactive chemicals injected into the body. These are noninvasive ways to see into the body, in contrast to surgery Surgery Surgery is the term traditionally used to describe procedures (called surgical procedures) that involve manually cutting or stitching tissue to treat diseases, injuries, or deformities. However... read more , which is an invasive procedure.
