The Muscular System
Each muscle pulls on a joint and moves the bones in a different way depending on the type of
joint. The pull of a muscle occurs when the muscle contracts, or becomes shorter and tighter.
Often, muscles work in pairs so that one muscle relaxes while the other one contracts. For
example, when the muscle at the front of the upper arm contracts, the muscle at the back of that
arm relaxes so that the arm can bend at the elbow. To straighten the arm back out, the muscle at
the back of the upper arm contracts, and the muscle at the front of the arm relaxes. Muscles also
help move food through the digestive system, fill and empty the lungs, and rotate the eyes.
1. Which sentence best introduces the ideas in the paragraph?
A. Cardiac muscle is found in the heart, and it helps keep blood flowing throughout the
body.
B. Smooth muscle lines many organs, such as the stomach and intestinal tract, and helps
these organs move fluids and solids through the body.
C. The human body has more than 650 tough, elastic pieces of tissue called muscle that work
with bones and joints to allow us to move.
D. There is no specific type of doctor who treats diseases and disorders related to muscles.