Aerobic: something that occurs in the presence of oxygen. anaerobic: something that occurs without oxygen because a person is using energy to do activities at a faster rate than the body is producing it. anxiety: intense worry and fear. arthritis: chronic inflammation of the joints. blood pressure: pressure of blood against the walls of blood vessels. cardiovascular fitness: how efficiently the heart and lungs can pump blood (which holds oxygen) to muscles that are being worked. endorphins: proteins in the brain that act as the body's natural pain reliever. endurance: a person's ability to continue doing a stressful activity for an extended period of time. exercise: a subset of physical activity, which is an activity that is structured and planned. heat stroke: a serious condition that causes the body to stop sweating and overheat dangerously. immune system: a body system that protects the body against illness. osteoporosis: a condition involving a decrease in bone mass, making bones more fragile. physical activity: any movement that spends energy. stroke: a sudden loss of consciousness, feeling, and voluntary movement caused by a blood clot in the brain. tendinitis: inflammation of a tendon. yoga: a series of exercises that incorporate regulated breathing, concentration, and flexibility.